What decides a good planet to colonize from a bad planet?
This entirely depends on what you are looking for the astro to do; production of fleet (production), boosting your economy, research, etc. For economy, you should be aiming for either an Asteroid or a Crystalline, though the latter is generally rare and hard to find. If it is a production base you want, Rockies are the best, though Metallics and Craters fill this role well too. Usually you will want to use a Gaia, Earthly or Arid for research bases, due to their large area and high fertility, though they suit most tasks pretty well.
