Can a planet orbit a black hole?

04 October 2016

Question

Just found your pod cast the other week. awesome podcast!

I was wondering if anyone has found a black hole that has captured another interstellar object. I am assuming that the creation of a black hole is so violent that anything that was originally in that solar system was blown away or consumed by the star prior to the formation of the black hole.

Answer

We put this question to Judith Croston... Judith - Yes. There are two bits to the question and so, if we think about how black holes can actually form, that sort of answers the second question about whether it would destroy the solar system.

So the best way we know about making a black hole is in what's known as a supernova explosion, which is when a really big star basically collapses and then has a really big explosion. For two reasons, if you had a solar system at that point, so if there were planets around that star, they would get blown away, they would get destroyed and that's partly because a lot of matter. there's actually less gravity and that means the planets can just go wandering off into space. But there are also shockwaves and all sorts of horrible things happening. So it's very unlikely that there would be a way for planets to survive when a black hole forms.

So if we found a planet, for example around a black hole, that would probably mean it had somehow had to get there afterwards. Either by forming around the black hole or by being captured, which is what the questioner is asking. So we haven't actually found any black holes that have planets orbiting around them, sadly.

Interestingly, there are some neutron stars that have planets around them which is kind of cool. But there are black holes that we think have captured other stars. So there are places where there are lot of stars very close together in the galaxy and we think that the chances of them banging into each other are such that it's quite likely, maybe, there's a star orbiting around a black hole that's actually been captured because they're really close together. And, actually those are some of the best ways we have of detecting the black holes in the first place is when the black holes pull stuff off the star and it falls in onto the black hole.

Caroline - So how could a star be near a black hole and not get sucked in by its immense gravity? How can a star orbit a black hole? It must surely arrive at huge speed or what happens?

Judith - Yes. So whether or not a star will get captured or fall into the black hole, as you say, it will depend on where it's coming from and how fast it's going. Now to actually get sucked into the black hole, it has to get incredibly close because, I think as I said a bit earlier, if you're far away from a black hole it just behaves like a star - the gravity just obeys the normal laws. If you want to actually get sucked into the black hole, you have to get pretty close and black holes are quite smalls, so the chances of that happening are, actually, pretty rare. It's much more likely that it will get captured in an orbit and go around it. And, in fact, we can see some stars orbiting the black hole in the centre of our galaxy and we can just watch them travelling around in these quite fast orbits and see that they are actually orbiting around the black hole, which is kind of cool.

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