[Barnacle Bill]

In Stellar Conquest there was a simple rule that if you eliminated a population above a certain size via bombardment, the planet became uninhabitable.

You could keep it just that simple, or flesh it out with details like:

1) Every time the planet is bombed and population dies, it inflicts a set damage to planetary conditions. If this goes negative, the planet is uninhabitable and anybody left dies.

2) In conjuction with #1, a ship system which can slowly improve planetary conditions from orbit. Perhaps this would be called a "decontamination module" and only improve planets with negative conditions.

3) Radioactivity could be tracked separately as a planet charactoristic. "Conditions" would refer to climate. The "habitability" or some such would be equal to conditions minus radioactivity, and would be the actual number used to determine population growth (I still think that domed populations should act as if conditions are 0, the actual conditions being applicable only to those who can breath the atmosphere, and atmosphere converters reducing the conditions to 0 as they approach the actual change in atmosphere). If radioactivity is >100 the planet is still uninhabitable until deconned per #2. There would be a decon facility which could reduce radioactivity on a planet which is still habitable. I recommend against creating any racial trait that lets you live on radiated planets, as people with that trait would just nuke ever planet until it glows.