Can black holes have planets?

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Can there be planets revolving around black holes? I'm not asking about black holes at the center of galaxies, but other black holes.
 

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  • #2
Gravitionally yes - but it's hard to see how they would get there!
 
  • #3
bcrowell
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I would have said no, since you'd think that the formation process, which is a supernova, would destroy any planets. But the same would seem to apply to a neutron star, and isn't there at least one neutron star known to have a planet?
 
  • #4
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I would have said no, since you'd think that the formation process, which is a supernova, would destroy any planets. But the same would seem to apply to a neutron star, and isn't there at least one neutron star known to have a planet?
Yeah there's a planet orbiting the pulsar PSR1829-10.. Apparently the supernova event left behind gas and dust which amazingly coalesced into the planet.
 
  • #5
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Yes they can, but said planets would also have to be quite far away from the black hole, otherwise they would have to have enormous rotation speeds in order to stay in a stable orbit. That and it becomes easier and easier to "knock" something into a black hole, the closer it gets.
 
  • #6
Vanadium 50
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A black hole's gravity is no stronger than a star's of the same mass. So you don't need planets any farther out than you do for regular stars.
 
  • #7
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Yes they can, but said planets would also have to be quite far away from the black hole, otherwise they would have to have enormous rotation speeds in order to stay in a stable orbit. That and it becomes easier and easier to "knock" something into a black hole, the closer it gets.
In other words :These objects stay within the orbit as long as they are out of the event horizon ?
 
  • #8
yes - outside the event horizon there is nothing special about a black hole.

Although in practice a planet would be lucky to survive either the red giant stage of the massive star and the Supernova explosion that created the black hole
 
  • #9
Borek
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Technically it can be possible to become a black hole first, catch a planet later.
 
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  • #10
Nabeshin
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yes - outside the event horizon there is nothing special about a black hole.

Although in practice a planet would be lucky to survive either the red giant stage of the massive star and the Supernova explosion that created the black hole
Well... not exactly. To err on the safe side one should say that at distances from the BH much larger than the EH radius, there is nothing special about the spacetime compared to that of a star. There are, however, differences when compared to the Newtonian picture of gravity.
 
  • #11
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You don't need a supernova explosion to create a black hole. Simple collapse will do. Whether or not a supernova explosion occurs depends on the mass. Furthermore it's not really an explosion of the whole star. It's merely material falling in due to gravity winning the tug of war over radiative pressures, rebounding against the compressed core and taking the outer stellar shells or outer layers with it.
 
  • #12
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The innermost stable circular orbit has a radius three times that of the event horizon radius. I would think that for a planet the Roche limit would also come into play. In addition to this, over long periods of time, orbits decay due to gravitational radiation, and this would happen faster if the planet was close to the black hole.
 
  • #13
bcrowell
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You don't need a supernova explosion to create a black hole. Simple collapse will do.
Hmm...I would be interested to learn more about this. The WP article on stellar evolution says: "Since the core-collapse supernova mechanism itself is imperfectly understood, it is still not known whether it is possible for a star to collapse directly to a black hole without producing a visible supernova, or whether some supernovae initially form unstable neutron stars which then collapse into black holes..." This makes it sound like the possibility of black hole formation be simple collapse is fairly speculative at this point. Anyone have any pointers to published papers?
 
  • #14
jimgraber
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Hmm...I would be interested to learn more about this. The WP article on stellar evolution says: "Since the core-collapse supernova mechanism itself is imperfectly understood, it is still not known whether it is possible for a star to collapse directly to a black hole without producing a visible supernova, or whether some supernovae initially form unstable neutron stars which then collapse into black holes..." This makes it sound like the possibility of black hole formation be simple collapse is fairly speculative at this point. Anyone have any pointers to published papers?
Search arXiv for "accretion induced collapse" if you want the serious stuff. You'll get over 100 hits.
Best,
Jim Graber
 
  • #15
bcrowell
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Search arXiv for "accretion induced collapse" if you want the serious stuff. You'll get over 100 hits.
Thanks for the pointer, Jim -- wow, the information is indeed a little overwhelming. I'm going to start a separate thread on this.

