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I see. My arrogance led me to assume human life. Yes, of course, those little pioneers can eke out a living just about anywhere.
Yeah. Considering life on Earth came into existence about as soon as it could, and unicellular organisms can live everywhere, life may be much more common than we think.
Then that should make us hopeful for Mars, Europa and Enceladus.
I don't know about Europa and Enceladus, but I'm pretty sure you're very
correct about being hopeful about Mars. Thanks to extremephiles and the fact that it's very hard to completely sterilize things, I think there might be (Earth)
life on Mars!
Well, about the original question, it might be possible for even a lone rouge planet to be habitable. I'm imagining it would have either a hot core to warm things up for liquid water, or have enough really heavy elements that this
happens often enough to keep the planet warm. Of course, the second option would be rare (and might kill the life), and the planet wouldn't stay habitable for quite as long as it would be if it was orbiting a star, but it still would count.
Of course, it might be hard to find
such a planet.