At the moment we have two (ok three if we count the Advent Loyal Titan) ways to Destroy planets.
1) Bomb them
2) Culture
3) Convert them with Advent Titan
my suggestion is this:
You can trade planets. Make offers to other players, and if they agree, you are the new owner of the planet.
Maybe it could be possiblie to combine this with Diplomacy, a peaceful way to get enemy planets.
As TEC it could be possible to force other players to trade them if your Culture is high enough.
