Each planet has a procedurally generated terrain using a noise/height map.
The 3rd planet from the star will always have Earth type colours, the others are random colours.
Sizes, orbit radiuses, number of planets, number of moons etc. are all randomly generated using a seeded random number, so you can get the exact solar system back by using the seed.
The solar system data is based on real-life values and you can see more info about each planet by clicking on the names in the gui panel.
The orbits are compressed, and the planets enlarged, in order to have a nice view of the solar system, but if you enable "True Scale" you will see the real scales and distances just to see how big space is :-)