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What a crazy idea. Some pitfalls:
- The condition near black hole are very unstable. If it enters some gas cloud, it will form an accretion disk, and planet will be fried. The black hole must be somewhere in intergalactic space.
- Time dilation. Does the Universe have enough age for life on such planet even emerge, not saying about evolution to something significant? Especially taking into account the time needed to form such massive black hole and throw it away from the galaxy.
As for your first notion, I am not saying it would be especially stable within galaxies, but that it is a thing that could be taken into account. I personally believe an accretion disk would more likely cause the planet to stall and fall into the black hole, so I do agree that said black hole would be somewhere in intergalactic space.
The time necessary to form supermassive black holes isn't that long if you go with the notion that all supermassive black holes were created along with the first stars from massive hydrogen clouds shortly after the Big Bang, as mentioned here, but in other sources as well. (http://www.astro.ufl.edu/~jt/teaching/he/talks/Lasso_Nestor.pdf)
The time necessary for life to emerge is unknown, though I doubt that it would go past simple single celled lifeforms due to time dilation, as you've already mentioned. The time necessary to eject a supermassive black hole from a galaxy isn't that long, especially since galactic collisions are especially violent events in which black holes sometimes get ejected into intergalactic space.
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Weird! That's some neat and crazy astrophysics there.
I think the paper is less about the plausibility of such a planet existing, and more a "for the fun of it" take on the usual idea of the entropy requirements of life -- that life requires a flow of energy from low entropy to high entropy; a hot sun and a cold sky -- and turning it upside down. Here we imagine a hot sky and a cold "sun". The black hole's gravitational field converts the CMB into the high temperature, low entropy source, and the black hole shadow acts as the low temperature, high entropy sink.
The idea works, and is fascinating, but it's among the least likely things to actually see in nature. To add to what SpaceEngineer said, it's rare for a black hole massive enough to have low tidal forces near its horizon get kicked out of its galaxy. Massive objects are harder to move. Rather than being kicked out of the galaxy or cluster, they tend to migrate to the center and sit there, king of the hill style. There is a reason why we find supermassive black holes at the centers of galaxies, and the biggest usually at the centers of large elliptical galaxies, at the centers of large clusters of galaxies. To throw one out of a galaxy would take the simultaneous close interaction of at least 3 supermassive black holes, and this is pretty unlikely.
So yeah, if these kinds of planets can exist at all, then it takes a long time for the universe to create them, and they are unfathomably rare.
True. I'm just putting the idea up here since no one mentioned it and I found it a particularly fun thing to think about. I am in no way certain that objects like these would be common, or even plausible for that matter, since I doubt that the exact conditions for the formation of such a system could be easily met. Rogue planets or planets from other systems would more likely fall into the black hole than fall at exactly the right angle to be caught at exactly the right distance to host life, and right at the moment the black hole was ejected from the galaxy.
It's just fun to think about, as you've already said.
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