Studio:Minmus' Worldbuilding Guide

Part 1: Stars

Worldbuilding is fun, and making guides about it is also fun. We will start with planets because they can be pretty important to the story, so it's important to understand how to create detailed and cool planets.

File:Ee.png

Ignore the Space Engine part

Stars

When building a star, you need to take into consideration whether you want planets to go around it, and if so, what kind and whether they are habitable. Stars have several classes, some of which are habitable and some are not. On average, 1000 stars will contain 900 main sequence stars, 96 white dwarfs and 4 giant stars. Among the 900 main sequence stars, there are 1 O star, 1 B star, 5 A stars, 30 F stars, 70 G stars, 98 K stars and 695 M stars. Let’s just add in a neutron star and a black hole, for good measure.

There are many different types of stars, which have different methods to make them.

Main Sequence Stars

The first step in building a star is to set the mass. The mass controls LITERALLY EVERYTHING ELSE about your main-sequence star. Main sequence stars go from 0.08 Solar Masses to 50 Solar Masses. Within this range, there are smaller ranges based on the different stellar types.

• P/O: >55 M_Sol
• O: 17-500 M_Sol
• B: 2.1-17 M_Sol
• A: 1.4-2.1 M_Sol
• F: 1.04-1.4 M_Sol
• G: 0.8-1.04 M_Sol
• K: 0.45-0.8 M_Sol
• M: 0.0748-0.45 M_Sol

By the way, only G, K, M and maaaybe F stars are truly habitable for long enough for intelligent life to evolve. Luckily, they’re more common than dirt! O, B and A stars can theoretically have habitable planets around them, but they wouldn’t be habitable long enough for intelligent life to evolve. However, they can still be colonies and stuff like that.

[Sidenote] also, smaller stars live longer. [end sidenote]

Now that THAT’S over with, Equation time!

Radius, Circumference, Surface Area, Volume, Gravity and Density

If you’ve chosen a mass less than our sun, the radius is given by:

M^0.8

If you’ve chosen a mass more than our sun, the radius is given by:

M^0.55

Now that you have your radius, circumference, surface area, volume, gravity, density can be found like so, in relative values.

C=R

SA=R²

V=R³

⍴=M/V

g=M/R²

If you wish, here are some absolute values.

C=2πR

SA=4πR²

V=4/3πR³

⍴=M/V

Luminosity

The intrinsic luminosity of your star is given by the mass-luminosity relationship:

L=M^3.5

This is a good fit for the main sequence (assuming this is in the present era of the universe), but it isn’t perfect. Here are some better ones if you want to be REALLY accurate:

L=0.23•M^2.3 (M<0.43)

L=M⁴ (0.43<M<2)

L=1.4•M⁴ (2<M<55)

L=32000•M (M>55)

Lifetime (time on the main sequence)

Your star’s time on the main sequence is given by:

T_ms= (M/L)

Multiplying by a solar lifetime, 10-11 billion years, will give your star’s lifetime in earth years.

[interesting side note] the least massive main sequence stars will remain on the main sequence for approximately 5.5 trillion years, which is a long time. In fact it’s about 400 times longer… than time itself. [interesting side note end]

Effective (surface) Temperature

The effective temperature of your star will be given by:

T_eff= (L/R²)^0.25

Multiplying by the temperature of the sun, 5778K, gives you your star’s temperature in Kelvin.

Then, use these charts to determine your star's approximate color.

Spectral Type	Color
I and WO	Deep Blue
P	Deep Blue
WC and WN	Blue
O	Blue
B	Deep Blue White
A	Blue White
F	White
G	Yellowish White
K	Pale Yellow Orange
M	Light Orange Red

Habitable Zone

If you want your star to have planets that support life similar to earth life, you must ensure your planets are in the habitable zone, unless you want to do something really weird.

The habitable zone extremities in AU are given by: HZ_inner=√(L/1.1)

HZ=√L

HZ_outer=√(L/0.53)

If you want to make life that uses methane as a solvent, and know how to find the habitable zone for methane, PLEASE TELL ME!

Frost Line

The frost line is the distance beyond which gas giants can form. It is given by:

FL=4.85•HZ

This is important, because gas giants always form outside this line, though they can and often do migrate inwards, like Jovoterra.

Post Main Sequence Stars

Post Main Sequence Stars are really weird, and are therefore very hard to calculate using simple mAtHs. However, we'll try.

Giants, Supergiants and Hypergiants, Method 1 (easier, but less accurate)

Giants, supegiants, and hypergiants are really weird, and are therefore very hard to calculate using simple mAtHs. However, we'll try. As an approximation, plug in the mass, the radius, and the luminosity, as these are the hardest to calculate. With these, you can calculate the other stats.

Circumference, Surface Area, Volume, Gravity, and Density

The circumference, surface area, volume, gravity, and density can all be calculated in the same way as with main sequence stars.

Surface Temperature

That can also be calculated the same way as main sequence stars.

Habitable Zone and Frost Line

Again, same as main sequence stars

Giants, Supergiants and Hypergiants, Method 2 (harder, but more accurate and gives more information)

To begin with this method, choose the mass in solar masses (M), the temperature in Kelvin (T), the radius in Solar Radii (R), and the percentage of fuel left (not hydrogen, helium) in the core (F).

Luminosity

This can be found with the following equation:

L = (T/5778)⁴ • R²

Lifetime Left

This, which can be useful for plot elements, can be found with this equation in million years:

T_left=((M•F)/L)•11200

Habitable Zone, Circumference, Surface Area, Volume, Gravity and Density

These can be derived from the same equations as main sequence stars. Ain't that convenient?

White Dwarfs

These are significantly easier to calculate than giant branch stars, and all statistics can be found by choosing the mass (M) and age (A).

Radius, Circumference, Surface Area, Volume, Gravity and Density

A white dwarf's radius actually gets smaller as it gains mass, and this ballpark equation is designed to reflect that.

R=(M^-1/3) • (0.95+(0.05/(1+(A^0.25)))

The Circumference, Surface Area, Volume, Gravity and Density can be found in the same way as main sequence stars, as they are both spheres.

Surface Temperature

A white dwarf's surface temperature is mainly dependant on the mass and the age. However, its equation is a little convoluted.

T_eff = ((M•10)^0.5) • (8/(1+A^0.25)

Luminosity

A white dwarf's luminosity is very small, and can be found with this equation.

L = (T_eff⁴) • (R/109.5•2)²

Habitable Zone and Frost Line

This can, again be found in the same way as with main sequence stars.

Neutron Stars and their Variants

Neutrons stars are insanely tiny, and this kind of messes with their statistics, which increases the convolutedness of the equations. However, all you need are the mass (M), and the age (A).

Radius, Circumference, etc.

The radius can be found by the below equation. Again, the Circumference, Surface Area, and so on (except density and gravity) can be found the same way as with main sequence stars.

R = (7/(M^0.2) • (1 + 1/((A+1)^0.07))

Density & Gravity

The density will be in Tons/cm³ and is given by this equation:

⍴ = (M/1000)/((4/3) • 3.141 • (1000R))

The gravity will be in m/s² and is given by the equation below:

g = ((((6.674•10^-11) • (M•1.9884•10³⁰))/((1000R)²)) • 27.959

Surface Temperature

This is given in Kelvin. Here's the equation.

T^eff=(7000000/(1+(A^0.5))) • M^0.5

Luminosity

Neutron Stars, given their tiny size, can have very large luminosities, even up to 1 solar luminosities! Equation:

L = (T/5778)⁴ • (R/696342)²

Black Holes

Black holes are even easier to calculate, especially if they are nonspinning. You only need the mass (M) and the Rotation (S), which is from 0 to 1, and defines the ratio between the black hole's spin rate and its maximum spin rate.

Schwarzschild Radius, Photon Sphere, and ISCO

The Schwarzschild Radius defines the event horizon. It is given by the following equation:

R = M•2.96 • (1-S)^0.585

The Photon sphere is the innermost possible orbit and is given by the following very simple equation:

P_s = 1.5R

The Innermost Stable Circular Orbit is the innermost possible S T A B L E orbit, for objects that have mass, and is given by the following simple equation:

I_orbit = 3R

Lifetime

The lifetime of a black hole is the length of time after which it decays from hawking radiation, assuming no infall of matter. Here's the equation:

L = M² • 2•10⁶⁹

Brown Dwarfs

Brown dwarfs are very common and very interesting, so it is useful to know how to build them.

Mass

Brown dwarfs have a mass between 13 - 75-80 M_♃. That was easy!

Radius

Sadly, the easiness ends here. Radius has a very complicated equation based on the Mass (M) and the Age (A).

R = ((2+((M^0.18)•(1/(1+(M•0.018)⁵)) • (1+((M•0.013)⁷)))/3) • (0.95+(0.05/((1+0.1A)^0.5)))

Doesn't that look insane? Luckily, we have it in our spreadsheet.

Temperature

T_eff = ((M^0.92)•30 + 250) / (0.8 + (A•0.0001)^0.5)

At least its shorter...

Luminosity

L = 0.1027198667R • (Teff/5778)²)²

They're getting easier! WOOOOHOOOO!

Density, Circumference, Surface Area, Volume, Surface Gravity, Escape Velocity

Again, same as stars.

Supernovae

Supernovae are insane and can be used for plot points. it is helpful to know what they can do.

Explosion Energy

If a star is greater than 200 solar masses, the explosion energy, in BILLION YEARS OF SOLAR OUTPUT, is given by the following equation:

E = 204.029 / (1 + (M - 200)²)

If the star is less than 200 solar masses, the explosion energy is given by the following equation:

E = M^0.7 • 5

Luminosity

The maximum luminosity of a supernova in solar luminosities is given by the following equation:

L = E • 3.1536•10¹⁶

Initial Temperature

The initial temperature of a supernova, in Kelvin, is given by this equation:

T_initial = (((M^0.5)•5) • 5)/30)^0.7)•1000000000

Part 2: Galaxies & Nebulae

Previously, we created a bunch of different types of stars. In Worldbuilding Guide Part 2: Galaxies and Nebulae we will create the objects which surround the stars, and make them not float in empty space. In this part we will be creating and discussing Galaxies, Nebulae, and Star Clusters.

Galaxies

Galaxies will be the largest contiguous objects in pretty much any worldbuilding project, and hugely vary in size, ranging from 1,000 to 100 trillion stars. They also hugely vary in appearance, with some being featureless yellow-white globes, and others having beautiful spiral structures.

What is a galaxy?

To begin, we will ask the question, what is a galaxy? In short, a galaxy is a huge system of stars, gas, dust, and dark matter, orbiting a common center of mass (usually a black hole), bound together by gravity. Galaxies come in many different shapes and sizes. The three main types of galaxies are Elliptical, Spiral, and Irregular. However, many other types exist, such as Dwarf Galaxies, Peculiar Galaxies, and Ultra-Diffuse Galaxies.

Dark Matter

Back when astronomers first started weighing galaxies, they noticed that there was nowhere near enough visible matter to hold the galaxy together. They had no explanation for this until a dude named Fritz Zwicky came along and proposed Dark Matter, a strange substance which doesn't interact with any of the fundamental forces except for Gravity and maybe the Weak Nuclear Force.

Dark Matter isn't something you really ever have to mention that much. But if you want to note down how much of it your galaxy has, a general rule is that most galaxies are ~85% dark matter, but you can vary this about 10% either way if you wish.

Distribution

Galaxies are not distributed randomly. They tend to form galactic clusters or groups. Galaxies in these groups are gravitationally bound and tend to influence one another. These groups also tend to gravitationally attract each other, forming massive structures referred to as Galactic Superclusters. Collections of superclusters can form even larger structures referred to as Filaments, which are visible at the scale of the entire observable universe.

Because of the distribution of galaxies, galaxies can collide relatively often. When two or more galaxies collide, they tend to trigger rapid star formation, one of the mechanisms by which a so-called Starburst Galaxy can form.

Spiral Galaxies

A spiral galaxy is the most common large galaxy type in the modern universe. A spiral galaxy is defined by its large density waves, causing large amounts of star formation and making the areas around the waves appear blue. Spiral galaxies are usually considered more habitable galaxies than other types, but this is debatable.

Structure

Spiral galaxies consist of a central bulge, a disk, large spiral arms (overdensities within the disk), and a sparse halo extending to intergalactic space. The bulge is generally considered not very habitable because of the higher densities, but this is again debatable. Similarly, the densest portions of the spiral arms are not considered very habitable because of the higher densities and much higher supernova rates.

Barred Spiral Galaxies

The most common subtype of spiral galaxies is the barred spiral galaxy. These are very similar to spiral galaxies, the difference being that the bulge is elongated into a long bar, with the spiral arms emanating from the ends. That's basically the only difference.

Lenticular Galaxies

Another common subtype of a spiral galaxy is a lenticular galaxy. This is a galaxy which is mostly starved of gas and dust. Lenticulars lack the prominent spiral arms and blue hue of normal spiral galaxies, and most planets in them are relatively old.

Math

Spiral Galaxies are often considered to have a "Galactic Habitable Zone", a region in which life is most likely to develop. This is usually at 0.47-0.6 times the radius in light-years.

The number of stars can be approximated by calculating the volume of a cylinder with the radius of the galaxy and a height of 1/100 the radius in light-years and multiplying by 0.004 for the disk. Then, add the volume of the bulge, which can be approximated as an ellipsoid and multiply by 0.0346. Once you add these together with your calculator you'll have the full number of stars in your galaxy.

Elliptical Galaxies

Elliptical Galaxies are spherical or elliptical in shape. They lack the gasses required to form new stars, so they are almost entirely composed of old stars (Stellar Population|Population II), giving them a reddish/yellowish color in contrast to the blue of spiral galaxies. Large elliptical galaxies usually have extensive systems of globular clusters surrounding them. Elliptical galaxies are thought to be the result of galactic collisions. 15% of galaxies are elliptical.

The smaller cousin of the Elliptical galaxy is the Dwarf Elliptical Galaxy: something in between an elliptical galaxy and a globular cluster. These dwarfs are often found as a satellite galaxy to a bigger galaxy.

The stars within the galaxy don’t orbit on a single plane, as in disk-shaped galaxies, but rather orbit randomly at varying inclinations. Profiles such as the Sersic Profile and Einasto profile can mathematically calculate the intensity and density of the galactic bulge, but these are rather complex and require you to work out things such as the scale height and central intensity of your elliptical galaxy, which is overkill and not very relevant overall. All that you really need to know is that the stellar density of the galaxy increases as you get closer to the galactic bulge.

Classification

The Hubble Classification System denotes ellipticals with the letter E, and dwarf ellipticals with dE. A number from 0 to 7 follows this, and describes the shape of the elliptical galaxy, with 0 describing a spherical galaxy and 7 describing a cigar-shaped galaxy. If you know the semi-major and semi-minor axes of the elliptical, you can calculate its "E number" as En where:

n = 10 • (1-b/a)

where a is the semi-major axis and b is the semi-minor axis of the ellipse.

About 1/10 of ellipticals have shell structures, where stars in the galaxy’s halos are arranged in concentric shells. This is a property exhibited only by ellipticals.

Math

The star count of elliptical galaxies can be approximated by finding the volume of the galaxy in cubic light-years (they're ellipsoids) and multiplying by 0.006 or something near that.

Irregular Galaxies

Irregular Galaxies are the most common of galaxies, and are mostly found in the form of dwarf satellites. Irregular galaxies are generally highly active galaxies, with high rates of star formation. Irregular galaxies, being shapeless blobs, are rather difficult to classify aside from this. Irregular galaxies are very rare to find larger than dwarf size.

Subtypes

Irregular galaxies have many subtypes. The most common of them is the Magellanic Spiral, which is a type of irregular galaxy which used to be a spiral. The most well-known real-world example of such a galaxy is the Large Magellanic Cloud, which has a single stubby spiral arm extending out from it.

Peculiar Galaxies

Peculiar Galaxies are essentially a drop box of all the random galaxy types which do not fit in the three main categories. As such, no general principles can be said about them.

Ultra-Diffuse Galaxies

Ultra-Diffuse galaxies are galaxies which have stars spread out in an extremely sparse fashion. These galaxies are extremely difficult to see, and aren't very useful in worldbuilding projects.

Interacting Galaxies

Interacting Galaxies are pairs (or larger numbers) of galaxies close enough to gravitationally affect each other. These galaxies can be practically any class except ultra-diffuse. Interacting galaxies generally are going through a starburst phase, with huge amounts of star formation.

Ring Galaxies

Ring Galaxies are very interesting. Instead of a disk, they have a large ring, separated from the core by a transparent gap with few stars in it. An example of this galaxy type is Hoag's Object.

Nebulae

A nebula is a large structure in the interstellar medium composed of interstellar gas. There are many types, and can serve as a very interesting backdrop for worldbuilding. They come in several types.

Planetary Nebulae

Planetary nebulae are the remnants of the final stages of stellar evolution for mid-mass stars(varying in size between 0.5-~8 solar masses). Evolved asymptotic giant branch stars expel their outer layers outwards due to strong stellar winds, thus forming gaseous shells, while leaving behind the star's core in the form of a white dwarf. Radiation from the hot white dwarf excites the expelled gases, producing emission nebulae with spectra similar to those of emission nebulae found in star formation regions. They are H II regions, because mostly hydrogen is ionized, but planetary are denser and more compact than nebulae found in star formation regions.

Planetary nebulae were given their name by the first astronomical observers who were initially unable to distinguish them from planets, and who tended to confuse them with planets, which were of more interest to them. Our Sun is expected to spawn a planetary nebula about 12 billion years after its formation.

Generally, one can make mature planetary nebulae mathematically by placing a sphere of gas around a white dwarf and picking a radius. It seems that the mean true expansion velocity of the nebular edge of a planetary nebula is approximately 42 km/s (source), so simply multiply the radius of the nebula by ~7138 to find the approximate age. If you don't want to do that, instead you can take an existing nebula (preferably a lesser-known one) and copypaste it. Thats basically what everyone does and nobody cares.

