While yes as far as I know currently all claimable planets and asteroids are claimed and you cannot take them from another player. However planets and asteroids DO open up sometimes. Most of the time that is due to the ineptitude of someone building and the planet goes uncontrolled, allowing anyone to claim some territory. I managed to grab myself an asteroid when slab decay first started.
If you watch the GNS newsflashes, you might see something like this
"Year 12 Day 94:: It appears today as though brute force is not quite enough to keep the citizens of Budpock down, as they have stepped up and taken power from the old leader, long hailed as a puppet of Mecrosa, and forced all Mecrosa-aligned politicians to resign their positions, ceding power to local control. "
Which is a long way of saying Mecrosa lost control of the planet and it could be fair game to claim. (However in this instance not only has every city been claimed on that planet, later that day the planet was back under control)
It will take lots of patience, a bit of research, a fast ship, and a deal of luck to be in the right place at the right time in case a planet opens up like that. Not impossible by any means though.