One of the things I did in FOTS was index a species environmental requirements. Since planets are also indexed based on solar position and solar type this tends to mean there are quite a lot of gas giants and frozen rocks. Depending on the technology available any world can be useful, but those frozen rocks can be very expensive to colonize. Since it's a game this makes inhabitable planets cheaper, but not inexpensive to colonize depending onnyour environmental requirements. To a race that likes frozen rocks half the worlds in a typical system might look like home though only a few are going to be valuable enough to really make an all out effort to colonize to their fullest potential.