Protoplanetary Nebulae

A protoplanetary nebula (PPN) is an astronomical object at the short-lived episode during a star's rapid stellar evolution between the late asymptotic giant branch (LAGB) phase and the following planetary nebula (PN) phase. During the AGB phase, the star undergoes mass loss, emitting a circumstellar shell of hydrogen gas. When this phase comes to an end, the star enters the PPN phase.

The PPN is energized by the central star, causing it to emit strong infrared radiation and become a reflection nebula. Collimated stellar winds from the central star shape and shock the shell into an axially symmetric form, while producing a fast moving molecular wind. The exact point when a PPN becomes a planetary nebula (PN) is defined by the temperature of the central star. The PPN phase continues until the central star reaches a temperature of 30,000 K, after which it is hot enough to ionize the surrounding gas.

Protoplanetary nebulae are really complicated, and are ridiculously hard to make from scratch. Here your best bet is to just yeet a lesser-known one.

Supernova Remnants

A supernova occurs when a high-mass star reaches the end of its life. When nuclear fusion in the core of the star stops, the star collapses. The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode. The expanding shell of gas forms a supernova remnant, a special diffuse nebula. Although much of the optical and X-ray emission from supernova remnants originates from ionized gas, a great amount of the radio emission is a form of non-thermal emission called synchrotron emission. This emission originates from high-velocity electrons oscillating within magnetic fields.

Supernova remnants again have really complicated structures, despite the fact that they should be just planetary nebulae but speed. Just go grab one from the real universe (not the crab nebula that one is so overused).

Ejecta Nebulae

An Ejecta Nebula is similar to a planetary nebula. It consists of large amounts of material ejected from high-mass stars before the end of their life. They are actually rather common and surround a decent fraction of high-mass stars, but they're mostly ignored in discussion about nebulae. They aren't even mentioned in the wikipedia page about nebulae!

To create one, you have two options: either grab one from the real world (including central star or it doesn't make sense) or describe it as a shell of a certain size around a late-stage star of your choice. Some ejecta nebulae have more than one shell!

Molecular Clouds

Higher density regions of the interstellar medium form clouds, or diffuse nebulae, where star formation takes place. In contrast to spirals, an elliptical galaxy loses the cold component of its interstellar medium within roughly a billion years, which hinders the galaxy from forming diffuse nebulae except through mergers with other galaxies.

In the dense nebulae where stars are produced, much of the hydrogen is in the molecular (H2) form, so these nebulae are called molecular clouds. Dense molecular filaments, which are central to the star formation process, will fragment into gravitationally bound cores, most of which will evolve into stars. Observations indicate that the coldest clouds tend to form low-mass stars, observed first in the infrared inside the clouds, then in visible light at their surface when the clouds dissipate, while giant molecular clouds, which are generally warmer, produce stars of all masses. These giant molecular clouds have typical densities of 100 particles per cm3, diameters of 100 light-years (9.5×1014 km), masses of up to 6 million solar masses (M☉), and an average interior temperature of 10 K. About half the total mass of the galactic ISM is found in molecular clouds and in The Silky Way there are an estimated 6,000 molecular clouds, each with more than 100,000 M☉.

A more compact site of star formation is the opaque clouds of dense gas and dust known as Bok globules, so named after the astronomer Bart Bok. These can form in association with collapsing molecular clouds or possibly independently. The Bok globules are typically up to a light year across and contain a few solar masses. They can be observed as dark clouds silhouetted against bright emission nebulae or background stars. Over half the known Bok globules have been found to contain newly forming stars. These are useful for hiding big secret things in worldbuilding.

These are even more insanely complicated, so the best way to go is to say "there's a big cloud of gas here" and maybe make a list of nebula names that fit for small objects, to allow for the existence of smaller nebulae within it without actually having to make them exist. Or of course you could clone a real one. Just don't do tarantula or orion please.

Open Clusters

An open cluster is a type of star cluster made of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center. This can result in a migration to the main body of the galaxy and a loss of cluster members through internal close encounters. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.

Young open clusters may be contained within the molecular cloud from which they formed, illuminating it to create an H II region. Over time, radiation pressure from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest of the gas away.

To create them in worldbuilding projects, one must create the most massive stars within them and say that they are located in a region of space x light-years across. One can also describe the cloud of gas that may be surrounding it. Open Clusters are one of the most simple and most common of deep space objects, and therefore are among the most tedious to make.

Globular Clusters

Globular Clusters are very dense, spherical collections of stars which orbit a galaxy as a satellite. They are found in the galactic halo or bulge, and galaxies can host from hundreds to thousands of them. The Milky Way hosts 150 to 158 confirmed globular clusters, while Andromeda may host as many as 500 clusters. Giant elliptical galaxies at the centres of galaxy clusters are by far the most abundant source of globular clusters: M87 hosts about 13,000 globular clusters.

Globular clusters are far denser than star clusters in the galaxy itself, and may have around 0.04 stars per cubic parsec in the outer envelope of the cluster, to as many as 100 to 1000 stars per parsec in the dense, globular core. Globular clusters consist primarily of old, low-metallicity population II stars. They range in size from a radius of about 10 parsecs to 50 parsecs.

To design one, basically say "its this big and has this many stars and is located here" and thats it for the overall structure. Obviously, its a good idea to write about the underlying lore or its pretty pointless otherwise.

Stellar Neighborhoods

A Stellar Neighborhood is defined as all stars within a certain radius of a given system. This is generally useful if you want to write about some sort of interstellar empire. Unless you have a lot of time on your hands, it isn't generally necessary to write about EVERY single star in your main star's Stellar Neighborhood, but it IS useful to know the general proportions of star types.

The most useful size for a Stellar Neighborhood in my opinion is ~10 parsecs. Within this distance of our sun, we have:

• 0 O or B-type stars
• 4 A-type stars
• 8 F-type stars
• 18 G-type stars (like the sun)
• 38 K-type stars (considered best for habitability)
• 249 M-type stars
• 5 L-type brown dwarfs
• 30 T-type brown dwarfs
• 11 Y-type brown dwarfs

Although the last three numbers are almost certainly a huge underestimate. When making your own stellar neighborhood, it is best to keep these rough proportions although you should feel free to fudge them a little. If you happen to have an O or B-type star within 10 parsecs of you, well SOMEONE has to have an O or B-type star within 10 parsecs of them! Don't fudge things too much, though.

Once this is done, arrange them randomly in a 10 parsec sphere, pick which ones will be the interesting ones, and write about them! All there is to it!

Conclusion

Now that we have mapped our universe at the largest scales, it is time to look at the smaller scales. It is time to design our solar systems in Worldbuilding Guide Part 3: Planetary Systems!

Part 3: Planetary Systems

_{Links to previous parts: Part 1 and Part 2}

Welcome to part 3 of the Worldbuilding Guide! Today, we will build some planetary systems.

Classical Planetary System

First, build a star. Go to part 1 for that.

Boundaries

Next up, we have to determine the inner and outer boundaries of your system. Too close to the star and planets will get ripped apart. Too far away from the star, they won’t be held in orbit. With that in mind, here are two useful equations to help determine these boundaries:

Inner limit (AU)

The inner limit is generally considered to be the Roche limit of the planet you are placing. Feel free to use that equation if you'd like to be thorough – it will be discussed in part 4 – but in general I=0.04M is close enough.

Outer Limit (AU)

The outer limit of planets is up for debate. In theory you can go as far out as the edge of the oort cloud and interstellar space, but planets actually forming there is implausible at best. The farthest distance that planets can form at is generally around 40 AU out at least around our sun, so a good guess would be given by O=40M. This is much, much less of a hard limit than the inner one.

Frost Line

The frost line is the distance beyond which gas giants can form, although they may migrate inwards later. It’s location in AU will be given by:

FL=4.85•√L

Habitable Zone

I believe this was referred to in Part 1.

Orbits

Orbits tend to be logarithmically spaced and are arranged so the ratio between adjacent stable orbits is between 1.4 to 2. First, place an orbit anywhere (but I usually place it at the location noted by √L AU) and then go inwards, dividing the distance each time by a number between 1.4 and 2 (calculators are encouraged) until you reach the inner limit. Then, go back to that first orbit, and multiply it’s distance by a number between 1.4 and 2 until you reach the outer limit. Note: It does not have to be the SAME number. In fact, it SHOULDN’T be the same number. More realistic that way. Now that we have our orbits, we can start making our planets! After all, what is a solar system without planets?

P-Type Binary System

P-type binaries are binary star systems in which any planets present will orbit around both stars at once, similar to tatooine.

The first thing you need to do is build not one, but two stars. The only consideration is that your Primary star is the most massive of the pair, otherwise it would be the secondary star. Also, if you want your solar system to be habitable, the primary should be less than 1.4 solar masses. Again, less is better.

Average Separation

The average separation of the stars is a crucial factor! Set the average separation of your stars between 0.12 and 6 AU, though if you want your system habitable, stick to the EXTREME LOW end of this range.

Barycenter

The barycenter is the center of mass of the two stars as they orbit around each other. We need to know where this is because any planets present will orbit around this point, rather than the actual stars. The average distance from the primary star to the barycenter is given by:

b1=a(Mb/Ma+Mb)

a = average separation

Mb= Mass of secondary

Ma= Mass of primary

The average distance from the secondary star to the barycenter is given by:

b2=a-b1

Binary stars usually orbit on elliptical orbits. Eccentricity is a measure of how elliptical an orbit is. Values here go from 0-1, with 0 being a perfect circle and 1 being a parabola. Select a value between 0-0.7 for each of your stars. However, habitable P-type binaries tend to have lower eccentricities, so i would advise sticking to the low end.

The maximum and minimum separation to the barycenter for each of your stars is given by:

maxsep=(1+e)•(b1 (or b2)) 

minsep=(1-e)•(b1 (or b2))

Note: each of these equations only works for one of the stars. You need to swap b1 with b2 for it to work for the other star.

However, what’s really important here, and will come into play later on, is the overall maximum and minimum separation of your system. Find this by adding the maximum separations for both stars and minimum separations for both your stars.

Last thing to note is that the stars should never come closer than 0.1 AU unless they're really tiny. If they do, they start to merge, and that really isn’t a good thing.

Moar Boundaries

The inner and outer limits for the system are given in a P-type configuration by:

I=0.04(Ma+Mb)

O=40(Ma+Mb)

Frost line

The frost line is given, in a P-Type configuration, by:

FL=4.85(√La+Lb)

Habitable zone

The habitable zone boundaries in a P-type system are given by:

HZinner=√(La+Lb)/1.1

HZavg=√(La+Lb)

HZouter=√(La+Lb)/0.53

Forbidden Zone! *ominous music*

The forbidden zone is a region of space where orbits are unstable because of the gravitational interactions between the two stars. The edges of the forbidden zone are given by:

Forb_inner=Minsep/3 (this boundary surrounds each individual star, not the barycenter)

Forb_outer=Maxsep•3 

Never ever EVER place planets within this zone! Period! If your habitable zone is partially covered by the forbidden zone, be careful in placing planets. 

IMPORTANT! 

In order to produce a p-type configuration, the outer edge of the forbidden zone MUST fall within the outer edge of the system.

Orbits (again)

Last step is to place your habitable planet and fill the other stable orbits in the system. The process here is literally the same as in the classical planetary systems, on page 5.

S-type Binaries

S-type binaries are binary systems where any planets present will orbit around only one of the two stars.

More stars

First, build 2 stars that, if you want them to be habitable, should be less than 1.4 solar masses.

Average separation

When you set the average separation of the stars, it should be somewhere between 100 and 600 AU. However, if you want your system to be habitable you should stick to the middle-high end. Also, eccentricity should be between 0.2-0.7. 

Next, use your average separation and eccentricity to calculate the barycenter and max-min separation the same way as on page 6 and 7.

Still more freaking boundaries

Now, calculate the inner and outer limits for both of your stars with the same equations as on page 5.

Forbidden zone

All we need to know for an S-type binary is the inner edge of the forbidden zone, given by:

Forb_inner=minsep/3

The goal here is to have the inner edge of the forbidden zone to be outside the outer limit of the system so the gravitational effect on each system by the other star is minimized.

Orbits

Now, calculate the habitable zone, frost line, throw in a habitable planet, and calculate the remainder of the orbits for each system.

Multiple Star Systems

Multiple star systems are systems with 3 or more stars. They can take a long time to build, so fair warning.You can have any number of stars in a multiple star system, but don’t go above 6, or it starts to just be a star cluster and not an actual system.

How many stars do we need to build in this book anyway?

First, choose any number of stars between 3 and 6. Systems with 3 stars are the most common, 4 stars less common, etc. so I would advise sticking with 3, 4 and maybe 5, because 6 star systems are, like, really rare. Take your stars and group them into subsystems with two or 1 stars in them such as: AB C, AB CD, AB CD  E, etc. Next, decide if each of your binary pairs are P-type or S-type. Now we can actually build our stars! However, your primary must be the most massive star, and B, C, etc. must be progressively less massive. Secondly, The first subsystem must be more massive than the second subsystem, etc. Thirdly, for a star to host habitable worlds, remember that it must be between 0.09 and 1.4 solar masses. Interestingly, if you want to build an uninhabitable star, it doesn’t have to actually be a star! You can build a brown dwarf, in which case the mass range is 0.013-0.08 Msol. Having considered all of these things, proceed and build your stars.

Configuration

Essentially, the multiple star system build is a mashup of the classical system, P-type binary, and S-type binary methods. 

Build your systems with the steps above and go to any section applicable to you. 

Quote-unquote “glue”

Now we need to ‘glue’ our subsystems together to make a fully fledged multiple star system. Firstly, find the total mass of each of your subsystems. You can pretend there’s a massive star at the barycenter of each subsystem with the combined mass of the subsystem. Then you can treat the two largest subsystems as 1 distant binary.

Average separation (again)

Now, set the separation of the subsystems to be anywhere between 1200-60000 AU. Then choose the eccentricity to be anywhere between 0.2-0.7. Then calculate the barycenter and max-min separation of the two subsystems. 

Note: If you have 3 or more subsystems, repeat this step, and treat the 4 stars you just did as 1 subsystem and repeat as many times as you need to. Just make sure that the total distance between the first two subsystems and the next one or two is at least 3.5 times the distance between the the first two subsystems.

Done!

Part 4: Intro to Planets

Planets are fun. They just are. Here, we will show you how to make all of them, to help make your system interesting.

Method 1

To start, choose the mass of your planet. Then, choose the composition by plugging in the respective percentages of Iron, Silicate, Water, Ice, Organics, Hydrogen, Osmium (y not tho?), Graphite, Diamond, Electron Degenerate Matter, and Neutron Degenerate Matter. Those last two will probably never be used, but it doesn't hurt! ;)

Then, plug in these values and the mass of the planet into our spreadsheet (the calculations are extraordinarily tedious without it).

The spreadsheet then finds the rest of the stats of the planet. That's kinda it.

Method 2 (less accurate, less tedious)

Pick the mass and radius, and plug them into this equation:

g=M/R²=R⍴

You'll get the gravity and the density. If you want the escape velocity, use this equation.

V_esc=Squareroot(M/R)

Done!

Orbits

File:Ellipse-def0.svg.png

An orbit is an elliptical path around a star or planet where a planet or moon travels around. Easy right? Pretty much! It’s a bit more complicated, though. They have a bunch of different statistics you need to figure out. However, they always look something like this, with the star at one focus.

Here is a rough guide to how to make orbits for different types of planets.

Hot jupiters

Semi-major axis (a)

The semi-major axis is, in essence, a planet’s average distance from the sun. To ensure that a hot jupiter isn’t a cold jupiter, it’s semi-major axis should be anywhere between the inner edge of the habitable zone, and the inner limit of the system. 

Eccentricity (e)

Eccentricity shows how ‘stretched out’ or ‘squashed’ an orbit is and go from 0 to 1, with 0 being a perfect circle, and greater values getting progressively more stretched out, with 1 being a parabola. Hot jupiters have very circular orbits, because gas drag from the protoplanetary disc was gradually circularizing it and dragging it inwards. Therefore, it’s eccentricity should be very close to 0, without being 0. 0.00x-0.0x should be about right.

Semi-Minor axis (b)

The semi-minor axis is the short axis of an ellipse, divided by 2. It is given by:

b=a√1-e2 

Because of a hot jupiter’s low eccentricity, however, the semi-minor axis will be very close to the semi-major axis.

Periapsis & Apoapsis (q & Q)

Periapsis, the closest point in an orbit to the parent object, is given by:

q=a(1-e)

Apoapsis, the farthest point in an orbit from the parent object, is given by:

Q=a(1+e)

Orbital Period (p)

This bit is really important when it comes to worldbuilding. Orbital period, or year, is given by:  

p=√a3/M

However, if you for some reason wanted to find the semi-major axis from the year, 

a=∛p2/M

You can multiply this value by earth’s year, 365.2422 days, to find your planet’s year in earth days.

Orbital Velocity (v_o)

Because hot Jupiters orbit very close to their stars, they have to orbit fast, right? Yup! This speed, or Orbital Velocity is given by:

v_o=√M/a

Multiplying this value by earth’s orbital velocity, 29.78 km/s, will give you your planet’s orbital velocity in km/s

Inclination (i)

Inclination is a measure of how tilted a planet’s orbit is from some reference plane. Most orbits are prograde (they orbit in the same direction as their star rotates). Prograde orbits lie between 0-90º of inclination, and retrograde orbits lie between 90-180º. Most planets orbit prograde, but interestingly, half of hot jupiters orbit prograde and half orbit retrograde! This doesn’t mean you can just pick anywhere between 0 and 180º though. For extra added realism, keep your planetary orbits within 10º of your reference plane, unless you have a specific reason why it isn’t. 

If you’re doing pen-and-paper worldbuilding, you can stop, but if you are rendering your system in 3D, keep reading. 

