Category:Planet

A planet is an astronomical object that meets the following three criteria: Planets form in the early stages of their parent star’s evolution. Not all of the giant molecular cloud which birthed the star is used up in its creation: some angular momentum is conserved and thus a protoplanetary disk forms around the protostar. This disk, composed of small rocky particles, slowly clumps into larger pieces via a process known as accretion. Eventually, through runaway accretion, a planetary system is born.
 * 1) Is massive enough to be forced into a spherical shape by its own gravity
 * 2) Is not massive enough to have ongoing hydrogen/deuterium fusion at its core
 * 3) Has cleared its orbit of any other astronomical objects (other than satellites)

Types of Planet
There are many types of planets, although they can be generalized into two groups: terrestrial planets and giant planets.

Terrestrial Planets
A terrestrial planet is a planet composed mostly of silicates and/or metals, such as Mercury, Venus, Mars and Earth. They have solid surfaces and may have molten cores and silicate mantles, along with possible geographical features such as mountains and volcanoes (if tectonically active) and hydrospheres.

They range in mass from 0.1 M⊕ to 10 M⊕ and are typically less than 2 R⊕. On the mass-radius graph (see below), this corresponds to between around the Fe band and around the H2O band, depending on the planet's type.

Giant Planets
Giant planets are massive planets composed largely of low-boiling point material (hydrogen, helium, water, methane, etc.). They are primarily divided into two main groups: Gas Giants (also known as Jovian planets for their similarity to Jupiter) and Ice Giants (e.g. Neptune and Uranus).

Most of these planets share a few common features.

Giant planets often have bands of counter-circulating streams of material known as zones (light-coloured, rising pressure systems) and belts (dark-coloured, sinking pressure systems). They move in opposite directions and create jets of very high speed winds (from Jupiter’s 360 km/h winds to Neptune’s mind-blowing 2000 km/h winds). These chaotic atmospheric conditions create violent storms (most famously Jupiter’s Great Red Spot).

Giant planets are also commonly surrounded by ring systems, and also tend to amass a large number of moons.

Giant planets can only form past the frost line of a system, where ices such as water, ammonia, methane, carbon dioxide, etc. can exist in solid form. Giant planets will therefore be found at least about 1 AU beyond the frost line, unless they have migrated inwards (see Hot Giants). Giant planets (with the notable exception of Eccentric Jupiters) will usually have very low eccentricities.

The mass range of giant planets is from about 10 M⊕ to 13 MJ and, due to electron degeneracy, giant planets of masses larger than 2 MJ will always have a radius of about 1 RJ. Radii of greater than 1 RJ can only be found in planets smaller than 1 MJ. On the Mass-Radius graph, this corresponds to the H/He and H bands.

Useful Equations
Regardless of the planet's classification, it is bound by the following equation relating mass (M, measured in MJ or M⊕), radius (R, measured in RJ or R⊕), density (⍴, measured in ⍴J or ⍴⊕) and surface gravity (g, measured in gJ or g⊕):



From the above, circumference (C), surface area (A), volume (V) and escape velocity (ve) can be derived using values relative to the Earth:









Equivalently, using values relative to Jupiter:









Finally, the mass-radius relationship of planets is not nearly as clear-cut as it is for stars. However, the relationship does exist, and a plausible planet can be created simply by reading a value from this graph. Each band on the graph represents a certain type of planet composition, which shall be discussed in greater detail on the pages of each planet type.