Cyborg IM1 wrote:
I really liked the Space book as the Infinite Worlds books. The world building stuff in Space 4e is some of the best I have seen.
The units are all English, rather than metric, but that is an easy conversion if needed.
The star system and planet generation sequences in GURPS Space
4th edition are very impressive, especially in the feature that allows them to work either backwards (start by choosing the air, water, and gravity you want on a planet then calculate/generate its bulk characteristics and a star system around it) or forwards (start with a star and generate a system around it including planets down to their surface characteristics. On the other hand the wealth of detail comes at the cost of lots of die rolls, table look-ups, and calculations.
I devised an Excel workbook that does it for you, pretty well (I had to make some small changes to avoid having to use macros). I think I've shared it before, but just in case: https://www.dropbox.com/s/6e1x1x28mh4vfv8/handbook_of_the_planets-beta23.xls?dl=0
I included an option to convert the output into metric rather than US units (from the peculiar units that the system uses internally, such as AU, gees, Earth-diameters…).