Longitude of the ascending node (Ω) and Argument of periapsis (ω)

Both of these values range from 0-360º and we are free to choose whichever value we want for these. Essentially, what these do is defining how the orbit is oriented in 3d space. You can do the same for the other types of gas giant.

Note: This same principle works for hot neptunes and hot saturns, in case you want to build those too.

Classical Gas Giants and SuperJupiters

While gas giants can only form beyond the frost line, migration may happen and a gas giant formed there could migrate inwards. For them,

a=(Frost Line +1.2 AU)-Inner limit

For these and gas dwarfs the 0.0x-0.00x rule for eccentricity still holds. 

The semi-minor axis, periapsis, apoapsis, orbital period, and orbital velocity you can just use the equations to find out. There are no special limitations for them. This also applies to gas dwarfs and eccentric jupiters.

For these, gas dwarfs, and eccentric jupiters, ensure that they orbit in a prograde fashion. For all four types of giant planet, pick an inclination between 0-90º. Again, you should still keep it within 10º of the reference plane unless you have a specific reason as to why it isn’t. For the gas giants in our solar system, their inclination ranges from 0.77-2.485º, so keep it low.

Gas Dwarfs 

Gas Dwarfs’ semi-major axes should go between the frost line and the outer limit of the system (40•M). For extra realism, try and pick a distant orbit for your gas dwarf.

Eccentric Jupiters

For all intensive purposes, they can go wherever you want. But if you want to make your solar system habitable, please leave it farther away than 2•HZouter.

These will have orbital eccentricities of 0.1 or higher. Again, if you want your solar system to be habitable, keep the eccentricity below 0.3, unless you REALLY know what you’re doing, which I don’t.

Terrestrial planets

Semi-Major axis

If you’re building a habitable planet, the semi-major axis must fall within the habitable zone, otherwise, it won’t exactly be habitable, will it?

Eccentricity

Habitable planets should orbit on nearly circular paths. Therefore, a safe eccentricity range is:

0<e≥0.2

/interesting sidenote the more planets there are in a system, the less the average eccentricity will be. The equation to find the rough average eccentricity is:

e_avg=0.584•N^-1.2

Note: this is not a hard-and-fast rule, but more of a guideline, something to consider when building your planetary system. Also, don’t use this equation with more than one planet. Don’t. /sidenote end

Periapsis and apoapsis

This will be given, as always, by q=a(1-e) and Q=a(1+e). If either of these falls outside the habitable zone, you should lower the eccentricity or change your semi-major axis, otherwise it would get a bit hot/cold at times.

Orbital period

Like before, (Thanks, Kepler!) the orbital period will be given by: p=√a3/M

Orbital Velocity

Calculate this, as before with vo=√M/a

Inclination

Your main habitable world in the system will likely define that system’s reference plane, so set the inclination of the main habitable planet to 0º and measure other planets in the system relative to this plane. However, you should generally keep planets on low inclinations, and save the high inclinations for oddball asteroids and KBOs, unless you have an explicit reason for doing so.

Argument of Periapsis and Longitude of the Ascending Node

If an orbit is in any way inclined, both parameters can have values between 0-360º. However, with non-inclined orbits, the Longitude of the Ascending node will automatically be 0º and the Argument of periapsis will be ‘undefined’. Why? Short answer is that it’s because of the orbit being exactly in line with the reference plane. Nah, just set it to 0 and undefined and you’re done. 

OPTIONAL STEP: you can go to your favorite orbital simulator, plop down your planet, plug in the numbers and boom!

By the way, for uninhabitable terrestrial planets, follow the same principle, just ignore the habitable zone and set the longitude of the ascending node and argument of periapsis to anywhere between 0 and 360º.

Dwarf Planets

Kuiper belt objects come in 5 classes based on their orbit: Resonant, Classical, Scattered Disk, Detached, and Sednoids.

Resonant Objects

Two objects are defined to be in resonance when the ratio between their orbital periods can be simplified to 2 small integers, such as 1:2, 2:3, 1:3, etc. We care about resonances because they help stabilize each other’s orbits, so that definitely helps stabilize a chaotic kuiper belt. These relationships are not approximate, they’re exact! 

However, not all resonances are created equal. In general, the greater the difference between the numbers in the ratio, the less stable it is. For example, 1:2 is a first order resonance and is very stable. 3:5 is a second order resonance and is less stable etc. The more stable a resonance, the more things we can put there. 

First, take your farthest convenient gas giant and find its orbital period. Next map out the resonances. In other words, take your planet’s orbital period and divide it by the resonance and repeat for each resonance. Choose at will and pick a good mixture of stable and less stable resonances. 

Now, convert the ratios to orbital periods using simple multiplication. Then convert the orbital periods into distances using: a=∛p2/M. Once you fill the figures for each resonance in, determine roughly how many objects should be at each resonance. This is important because dwarf planets need to have cluttered orbital neighborhoods. Keep relative stability in mind and choose at will. 

Now that we have considered Semi-major axis, MMR, Orbital period, and general population, we need to consider the final two unique parameters. Dwarf planets should have inclination ranges between 10º and 25º and eccentricity ≤0.25. However, given the rather large amount of stuff in the kuiper belt, some will break these rules. For example, 2005 TV189 has an inclination of 34º and 1996 TP66 has an eccentricity of 0.33. And remember, to stabilize a weaker resonance, increase your eccentricity. Now you can build your orbit with the same process and equations as on page 14, 15 and 16. Once you have plotted your orbit relative to the reference gas giant, don’t worry if they overlap. Thanks to the resonance, they are guaranteed never to collide.

Classical KBOs

To start, choose a semi-major axis within one of your asteroid or kuiper belts. Then choose an inclination up to 30º and an eccentricity up to 0.4. Now you can simply build your orbit with the same process and equations as on page 14, 15 and 16. That was much shorter than last time!

Scattered Disk Objects

Use the same process as above, simply choose a semi-major axis from 1.1-2x the distance to the outer edge of one of your asteroid/kuiper belts. 

Detached Objects

Detached objects are defined as objects with periapses at least 1.5x the distance of Neptune, or your outermost gas giant. When you calculate your orbit, make sure it has a relatively high eccentricity, but it’s periapsis doesn’t dip below 1.5x the distance of your outermost gas giant.

Disclaimer: Outermost gas giant does not count planet-nine like objects.

Sednoids

Sednoids are a subclass of detached objects with very high eccentricity and periapses at least 2.2x the distance of your outermost gas giant. Therefore, same process as detached objects, just with even higher orbits. YARG

Comets

Comets are small asteroids that generally have very elliptical orbits. There are many different types of comets, defined by their orbital periods.

Short-Period and Long-Period Comets

Short period comets, like Halley, are comets with orbital periods less than 1.5x that of the outermost gas giant. All you need to do is calculate an orbit with an orbital period less than 1.5x that of the outermost gas giant. Done! Long period comets have the same process, just their orbital periods are MORE than 1.5x that of the outermost gas giant. Short period comets are more likely to have inclinations less than 15º, but long period comets can have any inclination you want. 

Jupiter-Family Comets

Jupiter-family comets are comets that have semi-major axes equal to or less than the Frost Line. Calculate their orbits by choosing a semi-major axis smaller than the frost line, an eccentricity from 0.3-0.7, and calculating from there. Jupiter-family comets usually have inclinations under 5º.

Hyperbolic Comets

Hyperbolic comets are comets with an eccentricity greater than 1 and as such will never return to the solar system. They can either come from interstellar space or be ejected from the solar system by encounters with major planets. 

Asteroids

Asteroids can often be in very wonky orbits, and you need to take this into consideration when designing them for your solar system. There are different types of asteroids, but they aren’t really defined by orbit. First, choose basically any semi-major axis, within reason, and then choose an eccentricity less than 0.4. Any inclination will work, but stay under 30º. Again, pick any Longitude of the Ascending Node and any Argument of Periapsis. Then calculate the rest of the statistics with the method outlined earlier. 

Note: The physical characteristics can be calculated with the same method as minor moons.

Rotation

A planet's rotation can allow for some very interesting surface conditions. You should design your planet's rotation with these in mind.

Day And Night™

To begin, choose a sidereal (relative to the stars) rotation rate. CHOOSE WISELY, FOR YOU CAN NEVER TURN BACK! just kidding.

Once you have that, find the solar rotation rate with the following equation:

r_solar = |p/(p/r_siderial-1|

those bars mean Absolute Value.

Warning: The faster the planet spins, the stronger the storms are, and the slower the planet spins, the more drastic the temperature variations across the planet are.

Axial Tilt

A planet's axial tilt can determine a lot of things. Here's a chart.

As Axial Tilt goes up...
/\	\/
Average Temperature	Snow
Seasonal Extremes	Ice
	Humidity
	Clouds
	Albedo

Because axial tilt is determined by the vagaries of the last few large impacts in the early history of your planet, you can basically set it to whatever you want. However, keep in mind the conditions you wish your planet to have, and set your tilt accordingly.

We have finished the majority of the number crunching on your planet, but we should add cool looking stuff around it.

Part 5: Moons and Rings

Previously, we created stars, planets and their orbits around the aforementioned stars. However. All of these planets have something missing: Moons. Moons are very important to a vivid depiction of a world, and are also rather fun to make. They can also often be better at having life on them than their star-orbiting counterparts, as they can be protected by their parent planet's (possibly very strong) magnetic field.

Terrestrial Moons

Intro

For the purposes of this tutorial, we will classify moons by size and composition.

Size

All natural satellites can be either Major or Minor moons. Minor moons are small satellites that lack the gravity to pull themselves into a sphere, and as such they usually have funny shapes. These bodies, like the moons of mars, can be no larger than 200-300 km in radius. Major moons on the other hand, like our moon Luna, are gravitationally rounded, and they must have radii larger than 200-300 km.

Composition

All moons can be either predominantly rocky, like our moon, or predominantly icy, like Mimas. A grid such as this, although very simplistic, is a very useful tool for making worldbuilding easier.

Guidelines

Terrestrial planet moons generally follow a few simple guidelines.

1. Amount - For the most part terrestrial planets will have very few satellites if any. They may occasionally capture one or two asteroid minor moons, but rarely if ever have any major moons.

1. Habitability - if your planet is intended to be habitable, it should probably have a major moon. There are exceptions of course, as with tidally locked planets around red dwarves, but this is a general rule you should follow.

1. Distance - Close in planets will have less moons than that of more distant planets, all things being equal, as they have smaller hill spheres where a moon can orbit.

1. Composition - Inner system moons (planet SMA<Frost Line) will be predominantly rocky (with exceptions), and outer system moons (planet SMA>Frost Line) will be predominantly icy.

Major Moon construction

Major moons will not often occur around terrestrial planets, but they obviously can (example: Luna). Major moons surrounding terrestrial planets will generally only occur if the parent planet got whacked by another, preferably smaller, planet in its early history. This mostly disintegrates the parent planet, and the debris created by the collision forms a Synestia, a cloud of debris which will eventually recoalesce into the planet and its new moon.

To begin, choose a radius for your moon larger than 200-300 km, then choose the percentages of materials which form your moon. The main materials which form planets and moons are iron, silicate materials, and water ice. After you choose your percentages (ensuring they sum to 1), simply multiply your percentage by the corresponding density of the material, and add the three values together:

Densities of planet and moon-forming materials

Iron	Silicates	Water Ice
7.87 g/cm2	3.25 g/cm2	0.93 g/cm2

Next we can use the following equation to find the mass of the moon from the radius, which we already have, and the density, which we already have. M is the mass in earth masses, R is the radius in kilometers, and ρ is the density in g/cm².

M=(R/6371)^3 * ρ/5.51

We can then plug our mass and radius into the following equation to get our moon's surface gravity.

g=M/R^2

Minor Moon construction

Minor moons aren't spheres, so there's less we can really say about them without some 3d modeling program or something. The stabilization effect of minor moons on their parent planet's axial tilt is nonexistent, which means they don't affect planetary habitability in any way, which means we can do WHATEVER WE WANT WITH THEIR ORBITS!

Instead of going through the densities of different materials, just pick a reasonable-sounding number to avoid endless tedium. If it's rocky, go for something near 2 or 3 g/cm². If it's icy, go for something a little above 1 g/cm². If it has a lot of iron (which is uncommon), go for something a little less than 5 g/cm².

The moon is non-spherical, so we can have it be any shape we want. However, for ease of construction, I would advocate either copypasting the statistics of an unimportant asteroid (preferably one that's been imaged up close) or modeling your minor moon as an Ellipsoid. This means that we need to specify 3 axes, a, b, and c.

Sidenote: ellipsoids come in 3 distinct flavors. Choose one for your modeling at will.

Ellipsoid classes

Name	Dimension ratios
Tri-axial Ellipsoid	A>B>C
Oblate Spheroid	A=B>C
Prolate Spheroid	A=B<C

Now that we have density, size, and shape, we can find the mass in kilograms of our minor moon with the following equation, where ρ is the density, π is well, pi, and a, b, and c are the three axes.

M = ρ(4/3)πabc

Your number might seem like a lot but really it's absolutely tiny. We could try and find the surface gravity, but given the irregularity and tininess of minor moons, that's REALLY overkill. Even finding the mass is a little overkill, it's not entirely necessary. Knowing its composition, size, and shape is more than enough.

Once this is done, we can put our moon into orbit as below:

Orbits

To put our moon into orbit, we are going to know the physical characteristics of our planet, and its parent star, both of which have been covered in previous parts. Planets, like all astronomical objects, are encased in a dynamically defined sphere surrounding the object known as a Hill Sphere. Anything inside this zone is more gravitationally affected by this object than whatever its parent object is, allowing it to orbit the secondary object. Therefore, satellites must be placed within this zone, the outer limit of which in earth radii is given by:

H_s = a * ∛(m/M) * 235

However, this is not the only limit involved here. Many of you will know that moons cannot orbit arbitrarily close to their parent planet. The inner limit here is given by the Roche Limit, depending on the density of the moon as well as the planet's properties, and given by:

2.44 * R * (ρ_p/ρ_s)^(1/3)

Sidenote: Moons have hill spheres and roche limits too, calculable with the same formulae, so in special circumstances, you can place moons of moons.

Anyway, between the limits given with those two equations, you can place your moon! You will have to separately calculate roche limits for moons in some cases, but this will not usually be necessary unless you wish to place a moon unreasonably close to a planet.

You can place moons anywhere between these two limits, but its generally best to place most moons less than half the distance to the edge of the hill sphere, to ensure long-term stability.

The orbital period of our moon in earth days can be found by the following equation, where R is the radius of the orbit in earth radii, M is the mass of the planet and m is the mass of the moon.

P = 0.0588 * √(R^3/(M+m))

Now that we have semi-major axis and orbital period, we must decide whether our moon is regular or irregular. Major moons are almost invariably regular satellites, which means that the have low-eccentricity (>0.1), low-inclination (>10º), prograde orbits. Minor moons, on the other hand, with a few exceptions, are almost always irregular satellites, which means that they can have pretty much any sort of orbit, and are often captured. Irregular satellites can also result from a regular satellite getting too close to a major moon and getting thrown onto a weird orbit.

Knowing the general type of orbit our moon has, we can then decide precise values for inclination and eccentricity, then find the periapsis and apoapsis in the same manner as we do for planets. Next up: Gas Giant Moons!

Gas Giant Moon Systems

Rings!

Gas Giants very often have rings. They are usually placed from 1.34-2.44x the radius of the giant planet (plus or minus 0.2 in either direction if you want for both of those), or if you want to go overkill, calculate the roche limit of whatever moon you're breaking up and use that as the outer radius.

Moons

Most gas giants will have up to three wildly different groups of satellites, conveniently named A, B, and C, each with different properties.

Group A

Group A moons are irregular minor moons that orbit in the outer half of the ring system and just outside it, orbit 0.25-1.5 Earth Radii apart from each other, are from 10s to early 100s of kilometers in diameter, and, most notably, are held together by tensile strength rather than gravity. This has to be so because if they were held together by gravity, they would be ripped apart by the planet's Roche Limit.

The Reason why these moons will be so tightly packed together is because Mean Motion Resonances (discussed in part 3) stabilize their orbits. However, you dont have to lay out an absurd web of resonances. The 0.25-1.5 Earth Radii rule is enough because no one really looks closely at these anyway.

Another thing - Most of your Group A moons will be embedded within the rings. Because of this, they will leave some small gaps in the rings at their orbits, their width usually a little over ten times the diameter of the moon itself. Plot these.

Group B

The B group will contain a satellite system's most well-known and largest satellites by far. It consists of a handful of large, regular major moons. They can go from roughly 3-15 planetary radii from the gas giant's center, and no two major moons should go closer than 1 planetary radius from each other.

We can also include minor moons in our major moon layout, as either lagrangian companions or orbiting in resonance and in between the major moons. Real world examples include Methone and a few other obscure saturnian moons. There's no clear group that these fall into so we'll just call them group B2, and you should definitely throw them in!

It's a good idea to check whether any of your moons have a 2:1 Mean Motion Resonance which falls within the ring system. If so, make a nice big gap there of roughly 0.5 earth radii. Just remember when filling in the characteristics of your moons, give the moon causing the resonance a relatively high mass.

Group C

The final group, the C group, consists of a collection of distant, irregular minor moons orbiting on random orbits far from the main moon system. This means they orbit from ~20 planetary radii all the way out to the edge of the Hill Sphere, and orbit on highly inclined and eccentric orbits, some of which might even be retrograde. With few exceptions, they will be roughly 10s of kilometers in size.

Because most gas giants orbit relatively far from their star and are absolutely gigantic, you have a huge amount of room for your captured moons. You can set up several different classes, where members of the same class have relatively similar orbits. If you wish, you can go through each satellite and meticulously calculate them, but that is overkill, and you can just give a general idea with your classes.

Habitable Moons?

If we wish to make a habitable moon, we must move our gas giant into the habitable zone, and give it a large, earthlike moon. There are two ways to do this, either the gas giant migrates inward and holds onto an in-situ earth-mass-ish moon, or it migrates and captures a planet from the habitable zone on the way in. They both work, but neither is perfect.

