I'm not sure if this is working as intended, but this is a known...bug? Feature? Something.
I had it happen to me, as well.
No, they don't have any planets, and don't give them one. They won't develop it.
Given time you can work out an Alliance with them, too. Stranger still, when it happened in my game, the Peacekeepers also worked out an alliance with one of the other races: the Torians, who gave them ships.
It's always funny to hear them begging for aid with "the (insert race here) are too strong for us"...even though the Peacekeepers have wiped out their entire fleet.