Jool is the largest planet in-game. It is also the real life analogue of Jupiter.
JoolEditUnlike Earth's solar system, Jool is the only gas giant planet in its system. It has five moons, named Laythe, Tylo, Vall, Bop, and Pol. Tylo is the largest moon.
Jool can also be said to resemble Neptune in the fact that Eeloo (Pluto) crosses its orbit. It possibly takes its greenish hue from that of Uranus.
As expected, it is impossible for anything to actually 'land' on the surface, as the whole planet is a gas giant. However, before 0.18, it was possible to plant a flag on the 'surface' of Jool. This has since been removed. Some players have constructed types of floating bases in Jool's atmosphere, and these are commonly held up by RCS thrusters or small amounts of thrust by engines.