To ensure atmospheric retention, a habitable moon must be at least 0.25 earth masses. Since most moons are less than 1/5 that, a moon this size is very unlikely to form in-situ. However, captured major moons are very rare and would require something to slow them down into orbit which would be very likely to be destructive.

But the in-situ problem isn't the biggest problem: The bigger problem is that such a moon, having been formed beyond the frost line, will be icy. This means that when temperatures increase, the moon would melt, creating an ocean planet rather than an earthlike planet. Because of this, the capture-after-formation method is probably the most likely.

Great! However, there are still more hoops to jump through: Habitable moons should be placed around stars greater than 0.5 solar masses. Below this, the hill sphere of your gas giant will be too small to allow long-term stability in the habitable zone.

Once ensnared, ensure the moon orbits between 10 and 20 planetary radii, and ensure that its orbital eccentricity is less than 0.01. Just in case, make sure that the ratio between the moon's orbital period and the planet's orbital period is less than 1/9. However, all known captured objects are irregular satellites. That is, they have highly inclined and eccentric orbits, which means in theory you could have your habitable moon have a very wacky orbit. But this is unadvisable.

So yeah! We've done our habitable moon!

Oh.

You want something cooler.

Well you asked for it! Saturn's moons Janus and Epimetheus have a type of orbit known as a Horseshoe Orbit. Here's how it works: They both share near-identical orbits. Epimetheus, orbiting slightly closer, will be traveling farther and will approach Janus from behind. The closer they get, the more they gravitationally effect each other. Epimetheus slows Janus down, which makes it fall towards Saturn. Epimetheus likewise speeds up, increasing the size of its orbit. In other words, they switch places. This all repeats four years later.

You could set up something like this with two moons or even two habitable moons. It'd be unlikely as hell, but hey! It's possible, and would look awesome, with the other moon getting really close, then turning around and receding. That is the definition of awesome. If one of you doesn't do this I'm suing.

Part 6: Planetary Mapping

Previously we created the physical and orbital properties of every large object in the system. To avoid making all of your objects cueballs, we must now map them and give them cool surface features, which can also help with awesome settings and stuff. There are many different methods of mapping, and you will almost certainly have to use more than one of these.

Mapping with Temperature (Albedo)

Finding the temperature of a planet is a key part of the mapping process, and one which is very often overlooked. In general, the temperature of a planet averaged across all latitudes is approximated (APPROXIMATED) by this relation:

T ~ L^1/4/D^1/2

I say approximated because if you run it for Mars, for example, it's a bit off. If you run it for Venus, it's way off. Why is this? Because of a thing called the greenhouse effect, which is actually fiendishly complicated and really can't be put in a simple guide like this. Luckily, some nice people made this, which can calculate the temperature for us!

Albedo is a measure of how reflective a planet is. For example, a planet with a Bond albedo of 100 would reflect 100% of the light that hits it, and a planet with a bond albedo of 0 would reflect none of it. Therefore, lowering albedo increases absorption, thus warming the planet, and raising it lowers absorption, thus cooling the planet.

Conversely, raising the greenhouse effect traps infrared light, warming the planet, and lowering it traps less infrared light, thus cooling the planet. It's actually really difficult to predict exactly how much it does this though, and messing with it can bring about some really weird effects.

Therefore, it's best to mess with albedo, because that affects mapping way more than anything else does. And also it's best not to mess with greenhouse effect too much or else you get Venus, which is decidedly unbased. Now we could just stop here and log these in our sheet, but think about what is actually reflecting the light. Plants, mountains, rock, seas, clouds, and so on.

Spreadsheet time! Artifexian made a fantastic spreadsheet which is extremely useful for this sort of mapping. Simply plug in the different numbers in the green fields (the pink fields are very rough earth values for reference), and the albedo will show up in the bottom cell. Plug that into the calculator above, and if you dont like the results, mess around with the albedo some more.

Take the results and note them down for future reference. However, there are some huge simplifications in this spreadsheet: The ranges can be very wide to take into account different types of materials, meaning that you should probably choose something near the middle of the range and avoid the extreme edges. There's also a thing called the Fresnel Effect, which causes things at higher latitudes to reflect more light, which is ignored in the spreadsheet to keep the math from going completely out of hand.

Aside from these, cloud cover is assumed to be uniform across all biomes because uneven and ever-changing cloud cover basically requires a supercomputer to calculate in any reasonable length of time. Also, the reflectivity is based on the Electromagnetic Spectrum of our sun, as different materials reflect different wavelengths differently.

In summary, this method should never be the only method you use. It isn't a substitute for making plate tectonics, placing biomes in logical places, and so on.

Plate Tectonics

Earth's crust is broken into fragments. Drifting on the currents of the mantle, these fragments, aka plates, move relative to one another. Because of this, various landforms are formed at the boundaries where these plates meet, like volcanoes and stuff. The specific landforms depend on what type of plates are touching and what direction they are moving relative to each other.

Types of Plates

Plates can either be continental or oceanic. Continental plates are thick, Felsic in composition, and relatively low density. Oceanic plates are thin, Mafic in composition, and relatively high density. Plate boundaries come in three types: Convergent, Divergent and Transform.

File:O-c-convergent.png

An Oceanic-Continental Convergent Boundary.

Oceanic-continental convergent boundaries look like this. As the plates collide, the denser oceanic plate is forced beneath the less-dense continental plate. Features created here include a subduction zone (aka ocean trench), earthquakes, mountains, and volcanoes. A real example is the west coast of South America.

File:O-o-convergent.png

An Oceanic-Oceanic Convergent Boundary.

Oceanic-oceanic convergent boundaries look somewhat similar except everything's happening underwater. Expect to find more trenches, earthquakes, small mountains in the form of islands, and of course volcanoes. An example of this is the Mariana Islands.

File:C-c-convergent.png

A Continental-Continental Convergent Boundary.

The last type of convergent plate boundary is continental-continental. Here, both plates are composed of buoyant rock, so neither will want to fully subduct, causing a lot of folding, faulting, and buckling, leading to the creation of very big mountains and a ton of earthquakes. An example of this is the Himalayas.

Now there are two types of divergent plate boundaries: oceanic-oceanic and continental-continental. Oceanic-oceanic plate boundaries are characterized by a Mid-Ocean Ridge with a rift valley at its crest, earthquakes, and where the ridge pops out of the water, volcanic islands. Examples include the Mid-Atlantic Ridge. Continental-continental divergent boundaries are basically the same thing but on land. Examples of this include the East-African Rift Valley.

You could technically have a oceanic-continental divergent boundary, but given that new crust is created at these boundaries, it would rapidly become an oceanic-ocean boundary.

Then we have the transform boundaries: oceanic-oceanic and continental-continental. Continental-continental transform boundaries are where two continental plates slide past each other like the San Andreas fault. Expect earthquakes. Now, oceanic-oceanic transform boundaries are actually quite cool. They occur alongside divergent boundaries. If you look at the Mid-Atlantic Ridge, you'l find that the actual divergent boundaries are offset by transform boundaries, a natural consequence of plate motion around a sphere. Like before, expect earthquakes.

So there we are! We now have all the plate boundary types! I'll compile them into a table for you. Keep this table in mind because we will be using it a lot to create believable maps.

Plate Boundary Types

Type	Subtype	Features
Convergent	Ocean-cont	Trenches, earthquakes, volcanoes, mountains
	Ocean-ocean	Trenches, earthquakes, volcanoes, islands
	Cont-cont	Huge mountains, earthquakes, volcanoes
Divergent	Ocean-ocean	Volcanoes, ridge with rift valley, volcanic islands
	Cont-cont	Earthquakes, rift valley, volcanoes
Transform	Cont-cont	Earthquakes
	Ocean-ocean	Earthquakes

Actually Mapping

Planets are three-dimensional objects, so its a good idea to map in three dimensions. Most fantasy maps look fantastic on a page but look weird on a globe. As such, this method (designed by Artifexian) requires two main programs: Photoshop or GIMP, and GPlates, but it's just as easy to do everything that will be discussed in this tutorial by drawing on a piece of paper and a styrofoam ball, so there will be no specifics of each program here.

Start off with an initial concept for what your planet's landforms will be like. This helps avoid inadvertent earth-cloning (continents in the same basic positions as earth continents), which is fine if that's what you want, but it's kinda overdone.

Next, create an equirectangular (width/height ratio = 2:1) map of your planet, and, if you wish, overlay a 12x6 grid on it. Before continuing, note what distortions are present in an equirectangular map. The closer you get to the poles, the more s t r e t c h ed out your landforms appear side-to-side. With this in mind, sketch your plate boundaries on the map, never ever touching the top and bottom and making sure that whenever a boundary crosses the left or right edge of the map, it resumes at the same latitude on the opposite edge. For reference, Earth has 7-8 major plates, about 10 minor plates, and a bajillion microplates. Also, try to ensure that no more than three plates interact at any given point.

GPlates is useful for ensuring that the poles don't look like garbage, so use it. If need be, do some edits. After this is complete, mark in which plates will be continental and which will be oceanic. The continental plates need not be all land, just predominantly land, and vice versa for the oceanic plates. Remember, earthlike planets will need most of their surface to be covered in water, so its a good idea to make your largest plate oceanic.

After this, mark in the direction plates are moving relative to each other with arrows on a separate layer. The most convenient method for doing this is deciding where your planet's mid-atlantic ridge analog will be, and imagining how all other plates will interact.

Now for the fun part, filling in the continents! Just draw lines where you want your coastlines to be and make em look cool. You don't need artistic ability here, function over form is what we're going for. It's not like there's a shortage of youtube tutorials on the artistic side of mapmaking. Just go look em up if that's what you want.

With continents in, you now know exactly what is continental crust and what is oceanic crust, so we can mark in what is happening at each boundary. Based on the landforms you expect to see at each boundary, throw down various landforms according to the above table. Islands are really helpful for selling the believability of a world, so don't skimp on those!

Make sure to include volcanic hotspots, like Hawaii. These leave island chains in the middle of plates, stretching off in the direction that the plate is moving away from. However, on continents they can form large calderas, such as the Yellowstone Supervolcano. As an optional final step, we can use GProjector to make and export an utterly absurd amount of reprojections of our map.

Tada! We've finished the first few steps in proper mapping!

Realistic Climates

Now we have our plates and mountains and islands and whatnot. But we don't know what the land is like! Climate maps of the real world use a thing called the Köppen Climate Classification System. Here we will learn how to apply this system to our cool worlds. Also, to those tempted, do not make single-biome planets. They're rather boring and also aren't possible.

Closely Resembling Earth

To start, we will need a map, ocean currents, and wind patterns (the latter two are detailed here). Next we can figure out the relative precipitation and temperature variations.

Precipitation		Temperature
High	Low	High	Low
Low pressure areas (ITCZ, PF)	High pressure areas (Subtropical ridges)	Equator	Poles
Onshore winds	Offshore/parallel winds	Interiors	Coasts
Warm ocean currents	Cold currents	Low altitude	High altitude
Windward side of mountains	Leeward side of mountains	Large temperature variation inland	Small temperature variation near coasts
	Interiors of continents

The reason why I said "leeward" and "windward" side of mountains is due to a thing called Orthographic Lift, which means that the side of a mountain facing prevailing wind will receive a lot of precipitation, whereas the leeward side will recieve virtually no precipitation. Remember this table because we will be using it to help us refine our climate zones as we place them.

Basically the method is:

• Locate the rough zones
• Refine with temperature and precipitation in mind
• Compare and contrast
• Move onto the next biome.

The Hadley Cell

Start by marking off and ignoring all of your tall mountains, i.e. anything taller than 800 meters. Next, place your tropical climates. Tropical climates go in the equatorward half of the Hadley Cell, out to 15-20º north and south.

	Summer	Winter	Location	Features
Tropical Rainforest	Hot, wet	Hot, wet	low-lying areas, 0-10º N/S	Very dense forest, little underbrush, poor soil quality, very high biodiversity
Tropical Monsoon	Hot, very wet	Warm, dry	5-15/20º N/S, transition strip between rainforests and savannahs, along coasts with onshore winds as a fairly thin strip If any landmass is on a large continent occupying the 30º N/S line, north or south of a large equatorial ocean, place some monsoon stuff there on the coast.
Tropical Savannah	Hot, wet	Warm, dry	Low-lying regions outside of rainforest areas, 5 to 15-20º N/S, if there's a warm current, extend the savannah region out to about 25-30º N/S	Grasses and shrubs, isolated trees, frequent droughts, lower biodiversity
Hot Desert	Very hot, dry	Warm, dry	10-30º N/S, low-lying continental interiors, rain shadows, regions affected by cold ocean currents	Sand & rock, low vegetation, low biodiversity
Hot Steppe	Hot, low-dry	Warm, low-dry	10-35º N/S as a thin band around deserts and wherever you couldn't place deserts or savannahs

The Ferrel Cell

Ferrel Cell climate zones have two main subtypes: Humid Continental and Subarctic Continental. These subclasses and their placements and features are detailed below.

Ferrel Cell Biomes

	Summer	Winter	Location	Features
Humid Continental	Warm-hot, moderate	Cold-very cold, low-dry	For now, fill all low-lying continental regions in the Ferrel Cell with this biome. We'll finesse this later.	Temperate deciduous forests, prairies, grasslands
Subarctic Continental (Boreal/Taiga)	Cool-mild, moderate	Very cold, very low	45-75º N/S, skewing equatorwards where there's a cold current along the coast and/or offshore winds	Coniferous forests, poor soil quality, little undergrowth

Now, we fill in the subtropical climates, which will for the most part fill in the flanks of the Ferrel Cell continents. These come in six main subtypes:

Ferrel Cell biomes

	Summer	Winter	Location	Features
Mediterranean	Hot, dry	Mild, moderate	30-45º N/S in regions affected by cold currents	Evergreens, deciduous trees, olive, citrus, fig, etc., shrubland, wildfires, find a river valley in this zone and make it your planet's Sumeria
Humid Subtropical	Hot, wet	Warm-mild, moderate	25-45º N/S in regions affected by warm currents, should be on the opposite side of landmass from Mediterranean climates if done correctly	Palm trees, ferns, bushes, shrubs, grassland
Oceanic (Maritime/Marine)	Warm-mild, wet	Cool-cold, wet	40-60º N/S in regions affected by warm currents	No dry season, narrow temp range, conifers & broadleaves, cloudy, overcast, rainy, rainy, rainy, rainy, rainy
Temperate Rainforests	Warm-mild, wet	Cool-cold, wet	Thin band in coastal regions of Oceanic biomes	Dense forests with heavy moss/fern ground cover
Cold Deserts	Hot, dry	Cold, dry	Interiors of large continents, rainshadows, higher up than hot deserts
Cold Steppe	Warm, low-dry	Cold, low-dry	25-50º N/S, primarily as a thin transition zone from Cold Deserts, and in interiors and rainshadows, and as a transition away from hot steppes

The Polar Cell

There are only two climate subtypes to worry about here, Polar Tundra and Polar Icecaps.

Polar Cell Climates

	Summer	Winter	Location	Features
Polar Tundra	Cold, low	Very cold, dry	60-80º N/S	Permafrost, no trees, shrubs, grasses, mosses, lichens, low biodiversity, seasonal bogs and lakes
Polar Ice Caps	Very cold, low	Very cold, dry	75º+ N/S in polar oceans and in the center of large polar landmasses	Permanent snow & ice, no vegetation except mosses & lichen

Finishing up

If you want you can fill out your islands. Same rules apply as previously. From there you can eyeball what percentage of the planet's surface each biome takes up and use the albedo section to find out your planet's albedo.

This method ONLY HOLDS for earth-like planets and even then not always. Refer to the next part for what to do if your planet differs from earth in several specific ways, such as it being retrograde spinning.

Retrograde-rotating planets

For Retrograde-rotating planets, wind and ocean currents will run the opposite direction. All this flipping will mean that your climate zones will flip too. What was in the west is now in the east and vice versa. Reversing the wind direction will also reverse your rain shadows, so keep that in mind. Otherwise, place your biomes the same way.

Hot and Cold Planet Climates

Now that we know how to construct climate models for planets similar in temperature to earth, let's mess with the temperature a bit.

Cold Model

Twenty thousand years ago, during the last glacial maximum, Earth was 5-6º C colder than it is today. Place your tropical forests between 0-10º N/S, but with less coverage than you would put on for a standard earthlike planet. Place your tropical savannahs out to ~15º N/S. In regions affected by warm currents, extend the savannahs out to about 20-25º N/S.

Next, place your deserts. On a Cold planet like this one, place your hot deserts between 10-25º N/S in the interior of continents and regions affected by rainshadows, cold currents, and/or offshore winds. Cold Deserts are between 25-35º N/S, and can be placed in the same type of regions as hot deserts. They can also be placed higher up than their Hot counterparts, and at altitude in hot desert zones. Hot steppes and tropical monsoons are similar to modern-day earth, albeit monsoons are reduced a bit and steppes are only a transition between desert and savannah.

Place your Humid Continental climates between about 30-50º N/S, as a reduction from what your original climate map would have been if it had the same temperature as earth. Place your Subarctic Continental climate between 40-60/65º N/S, skewing the zone equatorwards in areas affected by cold currents and offshore winds.

Place your mediterranean climates between 30-45º N/S in regions affected by cold currents, and place your humid subtropical climates between about 25-35º N/S in areas affected by warm currents. Place your oceanic climates between 40-55º N/S in regions affected by warm currents, as well as at altitude in Humid Subtropical regions.

Place your Ferrel Cell cold deserts in continental interiors between 25-35º N/S in continental interiors and rainshadows. Deserts in general should cover more of your planet than if it had the temperature of modern day earth. Cold Steppes should only be thin transition zones between cold desert and surrounding areas.

