General Astronomy/The Planets

What qualifies as a planet has a somewhat variable definition, but in general, it should have the following properties:

1. It does not generate energy through fusion
2. It orbits around the Sun (or another star)
3. It is large enough that its self-gravity makes it spherical

To distinguish planets from the masses known as brown dwarfs, one might add a fourth condition: it formed by accretion rather than direct collapse from a nebula.

Planets have been classified in many ways. Perhaps the best physical classification actually uses four categories, each named after a 'typical' planet of the type: Terrestrial, Jovian, Neptunian and Plutonian.

Terrestrial planets have solid (rocky) surfaces, atmospheres that are thin in comparison to their diameters, and rocky interiors. They tend to have comparatively small masses. These include Mercury, Venus, Earth and Mars; Earth's Moon can be seen as a similar type of body.

Gas giants have thick atmospheres of gaseous hydrogen, metallic hydrogen mantles, and rocky cores. They have large masses. These include Jupiter and Saturn. They should, however be distinguished from the ice giants.

Ice giants planets have moderate hydrogen atmospheres, liquid (mostly water) mantles, and rocky cores. Their masses are intermediate. These include Uranus and Neptune. The Neptunian and Jovian planets are sometimes referred to as the "gas giant" planets, but the difference in interiors merits a separate classification.

Plutonian planets have icy surfaces, very thin atmospheres, and rocky interiors. Technically Pluto is the only such planet in our solar system, but similar bodies include most of the moons of the outer planets, notably Ganymede, Callisto, Europa, Titan, and Triton, as well as Pluto's own moon Charon. Another, more recent addition, is Eris (formerly known as 2003 UB313).