Planets (Reference)
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Planets Overview
Planets are a constant fixture within Scopes. They exist in various forms and classifications based on their size, formation, orbital parameters, composition and temperature
Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks
A protoplanet is a large planetary embryo that originated within a protoplanetary disk and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitationally perturb each other's orbits and collide, gradually coalescing into the dominant planets.
Mesoplanets are planetary-mass objects with sizes smaller than Mercury but larger than Ceres. The term was coined by Isaac Asimov. Assuming size is defined in relation to equatorial radius, mesoplanets should be approximately 500 km to 2,500 km in radius.
A dwarf planet is a small planetary-mass object that is in direct orbit around their Star, massive enough to be gravitationally rounded, but insufficient to achieve orbital dominance like primary planets of their respective star system.
Dwarf planets are capable of being geologically active similar to primary planets and moons
A terrestrial planet, tellurian planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate, rocks or metals.
Terrestrial planets have basic structure, such as a central metallic core (mostly iron) with a surrounding silicate mantle.
Terrestrial planets can have surface structures such as canyons, craters, mountains, volcanoes, and others, depending on the presence at any time of an erosive liquid or tectonic activity or both.
Those planets are typically Size: ≲ 1 R⊕ (Earth radius)
A coreless planet is a hypothetical type of terrestrial planet that has no metallic core and is thus effectively a giant rocky mantle. It can be formed in cooler regions and far from the star.
A super-Earth is a type of planet with a mass higher than Earth, but substantially below of ice giants, (f.e Uranus and Neptune), which are 14.5 and 17.1 times Earth's, respectively. The term "super-Earth" refers only to the mass of the planet, and so does not imply anything about the surface conditions or habitability.
Some super-Earth might be limited to rocky planets without a significant atmosphere, or planets that have not just atmospheres but also solid surfaces or oceans with a sharp boundary between liquid and atmosphere
Lowesr bound for Super Earth is \ge 2.5 R⊕ with up to 10 R⊕
A mega-Earth is a massive terrestrial exoplanet that is at least ten times the mass of Earth. Mega-Earths would be substantially more massive than super-Earths
An ice planet or icy planet is a type of planet with an icy surface of volatiles such as water, ammonia, and methane. Ice planets consist of a global cryosphere.
An ice planet's surface can be composed of water, methane, ammonia, carbon dioxide (known as "dry ice"), carbon monoxide, nitrogen, and other volatiles, depending on its surface temperature. Ice planets would have surface temperatures below 260 K (−13 °C) if composed primarily of water, below 180 K (−93 °C) if primarily composed of \ce{CO^{2}} and ammonia, and below 80 K (−193 °C) if composed primarily of methane.
On the surface, ice planets are hostile to life forms like those living on Earth because they are extremely cold. Many ice worlds likely have subsurface oceans, warmed by internal heat or tidal forces from another nearby body. Liquid subsurface water would provide habitable conditions for life, including fish, plankton, and microorganisms. Subsurface plants as we know them could not exist because there is no sunlight to use for photosynthesis. Microorganisms can produce nutrients using specific chemicals (chemosynthesis) that may provide food and energy for other organisms. Some planets, if conditions are right, may have significant atmospheres and surface liquids like Saturn's moon Titan, which could be habitable for exotic forms of life.
A carbon planet is a hypothetical type of planet that contains more carbon than oxygen. Such a planet would have an \ce{^{26}Fe}-rich core like the known terrestrial planets. Surrounding that would be molten silicon carbide and titanium carbide. Above that, a layer of carbon in the form of graphite, possibly with a kilometers-thick substratum of diamond if there is sufficient pressure. During volcanic eruptions,
it is possible that diamonds from the interior could come up to the surface, resulting in mountains of diamonds and silicon carbides. The surface would contain frozen or liquid hydrocarbons (e.g., tar and methane) and carbon monoxide.
A weather cycle is hypothetically possible on carbon planets with an atmosphere, provided that the average surface temperature is below 77 °C.
However, carbon planets will probably be devoid of water, which cannot form because any oxygen delivered by comets or asteroids will react with the carbon on the surface. The atmosphere on a relatively cool carbon planet would consist primarily of carbon dioxide or carbon monoxide with a significant amount of carbon smog.
An ocean world, ocean planet or water world is a type of planet or moon that contains a substantial amount of water in the form of oceans, as part of its hydrosphere, either beneath the surface, as subsurface oceans, or on the surface, potentially submerging all dry land. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid or thalassogen, such as lava (the case of Io), ammonia (in a eutectic mixture with water, as is likely the case of Titan's inner ocean) or hydrocarbons (like on Titan's surface, which could be the most abundant kind of exosea).
A hycean planet is a hypothetical type of planet that features a \ce{H_{2}O} ocean underneath a \ce{^{1}H}-rich atmosphere. The term hycean is a portmanteau of hydrogen and ocean.