In the Polar Cell, place your polar ice caps above 45º N/S with the lowest extent being in regions affected by warm currents or onshore winds. Surround your Polar Ice Caps with Polar Tundra. In regions where you have low-latitude ice caps, the tundra will be a thin transition zone. Around high-latitude ice caps, your tundra will be expansive. But don't go below 40º N/S.

Really Cold Model

Roughly 640 million years ago, Earth was enveloped in a thing known as Snowball Earth, aka the Cryogenian Period. During this time, it's debatable whether the entire planet was englobed in ice or whether there was a surviving band of open sea at the equator, but lets take the second interpretation because its more interesting to map. (the first interpretation would just be a solid ice cap color map).

To start, place Polar Tundra climates on all of your continental landmasses between 15º N/S. Then, place ice caps from 15º N/S to the pole, possibly extending it northward where there exists a continental landmass, and filling the space with more tundra. Make sure the locations where the icecap borders the sea isn't just a straight line at 15º latitude, but wobble it a bit, to make it look more interesting and more realistic.

That's kind of it. Wahey!

Hot (ish) Model

Because I'm tired of typing, I'll just give you a table with all biomes and their locations that you can follow, based upon Earth's climate in the Eemian Period, the last interglacial, when it was 2-3 degrees hotter than it is today.

Hot(ish) planet climate model

Biome	Location
Tropical Rainforest	0-10º N/S (reduction)
Tropical Monsoon	Transition between rainforest and savannah, Onshore winds, monsoon areas
Tropical Savannah	5-15º N/S, in areas affected by warm currents extend this to 25º N/S
Hot Desert	10-30º N/S, in continental interiors, rain shadows, regions affected by cold currents and/or offshore winds If you wish, you can choose a hemisphere to reduce desert coverage and the other to expand its coverage
Hot Steppe	Transition between hot desert and surrounding areas
Humid Continental	35-65º N/S (expansion)
Subarctic Continental	60-75º N/S, where there are cold currents or offshore winds extend to 50º N/S,
Mediterranean	30-45º N/S (expansion) in areas affected by cold currents 45-60º N/S in areas affected by warm currents, coastal regions only
Humid Subtropical	20-45º N/S in areas affected by warm currents
Oceanic	40-60º N/S in areas affected by warm currents, coastal regions only
Cold Desert	Ferrell cell interiors, rainshadows, areas affected by cold currents and/or offshore winds; altitude
Cold Steppe	Transition between cold desert and surrounding areas
Polar Tundra	70+º N/S
Polar Ice Caps	70+º N/S (reduction), interiors only

Note that this planet isn't truly a hot planet, it's only two degrees hotter than modern-day Earth. For a truly hot planet, the scenario below is a good fit.

Actually Hot Model

In a true hothouse climate the temperature gradient seen in the previous model largely breaks down, resulting in a planet that is near uniformly hot climates. The point of distinction is that no ice exists on the Earths surface long term cold weather can still happen (i.e. snow sleet and low temperatures) it is just far less common and ice can't remain stable on the surface melting or sublimating not long after its deposition. As such a hot house climate is defined by the complete lack of ice caps which means you will have far lower climatic variation globally with far weaker climatic circulation since there isn't a temperature gradient to drive the same sort of large scale wind patterns and currents, as such expect storms and cyclones to be largely responsible for heat transfer rather than sustained wind currents which will be far more likely to break up.

Biome wise it is a bit tricky to gauge what a hothouse climate would be like with modern vegetation since before the last Ice age grasslands didn't exist as grass as we know it didn't exist with the exception of the older bamboo like lineages. Modern grasses started to appear around 30 million years ago with the first true grasslands appearing only around 20 million years ago probably driven by nearby supernovae supercharging lightning activity on top of dry ice age climatic conditions and lower CO2 concentrations enabling grass to overtake forests for the first time in Earths history.

How grasslands would respond to a warming climate would be a critical clue in determining how climates would or would not differ from what we are used to. I would suspect that over all tropical forest cover would probably increase since it became the predominate biome wherever moisture was sufficient but there is no precedent for grasslands so it is hard to accurately gauge whether grasslands could maintain their dominance. I suspect not at least not without some form of allelopathic poisoning of the soil and or self conflagration strategies but evolution has a way of surprising us so the sheer possibility is there for world building.

Minutiae

Fossil Fuels and Ores

Fossil fuels and ores can play a huge part in the history of your world. Therefore, it is important to know where on your planet they would be placed.

Coal

Coal deposits are the remains of ancient tropical to subtropical swamps. Back when earth's climate was very warm and wet, plant life flourished in "coal swamps"; large, low-lying, waterlogged regions. The waters of these coal swamps were anoxic (containing little oxygen), meaning that plants that died and fell into the water didn't rot away entirely, but just got buried under more dead plants, eventually forming a compacted layer of organic material called Peat.

Peat is about 50% Carbon. If it were buried to a depth of ~4-10 km, pressures and temperatures present at these depths would concentrate it to about 60%, forming Lignite Coal, the lowest grade of coal. The more the coal is buried under successive layers of material, the more concentrated the carbon becomes and the higher the quality of the coal. Bituminous Coal is the most common and second-best type of coal, at roughly 70% Carbon concentration. The rarest and best type of coal is Anthracite Coal, which contains upwards of 90% Carbon.

So place your world's coal reserves (no need to specify which type) in locations which in the past were low-lying tropical to subtropical swamps. Place higher-grade coal in the foothills of mountain belts in these regions. Do not however place coal reserves in the interiors of mountain chains. The pressures there are such that 100% Carbon, aka Graphite, is created, which is famously not coal. Finally, place your peat reserves in modern-day low-lying wetlands.

Oil and Gas

Oil and Gas are hydrocarbons derived from the remains of ancient plankton, not dinosaurs. Much like the formation of coal, when the plankton died, they sunk to the lake or sea floor, accumulating and mixing with the clay found there, forming a muddy organic ooze. This was then buried and compacted, forming black organic shale, which is known to oil enthusiasts as Source Rock.

If the Source Rock is buried to a depth of roughly 2-4 kilometers, pressures and temperatures found at this depth produce Kerogen. Shale containing 25-75% Kerogen is referred to as Oil Shale. At greater depths, the oil is broken down to form Natural Gas, aka methane. Because of this, Oil can only form in a narrow window known as the Oil Window, typically less than ~6.5 kilometers. The Gas window is much larger, typically extending down to ~9 kilometers.

Oil shale source rock is not very permeable or porous, so drilling it doesn't yield much product. Luckily, oil and gas are less dense than water, so they'll try to escape the source rock and try to get above the water table. If not stopped, they will eventually reach the surface and create an oil/gas Seep. Therefore, for underground reserves, what's known as a Trap is required, and basically consists of a permeable layer of Reservoir rock beneath an impermeable layer of Seal rock. The oil and gas are generated in the source rock, rise up and collect in the reservoir rock because they are trapped by the seal rock.

Traps can be found at a variety of locations. Along the crest of tectonic folds, along fault lines, or near salt deposits.

SIDENOTE: Underground salt deposits can be found on land that was once subtropical deserts. Salt and lithium may also be "mined" in salt pans, large dried-up desert lakes and ponds where evaporating water concentrated the salt and lithium in the remaining basin.

Therefore, place your oil and gas reserves in regions that were once shallow seas and lakes, but were then buried by sedimentary deposits and underwent tectonic compression. Additionally, secondary oil and gas reserves can be placed in sedimentary basins along passive continental margins. A sedimentary basin is a depression that has been filled with sedimentary deposits, and a passive margin is where the land meets the sea sans plate boundary.

Finally, place oil deposits in Foreland Basins. Foreland basins are depressions on the continental side of a mountain belt, filled in by sedimentary deposits just like sedimentary basins.

Ores

Metals occur natively (in their pure form) or as minerals (bonded to nonmetallic substances). If a concentration of a metal in a mineral is significant and the metal can be easily extracted, the mineral is referred to as an Ore. When an economically significant concentration of an ore occurs, it is said to be an Ore Deposit. Ore deposits form in many ways, and are usually associated with volcanic activity.

In a Magmatic Deposit, ore minerals precipitate in an underground magma chamber. In a Hydrothermal Deposit, hot groundwater dissolves the minerals and carries them away, precipitating them into the surrounding rock. In a Submarine Vent Deposit, ore minerals precipitate onto the ocean floor via black smokers near divergent oceanic plate boundaries.

These are some of the ways they form, now where do they go? Most of your planet's geologically recent ore deposits will form around volcanically active plate boundaries, in the overriding plate in subduction zones as well as in hotspots. Most of these deposits will be Porphyry Copper deposits, which come in the subtypes of Copper only, Copper-Gold, Copper-Molybdenum, Molybdenum only, and Tin-Tungsten. In all cases, there will be minor amounts of Lead, Zinc, and Silver.

Next to these deposits, trailing away from the volcanically active region, place your Epithermal Gold deposits. Start with Gold, with minor amounts of Silver and Copper, then further out place Gold, with minor amounts of Silver, Lead, and Zinc, then further out again place Gold, with minor amounts of Silver, Lead, and Mercury.

Surrounding these, place your IOCG (Iron Oxide-Copper-Gold) Deposits. These are associated with slightly older rock, and are important sources of Uranium. These can also be placed in continental rifting zones. Next, place your Nickel-Copper and PGE-Chromium deposits. These should be very rare and should go in your Ancient Cratons; old, stable interior regions of continents, away from plate boundaries. By the way, diamond deposits can also go in these regions, just make them very rare.

VMS (Volcanogenic Massive Sulfide) Deposits are found in both new and old mountainous regions. These predominantely Zinc-Copper deposits formed on black smokers on ancient seafloors, where subsequent mountain creation pushed them upwards to mineable depths. Expect minor amounts of Lead, Silver, Gold, Cobalt, Tin, Selenium, Manganese, and Cadmium.

For similar reasons, place BIF (Banded Iron Formation) deposits in and around old mountain chains. Back when Earth's atmosphere was super oxygen-rich, oxygen dissolved into the seawater, changing its chemistry such that it no longer could hold iron in solution. The Iron precipitated onto the seabed as iron oxide, sank, and formed layered sedimentary deposits on the sea floor.

Place SedEx (Sedimentary Exhalative) deposits of mainly Zinc, Silver, and Lead in your continental sedimentary basins. These should be fairly uncommon. Finally, place Residual Mineral Deposits of mainly Aluminum in your rainforests and other similarly wet regions.

Sidenote, you can place Secondary Enrichment zones a little bit downhill from some of your existing deposits, caused when the water table leeches and concentrates minerals away from the original deposit. In particular, make your dominant Uranium deposit a secondary enrichment zone.

MVT, Mississippi Valley Type deposits, form when groundwater sinks beneath a mountain range, dissolves metals, then transports and concentrates them in a new deposit on the other side of a Foreland Basin. Throw down some of these, too! Note that MVT-type deposits can be hundreds of miles away from the original deposit.

Finally, we have Placer Deposits. This is where a mineral or native metal outcropping erodes, enters a river system, and gets concentrated downstream. This is why panning for gold and diamonds actually somewhat works.

And that is all! By the way this is overly simplified but it should be enough for your purposes, as even the overly simplified version is overkill-complicated.

Rivers

To create viable river systems, you'll need to figure out how water will flow over the topography of your map. To do this, mark out the ridges of your main mountain ranges and highland regions. These will be your drainage divides, and the areas they create will be your Drainage Basins. Next, roughly mark in your minor ridges. Using these, carve up your drainage basins, which will be quite large, into smaller river basins of varying size.

Next, place a primary river in each of your basins. Some tips: run your river from areas of high elevation to areas of low elevation. Run them in between your ridges and through your valleys. Make sure they take a fairly direct path, as any overly complicated courses with sharp turns will be rapidly eroded into the aforementioned fairly direct paths. Don't do splitting either except in the case of river deltas.

Next, add in some tributaries following the same rules as before. Then some tributaries of tributaries and maybe one more layer.

Note that in temperate or tropical regions, there will be many revers, which will be permanent, gaining rivers. That is, they will exist year-round and the volume of water flow increases in the downstream direction. Arid or Dry areas will tend to have fewer rivers, which will be ephemeral, losing rivers. That is, they only flow in the rainy season and the volume of water decreases in the downstream direction.

Features

Now that we have our rivers, now we should figure out what riverine features we would expect to see along their courses. We can divide a river into three parts, youthful, mature, and old age. The Youthful stage of a river is the stage during which it is in the mountains. This is where a river begins its journey. The gradient of the land is steep and the flow of the river is fast. The river channel is deep, narrow and V-shaped during this stage. Expect valleys, canyons, rapids, and waterfalls.

The Mature stage of a river is an in-between hilly stage. The gradient of the land is moderate, and so is the flow of the river. The river channel is wider, shallower, and U-shaped. Expect mild meanders, braided streams, narrow floodplains, and terraced floodplains. Finally, the Old stage is the last lowland stage of a river. The land has a shallow gradient, the river flows slowly, and the river channel is shallower and wider again. Expect extreme meanders, broad floodplains, oxbow lakes, yazoo streams, and deltas.

A river will begin at a source, usually a mountain spring or meltwaters from a glacier. If the water passes over relatively soft underlying rock, the fast-flowing river will carve out a V-shaped valley with interlocking spurs. If the underlying rock is relatively hard, slot canyons will form. If it alternates between hard and soft, expect stair-step canyons, like the grand canyon. If glaciation occurs, a glacier can erode a V-shaped valley into a U-shaped valley. The interlocking spurs are truncated, and hanging valleys form.

Rapids form whever a river passes over large rocky debris or a sufficiently steep gradient, and waterfalls form wherever an escarpment of hard rock is met. As the river leaves the mountains and enters its mature stage, the river's channel may become choked with mud and loose gravelly sediment. If this happens, the river will split into numerous interweaving channels. This is referred to as a Braided Stream and the deposits are called Braid Bars.

The river will also begin to meander slightly, though it won't be as curved as it will be in its old stage. On the outer curve of a meander, there will be a steep Cut Bank, and on the inner curve, the river will deposit gravelly sediment in a wedge-shaped Point Bar. Flooding in mature-stage river valleys can leave deposits of Alluvium, deposts of clay, silt, and sand, creating fertile Floodplains, which will be fairly narrow in the mature stage. If a river cuts down into a floodplain after its formation, it creates what is known as a Terraced Floodplain.

As the river exits the hills, it enters its old age stage, crossing the Fall Line. The Fall Line is the imaginary line delineating the upland and coastal regions, where rivers plunge, or "fall", at roughly the same elevation. Important cities or touns can often be found along the fall line as it marks the point at which boats can no longer travel upstream and the energy generated by the falls can be used to power various things.

Beyond the fall line, the river's floodplain will widen, and its meanders will become increasingly more pronounced. We will still find cut banks and point bars, only now the point bars will be soft and sandy. Also, meanders get curvier over time. If the curve gets too extreme, a Meander Neck is formed. Eventually, the river will cut through the weak meander neck, rerouting the river. The remaining meander is called an Oxbow Lake if it remains filled with water, and an Abandoned Meander if it dries up. A river may also split to form small river islands.

Interestingly for history, the ever-evolving paths of meanders can lead to very interesting territory disputes, as nation boundaries are frequently marked by rivers.

Deposition during flooding can lead to low ridges called Natural Levees. In regions where large natural levees form, low marshy swampland may form on the other side of the levees. Levees may also drop small tributaries from entering the main river, and adorably named Yazoo Streams flow parallel to the river in the floodplain.

File:TypesOfDeltas.png

All types of deltas with arrows representing the strength and direction of currents

Where the river enters the sea, a Delta may form. Sediment carried by the river deposits at the river mouth to form a Midstream Bar, causing the river to split. Each new distributary forms another midstream bar, and repeat. If the ocean currents are significantly stronger than the current of the river, no delta will form because the ocean current will sweep away the sediment, preventing the formation of a midstream bar. If the ocean current is only slightly stronger than the river current, deposition will occur, leading to the creation of a classic triangular delta. If the current hits the river mouth head-on, it will form a curved-sided, V-shaped Cuspate Delta. These are the most common instances. In rare cases, if the river current is much stronger than the ocean current, the river will form a Bird's foot Delta.

Lakes

Lakes can go basically anywhere along a river's course, although they are generally more common in highland regions. Mountain lakes can be placed in depressions caused by tectonic/glaciation activity and volcanic craters. Plunge pools from waterfalls are also classified as small lakes. In lowland regions they can occur where a river widens into a basin or in floodplain depressions. The important thing to note is that several rivers may flow into the lake, but only one may flow out. Also, little mini-deltas may occur where a river enters a lake (or, for that matter, a much larger river!).

Formulae

You can very roughly calculate how wide a river should be, in meters by the following formulae, where R is Rainfall (1 = 1,000,000 m³ per year), A is the area drained by the river (1 = 1,000 km²), B is the Provisional Width, and F is the depth factor.

B = R/4 • A • 0.1 W = B/F

Take the value from the first equation and input it in the second as B. If B is 20-100, F=2, if B is 100-200, F=3, if B is 200-500, F=4, and if B is greater than 500, F=5.

Also, turns out rivers don't meander randomly. The ratio Width of river:Radius of meander curve:Length of meander (one s-shaped length) = 1:2.3:11.

Exotica

Fun fact: Rivers sometimes cut through mountains. This occurs when a mountain-building event takes place in between the source and the mouth of the river. If the river erodes as fast as or faster than the rock is being thrusted up, the river is not redirected, and a Water Gap is formed.

Do all rivers empty into the sea? No. An Endorheic Basin is a depression in the land where water is allowed to flow in but cannot flow out. Instead, evaporation is the only mechanism by which water can reenter the hydrological cycle. In wetter regions, you can place salt lakes in these endorheic basins, in drier regions, you can place inland delta swamps, such as the Okavango Delta in the Kalahari desert.

Do all rivers need to be contiguous? No. Some rivers disappear and reappear. In Karst Regions, regions where a lot of limestone, dolomite, and gypsum are present, rivers may disappear down sinkholes and caves, flow for a while as Subterranean Rivers, then reappear downstream.

