Basically, the question is in the title. I know there are some real monsters in the universe, black holes that are billions of solar masses in size. I recall hearing that in some of the larger black holes, you could cross the event horizon of a black hole and still be fine, just can’t ever leave. I even recall speculation that the entire universe could be inside of a gigantic black hole, though I know this isn’t widely accepted.
So…could a planet orbit a black hole inside of its event horizon but remain intact? The question crossed my mind when I was watching a science show that was discussing planets orbiting a black hole and how it might be possible to get a very large number of these types of planets depending on how the black hole feeds, coupled with the old physics thought experiment of what happens if you replace the sun with a black hole of the same mass (answer…nothing except the whole freezing part…and, I know that the current planets wouldn’t be inside of the event horizon of such a black hole). So, could you theoretically have planets that are orbiting black holes inside of their event horizons?
Bonus question if it’s possible…what could conditions be on those planets? Could they have atmospheres? Life? Or would they be radiation blasted empty balls of rock? What about if it was a gas giant? Could one of those survive?
With a Schwarzschild type black hole, even before you get to the event horizon, there are no stable orbits. And once you cross the event horizon, you will be sucked into the black hole and doomed; even light cannot escape.
Maybe you could try to concoct something involving a rotating and/or charged black hole, but the burden of proof would begin with showing there could be any kind of stable orbit.
A planet could cross the event horizon of a supermassive black hole intact, but it wouldn’t be pleasant for very long.
The innermost stable orbit around a black hole (non-rotating) is 3 times the schwarzschild limit. So, something within the event horizon would not have a stable orbit, and would inevitably plunge into the singularity (or whatever lives at the center of a black hole.)
A rotating black hole may have some wiggle room, but once you’ve crossed the inner event horizon, there is no longer going to be any way to hold a stable orbit.
Wouldn’t the event horizon be the orbit for light traveling at light speed? Lower orbits would have to be faster than light, so matter wouldn’t be able to go fast enough to orbit.
The orbit at light speed is the photon sphere, which is actually outside the event horizon, 1.5 times the schwarzschild limit for a non-rotating black hole.
So, yeah, even photons can’t orbit closer to the black hole than that.
Or to put it another way, you can orbit closer than that, but you can’t have a closed orbit closer than that. Even falling straight down is, technically, a kind of orbit.
If a giga-hole swallowed a planet, the planet could stay intact briefly after crossing the horizon. But not for long.
That doesn’t go for the supermassive black holes though. IIRC, you can be 10’s of thousands of kilometers away and still fine yet have crossed the event horizon.
Doesn’t seem like you could orbit inside of the event horizon though. Sad really…I had this thought of planets still intact circling inside an event horizon but completely opaque to the outside world, and also with serious time dilation going on. Maybe a civilization hiding out until the end of time or something. Reality is so cruel. Oh well, I appreciate the answers all!
I mean, we really don’t know what happens within the event horizon. There is actually a fairly reasonable idea that it’s actually a solid wall of unimaginably hot fire. I’m not sure that I subscribe to that one, but it’s out there, and I don’t think we can prove it to not be the case.
At the same time, it’s possible that once across the horizon, you are now in a whole new universe, not falling towards the singularity, but instead popping into existence in a whole new place. Of course, if that place has different laws of physics, that could be bad for your personal structural integrity.
If you want to send your planet into the future, you might be able to find an orbit around a rotating black hole that is stable but highly time dilated, but make sure you double check your math!
Well, just to show how my mind works, I was watching a video on escaping the galaxy which was about an entire civilization trying to flee another civilization that is hunting them mercilessly. On a different screen, I was watching a video that was discussing the theory that there could be planets formed around a black hole. I was then musing on the question of this OP…what if you could have a planet orbiting within a supermassive black hole such that the planet was intact? Then the leap…what if this civilization who was fleeing for its existence decided, why not go there (or perhaps move a planet they have prepared into such an orbit)? Then essentially you will have escaped the threat, especially if you od it such that no one (especially the enemy) knows that’s where you went.
Just a thought experiment to amuse me in these waning hours of 2021. I do appreciate the answers, even if it burst my bubble.
Here’s another thing to consider. The Schwarzchild solution to the Einstein field equations describing the spacetime geometry of a non-rotating black hole (and the equivalent Kerr solution for a rotating one) has a very interesting attribute. When the radial coordinate r is less than the radius of the event horizon (i.e.- inside the black hole) what appears to happen is that the radial coordinate r and the time coordinate t change places. This suggests that once you cross the event horizon, the direction to the singularity is described by t, and r is now the timelike coordinate, not t.
It’s another way of saying that not only can you never leave (there is no spatial direction that points to “out”) but you can never escape the singularity, either by orbiting inside the black hole or by any other means. The universe outside the black hole is now your past, and the singularity is your inevitable future.
This sounds like a description of the Phantom Zone.
I’ve seen articles suggesting that if some of the most fundamental facts of the universe were just a tad different (as they well might be in other universes), then it would be impossible for quarks to glom together to form protons or anything else. The whole universe would consist of an expanse of vibrating complex probability fields and nothing more.
What a waste of a universe!
An implication seems to be that there could be any number of wasted universes like that. The one we happen to inhabit may be the extremely rare oddball universe where protons and more can exist. It’s just lucky for us that this is the universe we live in, because we couldn’t exist anywhere else. (Isn’t that essentially what the anthropic principle says?)
I think Prof. Strassler has a page, or several, about that.
XT I feel I should point out that something a lot like this is a pivotal point in Gene Roddenberry’s Andromeda. Minor Spoiler (it’s in the pilot) The battleship Andromeda is in a big fight. It ends up in orbit around a black hole- frozen in time. The orbit is not regular. Eventually, the Andromeda get’s close enough to normal space that it can be grappled and pulled out. For the ship and the people on it, no time has passed. For the rest of the universe, several centuries have passed and things have really gone to wrack and ruin.
I’ll have to check that out, though I can’t see how time would have completely stopped if they never went past the event horizon. Slowed, sure, but not stopped. Still, it sounds interesting.
My own fantasy thought for this thread was that a civilization on the run would colonize a planet inside the event horizon of a monster black hole and essentially be able to escape the universe (or some foe) forever…or until the black hole evaporated enough that the planet(s) in orbit crossed the event horizon back into the universe (if that’s even a thing). Sadly, reality once again destroys my fantasy musings. C’est la vie.
Honestly, I wouldn’t bother. It’s a cheesy and dated sci-fi, with Kevin Sorbo as the lead. His cringeworthy catch phrase of “Let’s bring it!” alone should have been enough to stop watching.
But, it was decent CGI for the day, and some of the support characters were fun. A Rev and Tyr spin-off would have been watchable.
Anyway, I did watch it back in the day, and it was due to “mumble mumble interaction with the artificial gravity mutter mutter”. Nothing to do with real science or physics.
