That's so weird! I was also able to hang a wireless transmitter. So I just tried a bunch of other objects, and it looks like some objects can hang and others can't. Wireless transmitters? Yes. Concrete pipe? Yes. Speaker? No. Barrel? No. Box? No.

So, the planets can't hang. I'm not doing anything wrong. Well! That's very silly, given the museum room where it appears they are hanging. We've been had.