And that's it. All the interesting stuff about rivers and how to make them!

Coastal Landforms

Coasts vary dramatically in terms of landforms and topography, making them difficult to classify. For now, let's say that in our world, coasts come in two types, rocky coasts and sandy coastal plains. To mimic earthlike conditions, we should say that our coasts alternate between these two types.

Rocky coasts dominate on active plate margins, near coastal mountain ranges, and near regions affected by glaciation. Sandy coastal plains will be found on passive margins and on areas of low relief. The split will basically be about 80-20 in favor of rocky coastline.

Rocky Coasts

The primary features of high-relief, erosional rocky coasts are cliffs and shore platforms. A cliff is a near-vertical rock exposure. If the top of a cliff is rounded via weathering, it is called a bluff. Cliffs and bluffs may be punctuated by coves and gorges.

Coves, small circular bays, form on concordant coasts. These are coasts where bands of rock of varying resistance lie parallel to the shoreline. A small gap is cut in the more resistant rock, and then a wide circular bay is carved out of the less resistant rock. The sea or a river can do this. Multiple coves can corm in sequence, sometimes merging. Gorges occur when a vertical strip of weaker rock running through a cliff is eroded by wave action. Simple!

Shore platforms are platforms of rock extending from a cliff base that are only visible at low tide. They can be horizontal or they can slope gently downward. If no platform is visible at low tide, the cliff is a plunging cliff. Shore platforms are rarely smooth and are usually pockmarked with depressions and protrusions, scored ridges, gullies, and dotted with rock pools that make neat ecosystems, as well as marine potholes–cylindrical bowl-shaped hollows.

Platforms form when cliffs retreat. Waves cut a notch in a cliff, which becomes overhanging. The overhang becomes unstable and collapses into the sea, leaving a small shore platform, rinse and repeat.

Rocky coasts generally start very irregular, with headlands and embayments everywhere. Nature will seek to smooth things out by preferentially eroding away headlands. As they are eroded, headlands leave some cool-looking landforms.

Wave action will carve sea caves into the sides of headlands. These may have associated blowholes, or marine geysers, which are holes in the roof of a sea cave that large fountains of sea spray are often ejected from. If a blowhole becomes too large, it may collapse, forming a sinkhole. The sea caves will eventually erode their way through the headland, forming a sea arch. This will soon become unstable and collapse, forming a sea stack.

The sea stack protects the adjacent shore from the waves, so a buildup of sand and other debris can form. This slowly builds out a thin ridge of sand and gravel between the stack and the mainland known as a tombolo. This sounds like a planet name.

The sea stack will soon erode to a sea stump and eventually disappear. In summary, the evolution of a rocky sea coast is this:

Headland>Sea cave>Sea arch>Sea stack w/ tombolo>Sea stump>gone

Finally, if the sea level rises and/or the land sinks, the coast is said to be submergent. If the sea level falls and/or the land rises, the coast is said to be emergent. Emergent rocky coast may feature extra high cliffs or uplift terraces, where former shore platforms are lifted out of the sea, becoming cliffs, and new shore platforms are created. You can also get raised sea caves hundreds of meters above sea level.

The major submergent rocky coast is the fjord coast, which is discussed in the glacier section. A lesser known one however is the Dalmatian coast. This occurs when tectonic activity creates a series of folds running parallel to the shoreline, a concordant coastline. Erosion occurs, breaking holes in the more durable rock and eroding the less durable rock entirely away, forming a series of rocky islands parallel to the new coastline. The prototypical example of this is the eastern coast of the Adriatic Sea. They look very cool and earth should have more of them.

Coastal Plains

The most obvious and beloved feature of low-relief, depositional coastal plains is the beach. There is actually a very high variety of beach types in various places on Earth that I encourage you to look up to avoid spamming generic long white sand beaches that are all clones of each other. However, I don't actually know that much about them and it would add ten paragraphs that I really don't feel like writing. Someone else who actually knows this kind of thing can though! In the spirit of simplification, we'll say that there are two types of beaches: Pebble beaches and sandy beaches.

Pebble beaches consist of coarse, cobble-like material and are usually found near cliffs. They can occur at any latitude but are more common near mid-high latitudes, as glaciers are great sources of coarse material.

Sandy beaches are composed of fine sediment like

checks notes

sand, and can be found basically anywhere, though they are particularly common in the tropics. The color of a beach is determined by the color of the sediment in its surrounding environment. Beige, white, pink, black, red, orange, and green are all supposedly possible colors for a beach. Beige is the most common color and is caused by high quantities of quartz and iron. White is caused by quartz, limestone, feldspar, and gypsum and are particularly common in the tropics, especially on tropical islands. Coral white beaches specifically are also found where there's lots of remains of corals and mollusks.

Coral pink beaches are quite rare and should be limited to tropical islands. The color occurs when the remains of reddish corals wash up onshore. Black, red, orange, and green are also quite rare and are usually the result of some form of volcanic activity. Thus, place them sparingly in your volcanically active regions. Black specifically is caused by basalt and obsidian, Red is caused by oxidation of iron from volcanic rocks, orange is caused by high iron content or orange limestone, crushed shells and volcanic deposits, and green is caused by high amounts of olivine.

Beaches often, though not always, have a dune system associated with them. The largest dune systems occur in the temperate zone where strong onshore winds are present. Refer the the wind map you will create in part 7 for this. Dune systems are usually less common in polar and tropical regions. As previously mentioned, coasts will alternate between rocky and sandy coast.

In some places, asymetrically curved beaches will link rocky headlands. These are known as logarithmic spiral beaches because their curves are exact logarithmic spirals.

Most landforms associated with coastal plains are caused by longshore drift, the transportation of beach sediment parallel to the shore along the coastline. An incoming wave would send water up the beach at some angle, then water goes back to the sea perpendicularly to the coast. This produces the longshore current, and a sawtooth motion will drag beach material along the beach in the direction of the current.

Where the coast indents at an angle of greater than 30 degrees, like at an estuary or bay, the longshore drift builds a sand spit into the open ocean. In some cases, this can stretch all the way across a bay and form a lagoon. An example of this is Venice.

Sand spits come in a load of flavors. Recurve spits form when a spit curves back towards the shoreline in response to changing wind and wave conditions. If multiple recurves occur, then it is called a compound recurve spit. Comet-tail spits form in the lee of islands where longshore drift acts on beaches on either side of the island. If two comet-tails merge, a looped barrier is created, enclosing a lagoon.

Lagoons are bodies of shallow, quiet water protected from the open sea by some sort of barrier. Paired spits form when a spit grows out of either side of a coastal opening, like a bay or estuary. Cuspate spits occur where two opposing longshore currents meet. They are triangular-shaped and wider than they are long. There's also tombolos, which we previously discussed. If several islands are connected in a chain to the mainland in this manner, we call it a tombolo cluster.

A barrier beach forms when a spit grows across most if not all of a bay, or where paired spits merge, or when waves deposit material in an offshore bar in such a way that the bar grows above the high water mark. The water bewteen the barrier beach and the shoreline becomes a lagoon. Storms may breach the barrier beach and create narrow inlets, dividing the barrier beach and creating a chain of barrier islands. Barrier islands can occur on most coasts, but most of them are on passive margins.

Like before, emergent coastal plains can feature terraces, called raised beaches. Submergent coastal plains form estuaries. Which leads us tooooooooooo

Estuarine Coasts

Estuaries are long and narrow tidal inlets that connect to the open sea. They function as a transition zone between the freshwater river and the sea, as freshwater and salt water mix. Their associated coasts are called estuarine coasts and are almost always submergent in nature.

Generally, estuaries come in four types: Drowned river valleys, bar built estuaries, tectonic estuaries, and fjords. Drowned river valleys are coastal plain estuaries that form when sea levels rise and flood the region around the river valley. They have a characteristic dendritic pattern, and the prototypical example is Chesapeake Bay.

Bar-built estuaries, also known as restricted mouth estuaries, are essentially the same except that they have significant barring across their mouths. The prototypical example is the Outer Banks of NC.

Tectonic estuaries form when tectonic activity causes large regions of coastal land to sink, which is then flooded by seawater. A classic example is San Francisco Bay. Finally, everyone's favorite, fjords! Fjords are long, steep-sided, flooded glacial valleys. They form in mid to high latitude regions that were once subject to ice age glaciation. After the glacier that carved the valley melts, its water enters the ocean, the sea level rises, the valley drowns, and look at that it's a fjord.

Organic Coasts

Organic coasts are coastlines on which living organisms control the landforms. Tidal flats, also known as mud flats, are regions of mud and silt which can be found in the intertidal zone in regions protected from strong wave action. These can especially be found in lagoons or on shores protected by barrier islands. Tidal flats occur in association with coastal plains, and go between totally submeged at high tide and exposed at low tide. A tidal flat will usually have several tidal channels flowing through it as well as a huge number of tidal creeks. If the tidal flat is high enough, salt-tolerant plants will start to colonize the flat, forming a coastal wetland.

Mangrove swamps and salt marshes are the two primary types of coastal wetland. Salt marshes are populated by salt-resistant herbs, grasses, and low shrubs. They can occur practically everywhere, but the largest ones occur in the temperate zone on low-lying, ice-free coasts, bays, or estuaries. They will also likely occur on the landward side of spits or such structures.

Mangrove swamps are dominated by salt-loving trees – your planet's version of the mangrove – and are limited to the tropics and subtropics unless your version of the mangrove has different temperature tolerances.

Coral reefs are by far the most beautiful of any organic coast structure. They form in clear, well-lit, warm water and so are limited to the tropics and subtropics. If a volcanic island builds itself up out of the ocean, a fringing reef may form around it. This is a coral reef that extends almost to the shoreline, and are also found on continental tropical coasts. As the island moves off the hotspot, it will slowly erode and sink. However, the coral reef will continue to grow, forming a small island surrounded by a lagoon and a barrier reef. Eventually the island will become fully submerged, forming an atoll, and finally the reef itself will finally collapse, forming a flat-topped submarine mountain known as a guyot. In summary, the evolution of an island and its reef is essentially this:

Island>Island + Fringing Reef>Island + Barrier Reef>Atoll>Guyot

Summary

In summary, your coastline essentially alternates between rocky and coastal plains. Rocky coasts dominate and mainly go on active margins, near coastal mountain ranges, in areas subjected to glaciation, and in regions of high relief, and coastal plains go on passive margins and in areas of low relief. These are some of their associated landforms:

Landforms associated with coast types

ROCKY COASTS	COASTAL PLAINS
Cliffs/Bluffs	Beaches
Platforms	Spits
Coves	Lagoons
Gorges	Tombolos
Sea Caves/Arches/Stacks	Tied Islands
Uplift Terraces	Barrier Beach / Islands
	Raised Beaches

Estuaries occur when river valleys flood, fjords go in mid to high latitudes where glaciers existed but do not exist now. Organic coasts are often associated with coastal plains, mangrove swamps go in the tropics, salt marshes go in the not-tropics, and of course coral reefs go offshore in the tropics. And now your world has beautiful, naturalistic shorelines! Bear in mind that this section is not exhaustive, as coastlines are insanely diverse. There is a lot of complexity that has been abstracted away for the sake of not writing a novel, so it is recommended to look into the various topographies mentioned.

Glaciers

Glaciers are large masses of ice formed of compressed snow that move slowly under their own weight. Broadly speaking, glaciers come in two forms: continental glaciers like antarctica, and mountain glaciers like the ones in Alaska.

Mountain Glacier Landscapes

Mountain glaciers are comparatively small (100s m-100s km long), only occur in and around mountainous regions, and generally flow from areas of high elevation to areas of low elevation, with the terrain dictating how they do so.

The largest type of mountain glacier is the mountain ice cap or icefield. These form in any large mountain basins or atop large mountain plateaus. They are relatively flat and tend to submerge mountain peaks and ridges. Any unsubmerged peaks and ridges poking through the ice are called Nunataks. Mountain Ice Caps will be drained through gaps in the topography through a series of outlet glaciers.

Sometimes these outlet glaciers will be valley glaciers. These are mountain glaciers that flow down what was once a mountain river valley. Debris from the surrounding valley walls will accumulate at the glacier's margins, forming lateral moraines. When two or more valley glaciers merge, their lateral moraines will also merge, forming one or more medial moraines. Due to compression within the glacier, subsurface debris may be forced to the surface, forming thrust or shear moraines. Finally, end moraines form when debris carried by the glacier gets dumped at the glacier's snout.

Other accumulations of debris may also be present on the surface of a valley glacier. Lobe-shaped deposits of rockfall, dirt cones, and large boulders called erratics are among these. The surface of a valley glacier may feature deep cracks known as crevasses. These are hundreds of meters long, tens of meters deep, and many meters wide. Crevasses form when glacial ice is fast-flowing and/or flows over steps in the underlying terrain. They may be visible, or filled with snow, or debris.

Valley glaciers may terminate within the valley itself, but if it terminates in an adjacent plain, it will form a lobe-shaped structure known as a Piedmont Glacier. Alternatively a valley glacier may flow into the sea as a tidewater glacier. Icebergs form as ice calves off the end of the tidewater glacier. Mountain glacier icebergs tend to be small, irregularly shaped pinnacle bergs.

Hanging glaciers are identical to valley glaciers except they form in hanging valleys. Bits of ice can calve off the snout and fall down the mountain, often triggering avalanches. Cirque glaciers are glaciers that form in cirques, bowl-shaped depressions that are carved into the side of a mountain. If two cirque glaciers form in adjacent cirques, they will carve into the intervening terrain and produce a knife-edged ridge known as an Arête or Freaking Awesome Ridge. Any upstanding spikes are called Gendarmes, and any troughs or passes are called Cols. Lots of french around cirque glaciers.

If three or more cirque glaciers are around a peak, they will carve a pyramidal structure known as a horn. The Matterhorn is a classical example of this.

Continental Glacier Landscapes

Continental glaciers are massive, blanketing vast swathes of continental crust. They overwhelm the underlying topography to such an extent that their flow is entirely unconstrained by terrain. Continental glaciers that cover areas greater than 50,000 km² are called ice sheets, and contiental glaciers that cover areas less than that are called ice caps. Both sheets and caps are dome-shaped and flow out radially from their thickest point, thinning towards their edges. Smaller local domes may be present depending on the underlying topography.

Ice streams, fast-flowing regions of the cap, will drain the glacier. They are commonly hundreds of kilometers long, less than 50 km wide, less than 2 km deep, and move at roughly a kilometer per year. Like mountain glaciers, expect lots of crevasses in the ice streams, especially near the margins of the continental glacier. Nunataks may also be present but nowhere near as frequently as in mountain ice fields. However, a large enough mountain range will probably stick up above the ice and look really stark and desolate and cool. When they do occur, expect moraines, rock falls, and erratics.

Standing in the middle of a continental glacier is really boring. It is a vast, flat white desert in all directions broken up only occasionally by some crevasses and rocky outcroppings. The margins of a continental glacier are much more interesting! If it terminates on land, it will do so via a series of lobe-shaped outlet glaciers. These act like mountain glaciers and so will manifest a lot of their features.

Strong, often hurricane-force winds will blow off the margins of a continental glacier. These Katabatic winds will create a sort of hazy look as snow and dust are blown off the surface into the air. They may also strip a region of its snow cover forming a Glacial Oasis, a snow and ice-free area in an otherwise ice-covered region.

If the continental glacier terminates in the sea, it will form a broad, flat ice shelf that is 100 to 1000 meters thick. Beyond the ice shelf the sea itself may freeze, forming 1-5 meter thick sea ice. Katabatic winds blowing over an ice shelf can rip apart sea ice, forming ice-free lanes known as Polynyas. Icebergs calved from ice shelves will tend to be Tabular bergs, icebergs that are many times wider than they are high.

These icebergs can be big. As in, JAMAICA big. Most of these icebergs will be a blue-white color, but occasionally they can be yellow (dead plankton/iron oxide rich seawater freezes onto berg), green (mixture of blue and yellow), black (pure, air bubble free ice/volcanic ash), or multicolored (seawater with specific particulate content freezes into cracks in the berg, later seawater with a different particulate content freezes into other cracks.

Glacial Placement

To get a complete picture of what is going on in your world, you will need to find where your glaciers are in your modern world and where they were in your world's last ice age.

Assuming your world is similar to modern day earth, your continental glaciers maybe placed 60-90º N/S in the polar ice cap climate zone. Sea ice will be present in the surrounding oceans, though coverage will vary by season. In winter, they will be present at ~70-90º N/S and at 45º N/S along coasts where cold currents flow. In the summer, this zone will be smaller, perhaps at 75/80-90º N/S, and 55-60º N/S along cold current coasts. Icebergs will go from the ice caps along your ocean currents. If you know those (see part 7), you will be able to identify any dangerous shipping routes.

Mountain glaciers can be placed in any of your mountainous regions at any latitude at high elevations. In general, mountain glaciers should not be placed in mountain rain shadows as the lack of snowfall in these regions will prevent glacial formation. The closer you get to the equator, the more your glaciers will be limited to poleward-facing slopes.

During the last ice age, assuming your world is similar to earth, place your continental glaciers between 45-90º N/S in the Polar ice cap climate zone. For mountain glaciers, place them at any latitude but this time the elevation limits are not as stringent.

Plotting both the last ice age and modern glaciers allows you to identify regions on your world that were once glaciated but currently are not, which will come in handy later.

Post-Glacial Landforms of Mountain Glaciers

Any of the landforms mentioned previously will become more exposed and dramatic once the mountain glaciers have fully receded. Mountain ice caps will smooth and round any peaks that were submerged in the ice. Any unsubmerged peaks–Nunataks–will be made more jagged and rugged due to frost shattering.

Cirque glaciers will leave behind cirques or carries, which will often be filled with a small, moraine-dammed lake known as a tarn. Tarns may be seasonal or persistent. Various types of valley glaciers will leave behind deep glacial troughs known as U-shaped valleys. These may be hundreds of meters deep and tens of kilometers long and may be host to a wide variety of landforms.

