The Solar System is the only system in the galacticraft modpack. Currently, the only celestial bodies in orbit around the star in the centre of the solar system are Mercury,Venus, the Overworld, the Overworld's moon, Mars, the Kepler Asteroid Belt, Jupiter, Saturn, Uranus and Neptune.
The Sun is the main source of light during the daytime.
Overworld/Earth is the planet that you first start off on.
The Moon is the first place you are able to visit. There are dungeons, a gray landscape with craters and Alien Villages.
Mars is currently available in the game. It's red and it requires a Tier 2 Rocket to get there.
Asteroids (the Kepler Asteroid Belt) is currently available in Galacticraft 3, via the use of a Tier 3 rocket, from Mars. Asteroids is not yet completed, in we have a Tier 4 rocket schematic planned, at least, which seems likely. Asteroids is made up of large "boulder" asteroids, with other "pebble" asteroids thrown carelessly around to jump across, to travel between asteroids. The 'landscape' is quite simply dappled brown rock, floating freely around with nothing else, and some asteroids hide secrets, like monsters.
According to the creator, Micdoodle8, a new, watery planet will be added eventually.