This is our second installment in our series of student questions shows and these questions come to us from Curtis High School.

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Just heard the show, and I'm really surprised about the kinds of questions asked.
Are these part of an astronomy class at that school?

I'm asking because most of the people I know have never heard of white holes, dark energy, etc.
so its really cool to see these high school students so interested in this stuff!!

What most thrilled me is the question about planets around black holes.
(this possibility had never even occurred to me)

Pamela said any life on a hypothetical planetary system around a black hole
would either freeze or get fried by radiation.

But what about a Europa-like world:
lots of shielding from a miles-thick ice sheet on the surface, and a comfortable ocean beneath,
heated above freezing point by tidal forces from a gas giant

Is a constellation like this possible/ viable/ likely?

What is the usual size of the accretion disks that are visible around black holes?

__________________
Keeper of the Jabberwock

It doesn't seem to be too likely clint, no. But I'd certainly imagine it's possible. There are really two cases to consider, the two you just mentioned and that Pamela described.

The easy one first: if the black hole is being nice and quiet, then assuming the planet is at a reasonable distance, this is no different to the free floating planets discussion that was had in the Astronomy folder. A moon, with a decent outer shell, supported by tidal and radiogenic heating should be alright.

If it's host to an accretion disc and is spewing out gamma and X-rays, that's a bit trickier. But as Pamela said, the disc will try to take the planet with it, so the first condition is we be far enough away that that's not an issue. That solved, we have the bonus that we're partially shadowed by the disc - it will be kind of (circular) wedge shaped, still reasonably flat and with most of the radiation coming from the large, flat surface area, not towards the planet hiding 'behind' the wedge. There may also be strong jets of material and radiation coming from the BH, but we could expect these to be more or less perpendicular to the plane of the planet's orbit.

So I think with a good, thick shell of ice, on a fairly wide orbit, you'd be alright. It's not a place you'd want to be coming and going from, if a disc was there, but it should be fairly safe on the inside.

Lord Jubjub: that depends on how big the black hole is, and what it's eating. I'm assuming above the BH is nothing more exotic than a few solar masses, for the sake of an easy life (and also explaining how the planet got there).

When you say it's not too likely,
you are probably referring to the issue of how the heck the planet got there in the first place, right?

I suppose it would have to have originated somewhere else,
and then somehow got near the BH and forced into an orbit.

Or is there any chance new planets could form around a BH?
Or old planets left over after surviving a supernova-turned-into-blackhole scenario?

Thanks for the link, by the way, very interesting thread

You're welcome! I enjoyed it too.

Partially, yes. But I mean that and everything else. We need a black hole of a reasonable size, that's not too active, that has a planet (from somewhere), that has a surviving satellite system, that has a moon in the right orbit, that is made of the right stuff, that is differentiated correctly, etc. etc. And that's before you even start on the thorny issue of obtaining life.

I'm being partially facetious, mind, because some of those things really aren't that unusal, or unlikely; but others of them may be extremely hard. And I've no objection to the thought that there must be other potentially habitable planets out there. But the more conditions you impose, the less likelier the odds for that particular situation get. So no, I don't think this is a good bet. But hey - the universe has managed to surprise us with just about everything else we care to look at, so who knows?

As to one way it might get there, you're right that starting somewhere else and getting caught there is a good idea. What's best known of the oldest known planet is a good model for this.


The very first confirmed extrasolar planets were found around a pulsar (PSR B1257+12) and appear to have formed after the supernova. How they got there isn't 100% clear, but it seems possible (e.g. Marzari et al, 2001) they were created by material sucked off a binary companion. The formation would happen very close to the pulsar, where the material was densest.

You could, in theory, do a similar thing around a black hole, but the need to stay beyond the last stable orbit (lest you get eaten!) gives you a lot less wiggle room. But never say never; it's probably possible to make something. However, I'm not convinced how massive you can get - the pulsar planets found so far this way are all somewhere around an Earth mass, give or take a zero. Making a gas giant with its own satellite system this way is, I would imagine, out.

With your other suggestion, the problem with old planets surviving the blast is that most of the mass of the star is lost during a supernova. It seems more likely any surviving planets would go flying off into space, the black hole no longer having the gravity to hold them. But without doing the numbers, I can't promise it's impossible to be left with something on a very large orbit. And since the absence of a star isn't a hindrance to life in this case, neither is being on a huge, eccentric orbit. So that'd probably work, if it could be managed.

I agree, this is definitely not one of first places to look for alien life forms.

But still, a decade (or so) ago we didn't even know for sure if there were other planets beyond our solar system.
Now we find them around pulsars, brown dwarfs, in binary systems...

And life might be quite possible (even if it's a remote possibility) even in such 'exotic' places as interstellar space and around black holes...

Sometimes it almost seems as if the universe is keen on outdoing our wildest imagination.

I thought the million miles Fraser suggested as a safe distance from a black hole was doubtful. From a three solar mass black hole, the gravity would be several g, making moving away from the black hole, require more energy than lifting from Earth's surface. A supermassive black hole might be 4 g at one billion miles and one g at 2 billion miles making it a difficult gravity well to climb out of. My guess is the accreation disk could still be distructive 2 billion miles from a super massive black hole. Please correct my guesimates if they are off by a lot. Neil

At 2 billion miles, we are looking at a minimum safe orbit between Saturn and Uranus.

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Keeper of the Jabberwock

I haven't checked those numbers, but supermassive black holes aren't what I was thinking of. That's a very hostile environment for keeping a planet in, for lots of other reasons beyond sustaining life.

For something more reasonably sized for this kind of problem, a black hole of around 11 solar masses could sustain an accretion disc with an inner edge somewhere in the region of 10^5 metres. That's perfectly reasonable for scrabbling a few rocks together a bit further out, or holding a giant planet on a stable orbit a bit further still.