The head will be a steep, almost sheer, rugged rocky face known as a trough end. Expect cool looking waterfalls. The floor of a U-shaped valley will for the most part be wide and flat. Where bands of soft rock are present, the valley glacier will have carved depressions into the rock tens to hundreds of meters of deep forming fjord-lakes, large lakes spanning most if not all of the valley floor, or paternoster lakes, smaller lakes that chain together like beads on a string.

Where bands of hard rock are present, valley steps will form on the valley floor. The up-ice, or stoss side of the step will be smoother and gently sloping, whereas the down-ice or lee side of the step will be steep and rugged. If a step spans the entire valley floor, it is called a riegel. Riegels may impound lakes and depending on their height may have rapids or waterfalls cascading over them.

Medial moraines, erratics, kames, and eskers may also be present on the valley floor. The remnants of the glaciers medial moraine may run as a ridge or mound of glacial debris, a till, down the center of the valley floor, though most do not survive glacial retreat. Erratics, large boulders dumped by the retreating glacier, may be randomly strewn across the valley floor. These can be anything from pebbles up to huge boulders weighing several hundred tons.

Kames are flat-topped small hills of till about 100 meters long and 10-ish meters wide. Eskers are long, winding ridges of till. In alpine regions, these can be tens of kilometers long, tens of meters wide, and over ten meters high. Kames and eskers typically occur together and are a product of glacial meltwater. Finally, a ground moraine or till sheet is a blanket of mixed till covering the valley floor. Basically, any and all material that didn't go into forming the features mentioned above gets dumped everywhere randomly, making a huge mess and forming a gently undulating landscape with low rises and ridges alternating with small depressions, kettles and kettle ponds. Kettles are small bowl-shaped depressions that form when a detached or buried bit of ice melts.

At the sides of the u-shaped valley, we can usually find the remnants of the glacier's lateral moraines. These ridges or mounts of till may be hundreds of meters high and wide, though most are way smaller than that and are prone to being eroded away over time. Alternatively, Kame terraces may be found. These are long, flat, bench-like deposits of sediment that line the valley sides. They are also a product of glacial meltwater. The walls of the valley will feaure alternating hanging valleys and truncated spurs. Truncated spurs are steep, inverted V-shaped cliffs that form when the interlocking spurs of a pre-glaciated river valley get bulldozed by the valley glacier. Hanging valleys will enter the main U-shaped valley at elevation. If it was glaciated, it will be another U-shaped valley and so will contain all the features above. Spectacular waterfalls often drain the hanging valleys into the main valley.

Near what was once the snout of the valley glacier, the remnants of the glacier's end moraine can be found as an arc-shaped deposit of glacial debris about 60 meters in height. This end moraine marks the furthest extent of the glacier so we call it a terminal moraine. A retreating glacier does not do so linearly, and may speed up, slow down, stop, or even advance depending on how hot or cold the climate at any given moment happened to be. If the glacier repeatedly halted during its retreat, there may be a series oend moraines called recessional moraines can form behind the terminal moraine. The longer the halt, the more material builds up, and the higher and larger the new end moraine will be. Expect lakes to be trapped between these structures.

Any time the glacier advanced, it would have bulldozed material at its snout, forming thick push moraines, which could make the landscape look like a rough plowed field or just increase the size of a recessional moraine. Expect the landscape here to be quite chaotic. And that is what deglaciated mountain landscapes look like.

Before we move on however, we must note that if a U-shaped valley gets partially flooded by seawater, it is called a Fjord, so expect them to be present any time you have a coastal mountain range that would once have been glaciated but currently is not. Most of them are deeper than the adjoining sea, with their terminal moraines forming a sill or shoal at their mouths. Often times, these ridges cause extreme currents, whirlpools, maelstroms, and saltwater rapids. By the way, a fully submerged U-shaped valley is called a marine trough but no one cares about those.

Post-Glacial Landforms of Continental Glaciers

Continental glaciers blanket all topography, highlands and lowlands. The landforms left behind in the highland regions of a continental regions will be identical to those we just discussed. The post-glaciated lowland regions will differ slightly however.

A good chunk of these regions will be dominated by knock-and-lochan topography. These are vast, low-lying regions spanning huge areas (often several hundreds of kilometers in width). THe landscape is gently undulating, with innumerable depressions and lakes, alternating with small rocky hills and outcropping.

These rocky hills may come in many forms. Domes are rocky hills shaped like domes, less than 100 meters in size. Their surfaces are smooth. Roche Moutonnées are again small hills of solid rock less than 100 meters again. Unlike domes, they are streamlined in the direction of the ice flow. Their stoss sides are gently sloping, and their lee sides are steep, rugged, and clifflike. A medium-sized RM between 100 and a thousand meters long is called a whaleback or a rock drumlin. The largest of them, over a kilometer long, are called Flyggbergs for some reason.

Crag-and-tails are large, rugged outcroppings of resistant rock, often volcanic types, with a smooth sloping tail of till in their lees. Expect large settlements to spring up around these. The crag is easily defensible and the tail is a great location for a settlement. Edinburgh castle in scotland is built on a crag, and the city's old town is on the corresponding tail.

Various types of ground moraine may occur in post-continental glacier regions. Degeer or Washboard moraines form whenever a continental glacier met a sea or lake. They're essentially the same as recessional moraines only they're underwater. They form as a series of parallel ridges perpendicular to the direction of the ice flow. They tend to be ~5m high, 10-50m wide, and spaced ~300m apart. If the land rises or the water level drops afterwards, the De Geer moraines become exposed, forming a visually stunning landscape. An example of exposed De Geer moraines is the Kvarken Archipelago in Finland. Look it up!

Rogen moraines dominate the interior of what was the continental ice sheet. They are arc-shaped deposits of till lying perpendicular to the direction of the ice flow with their horns pointing in the down-ice direction. They often occur in close, regularly spaced groups with each individual moraine being roughly 10-30m high, 100-300m wide, and between 300-1200m in length. Lakes tend to fill the depressions between them, giving the landscape a very chaotic appearance when viewed from above. Nearby the rogen moraines and/or in close proximity to the edge of the former continental glacier is drumlin fields are often present. These are smooth hills of glacial tills, elongated in the direction of the ice flow. Their stoss sides are steep and the lee sides are gently sloping. Drumlins are typically between 250-1000m long and 120-300m wide. They tend to occur in their thosuands in groups, forming a drumlin field or drumlin swarm. Look its the shire!

Also near the margins of the continental glacier, there can be tunnel valleys. Tunnel valleys are gorges between 2 and 4 kilometers wide, over a hundred meters deep, and 30-100 km in length. They form when subglacial melt cuts into the underlying bedrock. Tunnel valleys may become filled with sediment, forming dry valleys, or with water, forming lakes similar to the Finger Lakes in western New York.

Till plains are large, flat low-lying regions that are sometimes smoothly undulating, blanketed in a thick ground moraine. Till plains can be extremely fertile farmland, so put some farmland here. Just like mountain glaciers, we can also expect eskers, kames, and erratics, expect B I G. These eskers can be hundreds of kilometers long, hundreds of meters long, and over ten meters in height.

Kames in continental glacier lowlands may occur in kame fields, giving rise to kame and kettle topography. Erratics will be utterly massive, upwards of 20,000 tons. Finally, near the end of the continental glacier, we will probably find end moraine complexes. Again, like before, but b way bigger. The terminal moraine of a continental glacier can be hundreds of kilometers long, hundreds of meters high, and kilometers wide. Put cool castles on these.

Glaciofluvial Mountain Glacier Landforms

If your planet is like modern earth you will likely have many regions that are still partially glaciated with retreating glaciers. Retreating glaciers are melting glaciers, so expect landforms created by glacial meltwater to dominate in these regions. Meltwater flowing on the surface of a glacier will form superglacial ponds and meltwater channels. These bodies can drain through crevasses and moulins, round, near-vertical pothols in the surface of the glacier extending down to bedrock, roughly 10 meters in width. The sediment carried by the meltwater will be edeposited in these locations and will give rise to the kames mentioned previously once the glacier fully retreats.

Meltwater can also flow at the sides of a mountain glacier. Ice-marginal ponds and channels will be found here. Sediment would be deposited here, producing the Kame terraces mentioned earlier. Meltwater flowing at the base of a glacier can carve channels into the bedrock or into the ice itself. The former will produce small tunnel valleys, and the latter will produce a system of glacial caves, which will eventually be choked with sediment and become eskers.

Most meltwater will eventually make its way to the snout of the glacier. If it terminates on a plain, a system of braided meltwater rivers called an outwash plain or Sandur will form. Sediment will get sorted by the meltwater, so large debris will be found near the terminal moraine, and finer debris will be present further down the outwash plain, THere will probably also be kettle holes and lakes everywhere. If the glacier terminates in a valley, the width of the valley will become a mini-outwash plain, creating a "valley train". Lakes are common in front of glaciers. These may be moraine-dammed or dammed by the ice. If the dam breaks or the lake overtops its barrier, a glacial outburst flood occurs. These may be sporadic or seasonal and they are devastating. They can move over 300,000 cubic meters of water, 25 thousand tons of ice and another 25,000 tons of debris every second.

Glaciofluvial Continental Glacier Landforms

The glaciofluvial landforms of retreating continental glaciers are mostly the same as those present around mountain glaciers, except massively bigger. Instead of superglacial ponds and channels, expect full-on superglacial lakes and rivers with immense meltwater waterfalls castcading into giant moulins or over the edge of the ice margin into mega pro-glacial lakes. When these lakes outbursted, they were powerful enough to entirely rework the geography of the proglacial region into channeled scabland. Channeled scablands are vast flat expanses deeply scarred by meltwater channels, canyons, gorges, and so on. They have poor soil and essentially no vegetation.

The glacial outbursts were so cataclysmic that they reworked the shape of the landmasses themselves. The Straits of Dover were carved out by one about 400 thousand years ago.

Summary

For the most part, mountain and continental postglacial landforms are fairly similar, with scale being the main variable. Use the Cold climate model discussed here to find where the ice caps used to be during the last ice age. Fully glacial landscapes will go where glaciers actually are in the modern day, glaciofluvial landscapes appear on the edges, and postglacial landscapes occur where glaciers used to be but are not today.

Uninhabitable Planets

Uninhabitable planets, being much more common and varied than habitable planets, are also very useful to map. Uninhabitable planets can generally be split into these eight categories:

• Gas Giants/Dwarfs
• Selenas/Mercurians/Cereslikes
• Venusians
• Lava Worlds
• Martians
• Iolikes
• Icy Worlds
• Exotic Worlds

Gas Giants/Dwarfs

Gas Giants and Dwarfs are among the most common planet types, and they are very simple to map. One can simply plot out bands of varying widths, and then add randomly placed storms and other cloud features. If you wish you can put a polar hexagon on some of them, but this is not going to happen very often and the causes are unknown to scientists.

Selenas/Mercurians/Cereslikes

Selenas and Mercurians are another common planetary type that is also simple to map. They have very different compositions, but their surfaces look very similar and are therefore lumped into one category here.

Selenas and Mercurians are distinguished by the fact that they are geologically dead and are therefore pocked with craters and cracks. Because of how crater formation works, simply place any maria that are present first, followed by increasingly smaller craters. After this is done, you may place the occasional relatively large crater somewhere on the surface and add ejecta rays streaming away from it. In the case of mercurians, follow the above minus maria. Also, add more high cliffs and cracks than before.

A rare subclass of selenas are Ceres-like worlds, which are small planets with rocky and icy surfaces, possibly with ice volcanoes and other hydrothermal activity in evidence. Oh, and salt deposits. Put those in places.

Venusians

Venusian surfaces are characterized by having no present tectonic plates. Essentially, map them by randomly placing highlands and making everything super wrinkly, then add lots and lots of volcanic craters. As for clouds, just copy venus' texture no one's gonna notice. In all seriousness though, Venusian clouds are near-featureless, and are nowhere near as interesting to look at as in most pictures you've seen. Whatever cloudmap you have now, decontrast it by a lot.

Lava Worlds

For worlds that are entirely covered in lava, you're already done and you don't have to actually map anything. For worlds that are only partially covered in lava, make a selena map as above, but replace the mares with actual lava. Fill any lowlands in a similar way.

Martians

Martian worlds are defined by a lack of plate tectonics, completely dry terrain, and evidence of past water presence. As such, randomly place your highlands but smooth them out and place such things as eroded river paths and dry lakebeds. You may have very large volcanoes, place at will on a highland, but use sparingly. Finally, large highlands are likely to have large canyons running through them, similar to the Valles Marineris running through the Tharsis highlands. Place one of them if you like and make it look cool.

Iolikes

Io is a unique planet, a terrestrial world under such tidal stress that its volcanoes spout plasma into space around its orbit. Planets like it are very difficult to map, as nothing should realistically stay up to date for very long. There tend to be hundreds of constant volcanoes ranging in size from the very small to the enormous, with the largest of them supporting permanent lava lakes up to 200 kilometers in diameter.

Io, our example (false color). Note the large region of orange deposits surrounding one of the volcanoes.

To map a world like Io, a necessary first step is determining the tidal force relative to io itself to gauge how many and how powerful the volcanoes should be. After this, place points on your map corresponding to your largest volcanoes, as these will be functionally alone in driving the development of surface features. Impact craters will be nonexistent.

Place funky volcanic terrain ripped from Io's map at random around the map, including circles of ejecta deposits surrounding the largest active volcanoes, as shown to the right.

Asteroids

Asteroids tend to be undifferentiated lumps of material too small to pull themselves into a spherical shape. As such, their actual form is mostly a crapshoot and you can make a pretty decent model of one by grabbing some playdoh or similar material and messing around with it until you get a nice lumpy shape that looks right.

Conclusion

You will notice that parts of this tutorial call for things we haven't done yet. These things will be done in #Part 7: Atmospheres & Oceans, and then we have finally finished the physical properties of our universe!

Part 7: Atmospheres & Oceans

You may have noticed that some sections in the previous part call for parts of your planet like Atmospheres, Oceans, and other such things that were not addressed. This is because of poor planning on my part, but at least part 6 isn't the length of a novel.

Earthlike Atmospheres

Earth's modern atmosphere is composed of 78% Nitrogen, 21% oxygen, and 1% Argon, with trace amounts of basically everything else one could think of. However - it was not always like this. Its initial atmosphere was likely very thick and composed of hydrogen & helium. This was later blasted away and replaced with a secondary atmosphere composed of cometary material and volcanic gases (SO₂, N₂, NH₃, H₂O, CO₂).

Earth then cooled, oceans formed, life evolved, and blue-green algae began producing oxygen via photosynthesis, producing Earth's current atmosphere. This is generally how the atmospheres of habitable planets form.

Earth's atmosphere is divided into five layers: the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. The temperature variances in these layers are pretty complicated and almost totally irrelevant, so we wont go over them here. Anyway, thats an atmosphere.

Because of how insanely complicated the equations for atmospheres are, I usually use this spreadsheet from Artifexian. Basically plug in the relevant stats for your planet, check which gases will remain in the atmosphere, and then plug in some percentages. Done!

Eh... no. For earthlike planets you're generally quite limited in terms of which gases are present. In fact, it is a good idea to keep things the same or close to the same as Earth. Oxygen (so animals can breathe), CO2 (in trace gas form, so plants can breathe), and Nitrogen, so amino acids and DNA can be created. Even Argon is almost guaranteed to be there in some form, as it is created from the decay of certain isotopes of Potassium common in a planetary crust.

After filling in those four, just chalk up the remaining tiny percentage to unspecified trace gases unless you really want to drive yourself insane.

This atmosphere has a weight, and that is felt as pressure. There are lots of units one can use to measure this, but the most intuitive for most people is the atmosphere, which is equal to the sea level pressure on earth. The habitable ranges for sea level atmospheric pressures depends on the nitrogen/oxygen content, described by this graph:

File:Habitable-atmospheres-graph.png

For habitable planets, the partial pressure of nitrogen must not exceed 3 atmospheres, O2 must be between 0.16 and 0.5 atmospheres, CO2 must not exceed 0.02 atmospheres (although it should be below 0.005 atmospheres for no physiological stresses), and Argon, despite being an inert gas, should stay below 1.6 atmospheres. Stray outside these and you die an agonizing death. At least at sea level. You might be able to do something really cool with height, having only the highlands be habitable. I would read the hell out of that!

For the completionist, here are some maximum partial pressures for various noble and trace gases.

• He: <80 atm (extrapolated)
• Ne: <5 atm
• Ar: <1.6 atm
• Kr: <0.46 atm
• Xe: <0.21 atm
• NH₃ (Ammonia): < 0.0001 atm
• CO (Carbon Monoxide): < 0.0001 atm
• H₂S (Hydrogen Sulfide): < 0.00002 atm
• CH₄ (Methane): < 0.05 atm (toxic limit, flammable at lower pressures)
• NO₂ (Nitrogen Dioxide): < 0.000025 atm
• O₃ (Ozone): < 0.0000001 atm
• SO₂ (Sulfur Dioxide) < 0.000005 atm

Side Effects

High oxygen worlds are really fun. Things will burn faster, hotter, and more easily. More oxygen also makes respiration easier, so theoretically speaking you could get very big creatures, such as what happened during the Carboniferous period, when you had millipedes 2 meters long.

Scale Height

The spreadsheet linked above computes the average atomic weight, which no one gives a flying fridge about, and the scale height, which is very important. The scale height is the distance over which the pressure and density of an atmosphere fall by a factor of 1/e (e = 2.718..., an irrational number that we use for this for some reason). For every additional scale height, the pressure falls by an additional 1/e. Essentially, the larger the scale height, the more slowly an atmosphere tapers off to nothing. The scale height can be computed like so:

H = (RT)/(mg)

R = Molar Gas Constant (kg m² s^-2K^-1 mol^-1)
T = Temperature (K)
m = Molar mass (kg mol^-1)
g = Surface gravity (m/s²)

Something useful we can find with this is to find the atmospheric pressure at a given altitude, which we can use to make some very interesting settings. One that my friend suggested is an incredibly high-pressure world where the only habitable regions are the ultra-highlands, necessitating the development of powered flight very early on in history. Fun!

