Imagine we have a black hole and a star.

There are very close to each other so black hole slowly consumes the star.

It is possible to have planets in this system, and for them not to be consumed by this black hole but just follow their unique orbits. Could there even have life there?


It's possible to have planets orbiting a binary pair of stars, your scenario of a close orbit, sometimes called "short orbit binaries". See here, also posted above in comments. In such a binary-system, nothing can orbit an individual star, but at some distance, plants can orbit and some systems like this have even been observed, listed in the link. The orbital dynamics is the same for a star-black hole short-orbit binary.

Now, there are problems. Black holes form out of very large stars and the formation is one of the biggest explosions in the universe, a Type II supernovas, and that's not very friendly to any planets in orbit. A star might survive it, planets would be harder, though it might be possible for new planets to form from nebula material remaining after the nova (I'm just guessing there).

A black hole could also form from a Neutron star accreting matter, but you still have the problem that the formation of a Neutron Star also only happens out of a Type-II supernova, so such a system has a difficult beginning.

Edit: While some planets have been observed around Neutron stars, these appear to be quite rare. 2 Neutron Stars have been observed with planets, out of over 1,600 Neutron stars observed. A type II nova is very planet unfriendly.

Theoretically a close gravitational capture is possible, but those are very rare, as stars rarely get that close. There's many stars that are known to orbit the super-massive black holes at the center of our galaxy (Andromeda galaxy too), but stars and stellar mass black holes are much more rare. A few have been observed, but they don't appear to be common. Such a system would be easy to observe, so the fact that there are only a few that have been noticed is evidence to them being rare. Here's a few mentions of them. One, Two, Three, Four.

From the 4th article, which is from 2011, so more may be known now, but it says:

Only about 20 binary stellar systems are known to contain a black hole, out of an estimated population of around 5,000 in the Milky Way Galaxy.

A 2nd problem is that a star feeding a black hole would create an accretion disk which would be very radioactive and not ideal for life on an orbiting planet. Maybe the planet could have a very thick atmosphere that might protect it, but that would also likely reduce sunlight reaching the surface. The star feeding the black hole would also be losing mass, and over time, grow smaller and provide less light and heat to the planet. Ideally, you'd want it to be a very slow feed. It's pretty far from an optimal life on planet situation.

So it is possible to have planets in this system not to be consumed by this black hole but just follow their unique orbits

This part is certainly possible. Things can orbit a black hole at a safe distance without any problem. As for life, we don't know how common life is in other solar-systems so nobody can say how likely it might be, but it's theoretically possible, but, in my opinion, pretty far from ideal.

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    $\begingroup$ The first exoplanets were found around pulsars... $\endgroup$ – ProfRob Nov 27 '15 at 19:37
  • $\begingroup$ @RobJeffries I wasn't aware of that, but reading up on it, while they were the first exoplanets discovered, there are, as far as we can tell, quite rare. I think it's largely true that type II supernovas gets rid of almost all the planets either by ejecting them out of orbit or pretty much vaporizing them, though it's conceivable some planets could re-form out of expelled matter - not 100% sure on that though. $\endgroup$ – userLTK Nov 28 '15 at 11:09

Planet's that are not part of a solar system are called rogue planets. These planets are planets which are starless, and support themselves with gravity in space. It would most likely be very hard for life to form on these types of planets because there is no star to provide heat for the life. I'm not an expert on astrobiology though, so I can't answer much about life on these planets.

When a black hole consumes a star an accretion disk is formed around it. An accretion disk is a large disk of the stellar material that forms around the black hole as it draws mass from the star. If the black hole is big enough, it could swallow the star whole. If a black hole were to come into a range where it was drawing mass from the star, then all of the planets would most likely be destroyed due to the immense gravitational pull which they possess. If a planet would be orbiting the star, it would be extremely unlikely that it would not be destroyed.

It is almost certain that tidal forces would of destroyed the planet if it was orbiting the star, and unless the planet is large enough to support itself without a star it cannot become a rogue planet. I have no idea if a rogue planet can support life.

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    $\begingroup$ Except circumbinary orbits are certainly possible. $\endgroup$ – ProfRob Nov 27 '15 at 19:38

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