by contiguity
There are a few cards that let you steal planets in the expansion. However, I like the idea of "surveying" other player's planets before you can attack them. Here's the variant I'd suggest:Instead of surveying from deck, you can survey one of your neighbor's planets by using survey symbols equal to the military cost (or one less than that if leader). It gets placed closer to you to show you have surveyed it. (Only one person can do this per planet.)
On a future turn, you are allowed to attack it as if it were one of your own planets and add it to your planets (though its owner can still attack it as well). In any case, colonies are returned to their owner if applicable.