By the way, when the pressure is almost zero, you're in outer space and you can start throwing down space things there.

In summary, by keeping everything earthlike but slightly tweaking the percentages and pressures, we can get incredibly varied worlds! Who needs ridiculous gases?

Also add helium for abandoned advanced civilization worlds for funni

Not Earthlike Atmospheres

Not earthlike atmospheres have fewer restrictions. However, it is generally a good idea to avoid having more than like 3% oxygen, as it is rapidly absorbed into the planetary surface (unless you have an ice planet). Also, avoid rare things taking up too much of a percentage of the atmosphere.

Remember that unless the planet has a magnetic field or is under the influence of a different magnetic field, it is unlikely to have a thick atmosphere. An exception is Venus, which has such a thick atmosphere that it simply hasn't gone away yet, but there's still a large tail of gases streaming out behind it.

See also

• Niflheim from H. Beam Piper's Uller Uprising
• Clorox from Stephen L. Gillett's World-building

Atmospheric Circulation

A planet's lower atmosphere is essentially a massive circulation mechanism to transfer heat from the equator to the poles. This sounds simple, but it isn't because of the Coriolis effect.

Wam air rises at the equator, stopping at the tropopause at 12-15 km up. It then splits and moves poleward, beginning to deflect toward the east. at roughly 30º N it is essentially due east and as such can never reach the poles. It cools, sinks, and travels back along the surface towards the equator and westward. This is the hadley cell, and the prevailing winds within this are called the trade winds.

Something similar happens at 60º latitude. Relatively warm air rises to 8 km, moves poleward and eastward, sinks, and returns along the surface westward. This is the polar cell, and the prevailing winds within it are the polar easterlies because they blow from the east or something.

Between these cells is the Ferrel Cell. The prevailing winds are called the westerlies in this cell. The same structures occur in the southern hemisphere, but flipped because of the coriolis effect.

The zones where each cells meet are named like so: The two Hadley cells meet at the Intertropical Convergence Zone at the equator, the Hadley and Ferrel cells meat at the Subtropical Ridges, and the Ferrel and Polar cells meet at the Polar Fronts. Where warm air rises, there is a low-pressure zone, and where cool air sinks there is a high-pressure zone.

To apply this to your world, simply mark the ITCZ at the equator, the Subtropical Ridges at 30º N/S, and the Polar Fronts at 60º N/S and draw in the corresponding prevailing winds. Done!

Actually no its not. This only works for planets exactly like earth. A retrograde planet would flip the wind patterns to go the opposite direction, for example. This is neat, but what if we change the rotation rate? Generally, the slower a planet rotates, the fewer circulation cells it will have.

Specifically: planets with 1/2, 1/4, 1/8, and 1/16 times earth's rotation rate will have will have one cell per hemisphere. Planets with 1-2X earth's rotation rate will have 3 per hemisphere, as earth does.

Planets rotating 4x as fast will have 7 per hemisphere (equator-24º, 24-27º, 27º-31º, 31º-41º, 41º-58º, 58º-71º, and 71º-pole), and planets rotating 8x as fast will have five per hemisphere (equator-23º, 23º-30º, 30º-47º, 47º-56º, 56º-pole). No idea why planets with 4x rotation rate throw off the cool linear trend, but either way decide which is closest to your planet and mark it in as earlier.

Tidally locked planets would work like this: Powerful winds blow eastward from the subsolar point while less powerful winds would blow westward, cool winds would blow from the antisubsolar point onto the day side, and where all these winds meet there would be lots of precipitation and cloud cover. Highest temperatures would be north and south of this zone due to a lack of cloud cover. The east side will be warm and wet, while the west side will be cooler and drier. The night side surrounding the antisubsolar point will be extremely cold and dry, while the rest of the night side would be less extreme.

DISCLAIMER

This is obviously massively oversimplified. Real planets have pressure centers that change over time and are affected by the location of continents. If you want to achieve something this accurate, check this link. Personally I don't really care about being this accurate, and what we did above is sufficient.

Side Effects

As we saw with the tidally locked one, atmospheric circulation can determine wind patterns, precipitation, and biome placement, but we basically did that in part 6 because of my bad planning.

In an earthlike configuration, we can divide a planet into a hot zone, temperate zones, and cold zones. Anything which initially came from a given zone probably won't do that well in another. A plant from Peru planted in Canada will die quite rapidly, but an Italian crop planted in the US or Japan would probably be fine. Peru and Canada are in different zones, but Italy, the US, and Japan are in the same zone.

The same thing goes for animals, so no equatorial polar bears, and people. Empires are more likely to go conquering in zones similar to their homeland. Because of this, Jared Diamond (author of Guns, Germs, and Steel) suggests that zonal landmasses are more likely to develop advanced civilizations because they provide a relatively homogenous space for people to share crops and animals and acclimatize to each other's diseases.

These are not hard limits. Look at corn, the British Empire, or any historian. Atmospheric circulation still plays a role in the distribution of flora, fauna and civilizations. Also, where cells meet, maritime barriers are created. Sailors refer to the Intertropical Convergence Zone as "the doldrums" (no relation to the thing in the phantom tollbooth) because winds there tended to be fairly stagnant or flat out not existing at all. This meant that sailors could be stuck there for days if not weeks on end waiting for winds to change, leading to a higher chance of scurvy, delirium, starvation, cabin fever, and death.

The same is true for the subtropical ridges, or the "Horse latitudes", because it is said that sailors stuck there would throw their horses overboard to conserve water. You can come up with your own names for these though. These zones will impact how the seas are navigated on your world.

Expect tropical cyclones to form on equatorial oceans between about 5 and 20 degrees north and south, and then move with the prevailing winds. Thunderstorms occur anywhere where there is a cold air mass moving into a region of warm, moist air, like in the US. They can also occur when air moves upward in regions with an immense supply of water, like around the Intertropical Convergence Zone. They can also occur in mountainous regions.

If you number your wind cells from the equator, the cells with even numbers are where you would expect tornadoes to form, as they can be seen as a turbulent eddy that occurs as the result of the other two cells, and its winds are fickle and prone to variability. Also, tornadoes form where warm, moist air flows at low levels and cool, dry air flows up high. On earth this basically only happens in the United States, which is why that country recieves a full 75% of all tornadoes on earth.

This is going to involve some lord of the rings, so bear with me. Imagine we took Mordor, scaled up the Sea of Nurnen, and made it an estuary spilling out into an equatorial ocean. This fits all the thunderstorm criteria from earlier, and when we mix that all together we get EVER-LASTING THUNDERSTORMS! Our new Mordor can expect thunder and lightning for the vast majority of its year. Each night storms rage for about 10 hours or so, and at peak lightning would strike thirty times per minute. Try doing something like this on your world so we can all gaze in awe at it. Sounds super fantastical and awesome, right? Yeah its not, it happens on earth in Venezuela, in a funky place called Lake Maracaibo. It's amazing.

Ocean Currents

For ocean currents, we need a map that has oceans, continents, and winds. To begin, draw two currents close to the equator on either side. Split these currents equatorward and poleward. The equatorward fluow will loop back around to form an Equatorial Counter Current moving eastward. Split the flow of that at the next shelf and loop back to the first currents, and you have closed your first gyre.

The poleward currents should continue northward. At or just above the latitude where the next wind cell begins, those winds drag the currents into another loop. Once that hits the continental shelf, split the flow again, equatorward and poleward. The equatorward flow will close our tropical gyres, and the poleward flow will begin our next set. Repeat until you reach the edge of the last gyre.

If you have an open ocean at either or both poles, drop a westward flowing circumpolar current. If there are any gaps, fill them with smaller gyres or logically extend existing currents.

Color currents flowing away from the equator red, and flowing toward the equator blue. Keep the latitudinal currents that dont have any particular poleward or equatorward flow as black. Red is (relatively) warm, blue is (relatively) cold, and black is neutral. That is, warm currents carry warm water from lower latitudes to higher latitudes, cold currents carry cold water from higher latitudes to lower latitudes, and no significant heat exchange is occurring on the neutral ones.

Check to see if all the loops are closed and the spin directions make sense - that is, if one gyre spins clockwise, those bordering it spin counterclockwise, then clockwise, then counterclockwise, like a set of gears. Keep in mind that the faster your planet spins, the more air circulation cells there are, and therefore more ocean gyres.

If your planet is a waterworld, the currents will be banded. Think Jupiter's clouds but with water. However, if you want fancy gyres like earth has, you can have your continental shelves be only a little bit submerged as the current splits at the continental shelf, not the actual shore.

If we tidally locked our waterworld, we'd end up with a kind of spaceship lookin ocean encased in ice, with two big gyres in either hemisphere. If continents are present, the same rules from earlier apply.

Effects

Ocean currents effect seafaring societies. Explorers will discover things that ocean currents carry them to before other things, if they sail with the currents. They could also do what the polynesians did and sail upwind, because if something went wrong on the voyage it would be easy to get back home faster. This is why the polynesians reached the likes of Hawaii and Easter Island hundreds of years before they found New Zealand.

Ocean currents also cause latitudinal variations. We dealt with this in part 6. As an example, look at Florida and the Baja California Peninsula. One is dry and hot, and the other is wet and very hot. The culprit here is ocean currents. Cold water means cold air which means less evaporation, less clouds, and less rain. Whenever cold currents flow along landmasses, cooler and drier coastal conditions are created. The reverse is true of warm currents. Warm water means more evaporation, so expect wetter, warmer coastal conditions. It is worth noting these areas on the map.

In addition, expect cool coastal waters to be very nutrient-rich. These will be the planet's fishing hotspots (if there are fish). Also, expect coral reefs to form anywhere in the tropics where warm currents flow.

Lastly, there is an interesting phenomenon called El Niño Southern Oscillation events. These are irregularly periodic variations in winds and sea temperatures that occur in the pacific ocean. It has three main phases: Neutral, La Niña, and El Niño. Both of the latter can last several months, vary in intensity, and typically occur every few years.

In the neutral phase, or normal phase, the trade winds blow across the pacific. These winds push warm water across the ocean, piling it up in the west and drawing cool water to the surface in the east via a mechanism called upwelling. The temperature differences cause air to rise in the west and sink in the east, creating a huge circulation system known as Walker Circulation.

In the La Niña phase, everything gets turned up a lot. Trade winds blow harder, warm water piles up in the west, cool water piles up in the east, and Walker Circulation strengthens, causing trade winds to blow even harder and boom look at this feedback loop. La Niña brings storms and flooding to the west and heat waves and droughts to the east.

In the El Niño phase, everything reverses. The trade winds weaken, allowing warm water to drift back towards the east. This breaks down Walker Circulation, causing the trade winds to blow even weaker, allowing more water to gather in the east, and oh look its another feedback loop, except everything is flipped. El Niño does exactly the same thing as La Niña, except reverse the direction.

If your world has a large open ocean akin to the pacific, expect equatorial landmasses on its periphery to be subject to ENSO events. Bear in mind that directions will flip if your planet spins retrograde. Oceans - done!

Tides

What are they?

Lets take the simple earth-moon system as an example. Every point on earth is gravitationally attracted to the moon. The closer the point is, the stronger the attraction. Also, because the earth-moon system orbits around its barycenter, every point would be subject to centrifugal forces as well.

Combine these two vectors, and you get a tidal bulge as the ocean deforms along them. As Earth rotates through the bulges, it experiences tides as you probably know them - High tide, 6hrs later low tide, 6hrs later high tide, and so on. These are called Semidiurnal Tides - two high and low tides per day.

The sun also produces a tidal bulge, just weaker. When the two bulges are pointing the same direction, you end up with a "spring tide", as the high tide becomes really high. When the two are pointing perpendicular to each other, you get a "neap tide", as all the tides are more boring. No idea why they are called this.

This sounds simple, right? Nope. If it was really this simple, then all areas on earth would experience equal semidiurnal tides. Sure, most areas do experience such tides, but there's also "mixed tides" where everything's kind of wonky, and weirdly, "diurnal tides", one high tide and one low tide per day. There are also wildly varying tidal ranges across all the coastlines.

Imagine the ocean as a big bucket, and the coastlines as the bucket's rim. As the bucket rotates through tidal bulges, we get alternating high and low tides on either side of the bucket, and because of the coriolis effect the whole system would rotate. Tides are essentially standing waves centered on Amphidromic Points, a point which is unaffected by tides. In reality earth has several of these amphidromic systems, and they interact with one another to give rise to really complicated tides.

Why is this? Imagine waves as having a speed limit. If they exceed the speed limit, they split into smaller waves, creating more amphidromic points. The shallower the ocean, amongst other factors, the lower the speed limit. Therefore, big deep oceans like the pacific will have relatively few amphidromic points, whereas shallower areas like the North Sea will have many. Also, the wider the bucket, the more extreme the tidal range can be, which is why all but the biggest lakes do not experience noticeable tides. Even the mediterranean isn't big enough!

Landforms like the Bay of Fundy are great at generating huge tides. It is shaped like a funnel and gets gradually narrower and shallower towards the shore. This amplifies the effect of the tides. In addition to this already contrived system, the natural back-and-forth sloshing of water in the bay is aligned near-perfectly to the tides, further amplifying them. The tidal range in the mediterranean is a few pathetic centimeters, but in the bay of fundy it can be 116 meters! That's like 14 small cars stacked on top of each other. That's just vertical, if you consider the slope it can look like a hell of a lot more than that.

Finally there's the Tidal Bore, a rare phenomenon that only really effects regions with large tides. The incoming tide floats anywhere that feeds the effected bay, forming a wave that flows upstream, against the current. The largest tidal bores in the world happen in the Qiantang river in China. They travel upstream at speeds of up to 40 km/h, they get up to 9 meters high, you can SURF ON THEM, and they produce a low, trainlike rumble as they approach. Tides are awesome and also weird and complicated and hard to calculate.

How to Calculate Them

Artifexian made a very nice spreadsheet here, go ahead and make a copy.

In general, to calculate the tidal forces exerted by a major moon on your planet, use this equation:

T = (2230000 • M • D)/R³

T = measure of the tidal force on the planet
M = mass of satellite (in Earth masses)
D = diameter of planet (in earth diameters)
R = semi-major axis of satellite's orbit (in earth diameters).

If you want to calculate the forces exerted by a star on your planet use this one with these inputs:

T = (0.46 • M • D)/R³

T = measure of the tidal force on the planet
M = mass of star (in solar masses)
D = diameter of planet (in earth diameters)
R = semi-major axis of planet's orbit (in AU)

The constants are caused by unit conversion. Run the numbers, take the output of the lunar equation as your standard tide - positive, high tide, negative, low tide. Sum the lunar and solar tidal equations to get spring tides. Again, positive value is high tide and negative value is low tide. Subtract the solar from the lunar tidal equation to get the neap tide. Finally, multiply each by 0.54 to convert them to meters. Remember, these are tides in the deep open ocean. Actual tides will vary massively depending on local geography.

Everything we just did is massively, massively oversimplified. To get truly accurate tides, you'd need to run Laplace's Tidal Equations, which are

uh

involved?

So we fudge a bit. No big deal. If you want you can check if anything is tidally locked with this equation.

E = (T • A)/M

With these inputs:

• E = total tidal effect
• T = mass of all tidal forces acting on the world
• A = age of the star system (in billions of years)
• M = mass of the world (in earth masses)

Round the number you get to the nearest integer. If its 50 or above, the object is tidally locked. The object will be locked to whatever exerts the most tidal force on it. However - if you notice that the moon-star equation is producing larger numbers than the moon-planet equation, you should probably recheck the numbers or move the moon in by a lot. Hilariously oversimplified tidal calculations DONE.

What about habitable moons? same equations apply, just with slightly different inputs. M is the mass of the planet in the lunar tides equation, not the moon in this case. There is a catch. The tidal force exerted by a planet on its moon will be very large. However, to actually experience tides, the moon has to rotate through the tidal bulge. The moon is probably tidally locked, so that isn't happening. Double planet systems suffer from a similar issue, so you're stuck with just stellar tides. For those who want to handwave this, there's a double planet and habitable moon tide spreadsheet linked above.

What about multiple moons? Same equations as before just run them for each major moon. Ignore the pathetic asteroids though. Each of these will produce a different tidal bulge, and when they all intersect you'll get insane tides, and generally there'll be complicated relationships between them. When all the moons align in the sky, the tides will be seriously huge. If you make that happen when the planet is closest to its star, the tides will be bigger again. If you contrive a Bay of Fundy clone, they'll be bigger again. If you make it so that a storm hits there at high tide, there won't be enough measuring sticks in the universe to measure the absurdly insane tides you've just created.

What about multiple star systems? If you have a P-type orbit, just run the solar tides equation from earlier and simply combine your stellar masses. If you have an S-type orbit, ignore the other star and let it wallow in depression over not affecting the tides enough to be noticed.

The Intertidal Zone

The intertidal zone, the region above water at low tide but below water at high tide, are super productive ecosystems. There are so many life-forms that can live here that would be fun to write about. Animals that forage on these can be hunted as well. Therefore, settlements nearby the shore will do quite well.

However, intertidal zones can affect life on a more fundamental level. No tides, no intertidal zone. No intertidal zone, no land animals! Intertidal zones act like a halfway house for creatures moving out of the water to colonize the land. The smaller it is, the harder it would be for creatures to make the initial leap onto land. If there is none, then life may take considerably longer to do so and may never do it at all. Who would make all the measuring sticks we need then?

Conclusion

In this part, we finished some things we didn't do in the last part, and are now officially done with the physical properties of the universe. Next, we will begin writing about life in the universe in #Part 8: Life & Evolution.