Haven't yet reached R20, trying to use the 2 week protection window as a buffer to get a bunch of research done. I got saddled with a few garbage planets due to not understanding the no-takebacks policy inherent in colonization, and a question occurred to me:

If I'm full on planets for my current rank, what happens if I invade an enemy planet? Can I take more planets than I can colonize via brute force, or am I stuck with X planets per rank regardless of how I got them? Also, are there any benefits to attacking an enemy planet that you don't have room to colonize (mineral or artifact theft for example)?

Mostly I'm curious not just because I found a planet I might try to invade at R20, but also because I was thinking of dropping some $ on a batch of GP to improve my one good planet and I need to know how great the incentive to steal that planet might be...

Invasion/colonizing new planets limit counts the same way. If you can't colonize due to limit, you can't invade. Also take note invading takes more energy (invading + taking down planet population) + double the amount of credits compared to colonize
Not only that, invasion can fail, (24hr timer)
And, owner of the planet may retaliate or lay traps on the planet, creating complications.
By all means, avoid invasions until you are ready for battles.

You get to keep the structures on the planet. If you do not have the required research, the structures will do you no good (no production/stats)
Note that invasion is an EXTREMELY big offense for most players, so expect full force retaliation.

Ok, thanks for the help.

on a side note, If you've scanned a hostile planet, and then the owner gains a level, is that level gain reflected in your star chart? I figure I'd probably be safe taking a planet from an inactive player, but if the lack of level gain over the last week is simply because the game didn't show it, that would stop me.

I think the level gain is reflected instantaneously, as long as reopen the target planet. Not sure though, but it works that way for my own planet

Yes, the player's current level IS updated as they increase in level.