A fun paper I came across a while ago - The Steppenwolf: A Proposal for a Habitable Planet in Interstellar Space. The question at hand is, what are the optimal conditions for a planet ejected from its star system to retain liquid water?
Unfortunately it has been a few months since I read through the paper itself, so I've forgotten a lot, and I'm not willing to do it again just now. However, the results are surprisingly optimistic. A terrestrial planet with an Earthlike composition would be able to retain liquid water under a layer of insulating ice for significant periods if its mass were approximately 3.5 times that of Earth. And since more massive planets could potentially retain a higher mass of volatiles I wouldn't be surprised if isolated planets below that threshold retained liquid water too.
Meanwhile, this sentence - "If a rogue planet had about ten times higher water mass fraction or a thick cryo-atmospheric layer, it would need to be only ~0.3 times the mass of Earth to maintain a liquid ocean." - seems a bit more theoretical, since I am not sure are likely to encounter planets of such low mass with such large amounts of the appropriate materials. Unless I un-forget about the possibility of predominantly icy planets.
The whole paper is fascinating and worth a read. It's just a shame it is so difficult to detect these planets if they are out there. I am not sure we ever will without some form of interstellar travel (if nothing else, even slow automated exploration would give our descendants more vantage points from which to potentially observe one). Or we could get lucky and find one in the neighbourhood. Not betting on that.