-Ben
 
  • #16
One poster confused rotation speed with orbital velocity, and another talked about gravitational radiation perturbing an orbit. The gist of orbital velocity is that orbiting objects are always 'falling' with a certain Vf=Vo(sin alpha) where alpha is dependent on the radial distance from the gravitational center and the Maxwell's gravitation equation..that component of the orbital velocity normal to the surface colinear with the center of gravity of the orbited object and induced by the gravitational force exerted on the object at every instant at the instantaneous altitude at that instant...been awhile since I had my orbital mechanics class. In a nutshell, the orbital velocity is always greater than the falling velocity due to the action of the aformentioned gravitational force at the orbital distance. Now at the event horizon, the escape velocity, or point of orbital equilibrium, Vf= (-3(10^6))=c, so any orbital speed or Vo>>c which Einstein would not like, nor his latter day lackeys. So another limit, will be Vo<c at its corresponding altitude will be the lower limit. However, again to keep Einstein's lackeys happy, this is not reachable either due to the expansion of mass of the orbital object as it gets closer to the black hole, so this will have to be calculated as well, yielding a very large orbital radius before any true equilibrium is reached. Then and only then can Roche be applied to clear tidal forces from planetary disintegration, however, this disintegration will be localized to the planet's surface, constraining it to a liquid state with concomitant global heating. You do not live there, you would not care. Who'd want to live in such a Hadean place anyway but we are talking 'suppose' here.
Now on the other hand, if we have gravitational radiation as an accomplished face and accept that as true, then the 'black hole' is not truly black, like Steven Hawking says but in a different way. The place is leaking this radiation, so therefore gravitational radiation must have a velocity of >c or it would not escape the black hole to 'radiate'. Radiation of whatever form is energy, and because of this, if we accept that the black hole is an adiabatic system, then the place will eventually dissipate if it is not fed.
However if the black hole is large enough to exceed some horrible limit in size and tears a hole in the universe, then a collapse of space >>>>c can occur in that area with unknown results.
This leaves out the question of breakdown of physics underneath the event horizon, where even Einstein has no supporters and orbital Vo>>c be even hold sway?
 
  • #17
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The gist of orbital velocity is....Vf=Vo(sin alpha)....Maxwell's gravitation equation...been awhile since I had my orbital mechanics class. In a nutshell, the orbital velocity is always greater than the falling velocity...point of orbital equilibrium, Vf= (-3(10^6))=c, so any orbital speed or Vo>>c which Einstein would not like, nor his latter day lackeys.... expansion of mass .... Roche be applied.....constraining it to a liquid state with concomitant global heating....Hadean place....therefore gravitational radiation must have a velocity of >c.... if the black hole is large enough to exceed some horrible limit in size and tears a hole in the universe, then a collapse of space >>>>c can occur in that area with unknown results.
Wow.
 
  • #18
Wouldn't the planets get sucked in? Assuming that they are within the event horizon. If they were outside of it, not too sure...
 
  • #19
russ_watters
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If a planet is inside the event horizon, yes, it would be destroyed. But the event horizon is much smaller than a star of the same mass.
 
  • #20
As a skater drawing her legs in while doing a spin and then accelerates her spin in a demonstration of conservation of angular momentum, so too would planets gradually losing orbital distance if they are too close to the star remnant. However, this would be the case with normal solar systems as well if a planet was too close and got 'sucked in'. I wonder if the jets from the black holes would have sufficient light to illuminate their planets at all...just a thought.
 
  • #21
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"Since the core-collapse supernova mechanism itself is imperfectly understood, it is still not known whether it is possible for a star to collapse directly to a black hole without producing a visible supernova, or whether some supernovae initially form unstable neutron stars which then collapse into black holes..."
See references from

http://arxiv.org/PS_cache/arxiv/pdf/1010/1010.5550v1.pdf [Broken]

Note also that the answer probably varies varies wildly with the composition of the star.

This makes it sound like the possibility of black hole formation be simple collapse is fairly speculative at this point. Anyone have any pointers to published papers?
If supernova explode with anything like the neutrino reheating model, then it's pretty easy to get the supernova to to straight through to a black hole in simulations. You just pile on enough material so the shock dies.
 
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  • #22
russ_watters
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As a skater drawing her legs in while doing a spin and then accelerates her spin in a demonstration of conservation of angular momentum, so too would planets gradually losing orbital distance if they are too close to the star remnant.
I don't see how those two scenarios are related.
 
  • #23
I don't see how those two scenarios are related.
Take a look at his other post and you may understand his angle better.
 
  • #24
FtlIsAwesome
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Yes, if they were the remaining cores of jovian planets or they were captured, assuming the star goes supernova.

Could it be possible, for a black hole to have a companion main sequence star, and have a habitable planet (habitable by aliens, not neccesarily humans)? The scenario I'm thinking of is a planet orbiting a black hole, which is in the habitable zone of a K class star. There is also a neutron star orbiting nearby.
This scenario might result from the three stars capturing each other, and the planet is pulled away from the K star to orbit the black hole, but it might not be feasible.

If this is off-topic, please tell me.
 
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  • #25
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a planet to orbit a black hole has to be formed with the black hole lucky enough to be outside the sucking radius of the black hole ! !! as if it come out from space towards the black hole it has a tremendous probability to get sucked in before it orbits or until we discover a method to spot it ! so i guess if planets do orbit a black hole , it might be a temporary one , as they would end up crashing the black hole after a while
 

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