Hycean planets could be considerably larger than previous estimates for habitable planets, with radii reaching 2.6 R🜨 and masses of 10 M_E.
The planetary equilibrium temperature can reach 430 K (157 °C; 314 °F) for planets orbiting late M-dwarfs.
An eyeball planet is a type of tidally locked planet, for which tidal locking induces spatial features (for example in the geography or composition of the planet) resembling an eyeball. They are terrestrial planets where liquids may be present, in which tidal locking will induce a spatially dependent temperature gradient (the planet will be hotter on the side facing the star and colder on the other side). This temperature gradient may therefore limit the places in which liquid may exist on the surface of the planet to ring- or disk-shaped areas.
Because most planetary bodies have a natural tendency toward becoming tidally locked to their host body for a long enough timeline, eyeball planets may be common and could host life, particularly in planetary systems orbiting red and brown dwarf stars which have lifespans much longer than other main-sequence stars.
A steam world is a type of exoplanet with a steam \ce{H_{2}O} atmosphere
An iron planet is a type of planet that consists primarily of an \ce{^{26}Fe}-rich core with little or no mantle. Mercury is the largest celestial body of this type in the Solar System, but larger iron-rich exoplanets are called super-Mercuries.
\ce{^{26}Fe}-rich planets are smaller and denser than other types of planets of comparable mass. Such planets would have no plate tectonics or strong magnetic field as they cool rapidly after formation. These planets are not like Earth. Since water and iron are unstable over geological timescales, wet iron planets in the goldilocks zone may be covered by lakes of iron carbonyl and other exotic volatiles rather than water.
Chthonian planets are a class of celestial objects resulting from the stripping away of a gas giant's hydrogen and helium atmosphere and outer layers, which is called hydrodynamic escape. Such atmospheric stripping is a likely result of proximity to a star. The remaining rocky or metallic core would resemble a terrestrial planet in many respects.
A lava planet is a type of terrestrial planet, with a surface mostly or entirely covered by molten lava. Situations where such planets could exist include a young terrestrial planet just after its formation, a planet that has recently suffered a large collision event, or a planet orbiting very close to its star, causing intense irradiation and tidal forces to melt its surface.
Lava planets would orbit extremely close to their parent star. In planets with eccentric orbits, the gravity from the nearby star would distort the planet periodically, with the resulting friction producing internal heat. This tidal heating could melt rocks into magma, which would then erupt through volcanoes. This would be similar to the Solar System moon Io, orbiting close to its parent Jupiter. Io is the most geologically active world in the Solar System, with hundreds of volcanic centres and extensive lava flows. Lava worlds orbiting extremely closely to the parent star may possibly have even more volcanic activity than Io, leading some astronomers to use the term super-Io. These "super-Io" exoplanets may resemble Io with extensive sulfur concentrated on their surfaces that is associated with the continuous active volcanism.
Pulsar planets are planets that are orbiting pulsars. They are extremely rare. Only special processes can give rise to planet-sized companions around pulsars, and many are thought to be exotic bodies, such as planets made of diamond, that were formed through the partial destruction of a companion star. The intense radiation and winds consisting of electron-positron pairs would tend to strip atmospheres away from such planets.
The formation of planets requires the existence of a protoplanetary disk, most also require a "dead zone" within it where there is no turbulence. There, planetesimals can form and accumulate without falling into the star. Compared to young stars, pulsars have a much higher luminosity and thus the formation of a dead zone is hindered by the ionization of the disk by the pulsar's radiation, which allows the magnetorotational instability to trigger turbulence and thus destroy the dead zone. Thus, a disk needs to have a large mass if it is to give rise to planets.
A blanet is a member of a hypothetical class of exoplanets that directly orbit black holes.
Blanets are fundamentally similar to other planets; they have enough mass to be rounded by their own gravity, but are not massive enough to start thermonuclear fusion and become stars. In 2019, a team of astronomers and exoplanetologists showed that there is a safe zones around a supermassive black holes that could harbor thousands of blanets in orbit around it.
Blanets are suspected to form in the accretion disk that orbits a sufficiently large black hole.
Planemo si often used as term for isolated planetary-mass object (iPMO), which is an interstellar object of planetary mass which is not gravitationally bound to any star or brown dwarf.
Gas Giants
A gas giant is a giant planet composed mainly of \ce{^{1}H} (Hydrogen) and \ce{_{2}He} (Helium).
gas giants can be divided into five distinct classes according to their modeled physical atmospheric properties, and hence their appearance:
- ammonia clouds (I)
- water clouds (II)
- cloudless (III)
- alkali-metal clouds (IV)
- silicate clouds (V)
Jupiter and Saturn are both class I. Hot Jupiters are class IV or V.
