In Kerbal Space Program, there are six planets, and these planets are orbited by moons or other objects. Each planet has it's own atmospheric differences, surface differences, gravity, and physics. These planets, in alphabetical order, are:
Moho is the closest planet to the sun. It is a analogue of Mercury, and has no moons. It has an orbital inclination of 7.0 degrees, and 1 mohoian year is 25 kerbin days, and 15.5 hours. It is impossible to achieve a geosynchronous orbit around Moho. Its equatorial radius is 250 Kilometers.
Eve can be considered a analogue of Venus, but it varies in some aspects. It has a strange purple color, but its landscape has plateaus, and what appear to be Maria in the dark purple spots. It has one natural satellite, named Gilly. Gilly has a highly elliptical orbit, and high orbital inclination, in a way that in resembles the Dwarf Planet Pluto. Gilly seems to be a captured asteroid, or may have been created when a large object struck Eve at an angle. You can achieve synchronous orbit around Eve. It is slightly larger than Kerbin, with an diameter of 700 Kilometers. A Year on Eve is 164 days.
Kerbin is the home planet of the Kerbals, and the home of the Kerbal Space Center. Kerbin has two moons; a real moon analogue named (the) Mun, and a smaller moon named Minmus, that has a slight orbital inclination. Kerbin's diameter is 600 kilometers. A year on Kerbin is 426 days (kerbin rotations).
Duna is a planet similar to Mars, but some noticeable features are the unusually large polar ice caps, and it has a relatively small diameter of 320 km.
Jool is the fifth planet from Kerbol and has the most moons, totalling with 5. It's a green gas giant, which, because of this, can destroy and kill Kerbals/ships/spaceplanes.
Laythe is the first and closest moon of Jool. Laythe is a blue, watery planet with sandy islands on it. It also has a breathable atmosphere and the Kerbals can remove their helmets on the moon.
There is an easter egg on Laythe; The Sagan Sea, referring to Carl Sagan.
Vall is the second and second-closest moon of Jool. It is an ice planet that is an analogue of Europa, an ice planet.
There is an easter egg on Vall; The Vallhenge, a Stonehenge-like set of blocks. Some have wedges on them!
Tylo is a rocky moon that is supposed to be simular to the Mun. It is the third largest moon and is literally the largest "planet" [freaking LARGER than Kerbin.] It is an analog of Ganymede.
In real life, Ganymede is only larger than Mercury.
PS: There's an easter egg on Tylo. A cave-like formation that between 0.2.1 and 1.0.5 [supposedly] that was hidden beneath the surface. I think it's still up there to see for your own eyes!
Bop is a captured asteroid that is the fourth and second-smallest moon of Jool. It is an asteroid that's supposedly an analog of any of the 59 minor moons of Jupiter [not one of them is similar, supposedly]. This asteroid moon is home to the Deep Space Kraken.
Pol, literally named after it's simularity to a grain of Pollen, is the fifth and smallest moon of the Jool system.
Dres and Eeloo Edit
Dres and Eeloo are dwarf planets. They are supposedly an analog of;
Dres: Eris, Sedna [confirmed both]
Eeloo: Pluto [supposedly, not final]
Kerbol is the star and NOT a planet/moon. Simular to Jool's gravity, Kerbol can pull things that go near it's surface.
- Laythe is nearly the size of Kerbin.