A cold hydrogen-rich gas giant more massive than Jupiter but less than about 500 M_E (1.6 M_J) will only be slightly larger in volume than Jupiter. For masses above 500 M_E, gravity will cause the planet to shrink.
Kelvin–Helmholtz heating can cause a gas giant to radiate more energy than it receives from its host star.
Gas dwarfs
A gas dwarf is a planet with a rocky core that has accumulated a thick envelope of \ce{^{1}H} (Hydrogen), \ce{_{2}He} (Helium) and other volatiles, having as result a total radius between 1.7 R⊕ and 3.9 R⊕.
A super-Jupiter is a gas giant exoplanet that is more massive than the planet Jupiter. For example, companions at the planet–brown dwarf borderline have been called super-Jupiters
some are hot, some can be cold. Even though they are more massive than Jupiter, they remain about the same size as Jupiter up to 80 M_J. This means that their surface gravity and density go up proportionally to their mass. The increased mass compresses the planet due to gravity, thus keeping it from being larger.
Hot Jupiter is a class of gas giant that are inferred to be physically similar to Jupiter but that have very short orbital periods (P < 10 days)
Their mass is usually that of 0.36-11.8 M_J; orbital periods of 1.3–111 days (upper mass limit ≈13.6 M_J to avoid \ce{^{2}H} deuterium fusion).
The Orbital Shape is nearly circular, the tidal forces (and stellar perturbations) quickly damp eccentricity.
Their density is often “puffed up,” with very low densities (e.g. TrES‑4b at 0.222 \pu{g/cm^2}) due to intense stellar irradiation, high atmospheric opacity, possible internal heating, and proximity to Roche limit.
As mentioned above, due to proximity to Roche limit, they are tidally locked -> permanent day and night hemispheres.
Hot Jupiters are most commonly found around F and G type stars; much rarer around K and M dwarfs (observational biases notwithstanding).
Ultra-hot Jupiters are hot Jupiters with a dayside temperature greater than 2,200 K (1,930 °C; 3,500 °F). In such dayside atmospheres, most molecules dissociate into their constituent atoms and circulate to the nightside where they recombine into molecules again.
An ice giant is a giant planet composed mainly of elements heavier than \ce{^{1}H} (Hydrogen) and \ce{_{2}He} (Helium), such as \ce{^{8}O} (Oxygen),\ce{^{6}C} (Carbon), \ce{^{28}Ni} (Nitrogen), and \ce{^{6}S} (Sulfur).
the term "ice" refers to volatile chemical compounds with freezing points above about 100 K, such as water, ammonia, or methane, with freezing points of 273 K (0 °C), 195 K (−78 °C), and 91 K (−182 °C), respectively
Neptunian exoplanets are similar in size to the ice giants Neptune and Uranus in the Solar System. Neptunian exoplanets may have a mixture of interiors though all would be rocky with heavier metals at their cores. Neptunian planets typically have \ce{^{1}H} (Hydrogen) and \ce{_{2}He} (Helium) dominated atmospheres.
Sub-Neptune refers to a planet with a smaller radius than Neptune even though it may have a larger mass or to a planet with a smaller mass than Neptune even though it may have a larger radius like a super-puff.
Neptune-like planets are considerably rarer than sub-Neptune sized planets, despite being only slightly bigger. This "radius cliff" separates sub-Neptunes (R < 3 R⊕) from Neptunes (R > 3 R⊕).
A Mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet less massive than Neptune but resembling Neptune in that it has a thick \ce{^{1}H}-\ce{_{2}He} atmosphere, probably with deep layers of ice, rock or liquid oceans (made of water, ammonia, a mixture of both, or heavier volatiles).
Without a thick atmosphere, they would be classified as an ocean planet instead.
A super-Neptune is a planet that is more massive than the planet Neptune. These planets are generally described as being around 5–7 times as large as Earth with estimated masses of 20–80 M_E; beyond this they are generally referred to as gas giants. A planet falling within this mass range may also be referred to as a sub-Saturn.
A hot Neptune is a type of giant planet with a mass similar to that of Neptune or Uranus orbiting close to its star, normally within less than 1 AU
There also Ultra-hot neptunes. LTT 9779 b (Cuancoá) is the first ultra-hot Neptune discovered with an orbital period of 19 hours and an atmospheric temperature of over 1700 °C.
Ultra hoy neptune is a planet with dayside temperature greater than 1,700 °C
A super-puff is a type of exoplanet with a mass only a few times larger than Earth's but with a radius larger than that of Neptune, giving it a very low mean density. They are cooler and less massive than the inflated low-density hot-Jupiters.
A helium planet is a planet with a \ce{_{2}He}-dominated atmosphere. This contrasts with ordinary gas giants such as Jupiter and Saturn, whose atmospheres consist primarily of \ce{^{1}H} (Hydrogen), with \ce{_{2}He} (Helium) as a secondary component only.