# What would a binary black hole look like from the surface of a planet orbiting that system

If it is even possible, what would the Goldilocks zone be like - in fact is such a system even possible of holding life - around a binary black hole? Could it have planets orbiting it?

EDIT 1: Oh well, no star no life. So now, how would it look like if there is a star that orbits the binary black hole system? Would there be a Goldilocks zone? Would it be stable? Taking note that the planet will still be orbiting the binary black hole and not orbiting the sun.

• Two questions: one, are the black hole(s) natural or artificial? Although a black hole equal in mass to the Sun could exist, all observed stellar-mass black holes are at least 5 or 10 solar masses, since they tend to form from the collapse of massive stars. Two, what is the geometry of the system like? A main sequence star in binary orbit with a black hole, and the planet in circumbinary orbit around both? Or is the planet in orbit around the star, and that system is in orbit around the black hole? – 2012rcampion Feb 8 '15 at 19:34
• @2012rcampion I guess they would have to be natural, as I would not quite see any point to artificially create a binary black hole system. As for the orbits, it is 2 black holes orbiting each other like 2 stars, then 1 star would be orbiting the 2 black holes, and then the planet orbiting the 2 black holes as well. Is it possible for the planet to be stably orbiting near enough the star? – grimmsdottir Feb 9 '15 at 4:03

Well I'm pretty sure there is no 'Goldilocks' zone around a black hole. That zone is where there is enough solar radiation to warm the surface to a life support temperature (but not too hot either).

A black hole doesn't emit solar radiation. as a matter of fact it sucks in light (hence the 'black hole' nomenclature. What it does release is (theoretically) Hawking radiation. Now I suppose it is always possible that somehow something has evolved to live off this radiation but it seems unlikely.

How would it look? likely two largish black holes in the sky (always night) possibly each with a corona that out lines the event horizon. A negative sun. However, the black holes are significantly smaller than a star and might not be much more than a couple small light rings. with enough atmosphere you might not even be able to see them at all.

EDT (adding my comment from below) still to be in the Goldilocks zone the planet would have to orbit the sun, the sun could be in some kind of dance with the two black holes, but the view would still be about the same when they are on the night side of the planet. With the sun nearby they might show up a little better with a brighter corona

Also this question might help with some idea of the issues of orbiting multiple stars Can a planet have a figure-8 type of orbit around two separate stars?

• Alright then, maybe I should change my question a little so that there can be a Goldilocks zone? Suppose now that within that binary black-hole system, there is a star there as well, orbiting the 2 black holes like a planet, how would it then lock like, from another planet, orbiting the 2 black holes stably, not the sun – grimmsdottir Feb 3 '15 at 3:38
• @grimmsdottir still to be in the goldilocks zone the planet would have to orbit the sun, the sun could be in some kind of dance with the two black holes, but the view would still be about the same when they are on the night side of the planet. With the sun nearby they might show up a little better with a brighter corona – bowlturner Feb 3 '15 at 3:41
• Black hole does not "suck in light". Its gravity suck in matter from around it, and it is called black because escape velocity is faster than speed of light, so no light from its surface can escape its gravity - but as matter is crushed into it, black hole does emit light from the outer area (well above from surface, where escape velocity is just speed of light). – Peter M. - stands for Monica Feb 3 '15 at 12:27
• The accretion disks surrounding black holes actually throw off quite a bit of radiation, more than enough to warm a nearby planet. Of course, their radiation peaks in the x-ray band, which would probably kill everything on said nearby planet. – ckersch Feb 3 '15 at 22:40
• More x-ray cooking... – bowlturner Feb 4 '15 at 0:01

These black holes are going to emit Hawking radiation. Not a lot, but a bit. The power emitted by a black hole with mass $$M$$ is $$P=\frac{\hbar c^6}{15360 \pi G^2M^2}$$ Each black hole will emit that amount of power, so - not accounting for the fact that they may eclipse each other (from the view of the planet) from time to time, the luminosity of the system should be twice that value. Let's say that each black hole has the mass of one Sun. We then find that the total power emitted is $$P=1.8\times10^{-28}\text{ W}\approx4.7\times10^{-55}L_{\odot}$$. Without another source of light, there's simply not going to be a habitable zone.

If you do have a star in the system, then you may be able to have a habitable zone, but it would be solely due to the star. In this case, there are several problems we have to consider:

• We now have three massive bodies in the system (four if you count the planet). This is likely to be unstable, unless the two black holes are in a tight orbit and are effectively one object.
• If the star is near the black holes, gas may be accreted by the compact objects, resulting in an accretion disk. This disk will be a source of high-energy radiation - a problem for life.
• If the planet orbits the black holes and not the star, it will almost certainly be outside the star's habitable zone.
• Note that the black hole is actually absorbing energy from the CMB it is so cold: about 330 W! – 2012rcampion Feb 5 '15 at 2:24
• @2012rcampion Damn, you're the guy who always gets the details right! :-) Yes, I had considered that. It shouldn't affect the zone - or lack thereof - though, right? – HDE 226868 Feb 5 '15 at 2:25
• Correct, the 'Goldilocks zone' is still nonexistent: anywhere outside the black hole (r=2e-8 AU) the planet receives more energy from space than it does from the black hole, not enough to support life (as we know it). – 2012rcampion Feb 5 '15 at 2:29

Would there be a goldilocks zone?

Probably around the star, but not in the way of the black holes.

Would it be stable?

No, I don't think it would.

Black holes are massive(read:heavy), as are stars. All massive objects produce gravity and are affected by gravity. All three would interact in a rather chaotic mess, as would every planet in orbit around them.

What you're describing will fall under into the category of n-body problems. The maths is a little heavy (pun intended), but the crux of it is that stable solutions are few and far-between. Your star may currently be orbiting the black hole, it won't be for long!

• One possibility for a stable orbit would be to have the star orbit the black hole binary and the planet orbit the star. There's quite a few stable systems of binaries orbiting each other that work in this way, like this one: en.wikipedia.org/wiki/Nu_Scorpii – ckersch Feb 4 '15 at 2:25

# How could it be possible?

Obviously the black holes cannot be the source of light for the planet, so we need four bodies: The two black holes, a star and a planet. Moreover, the star should be in (at least approximately) constant distance to the star if it is supposed to support life.

Now, how could that happen? Well, the system of two black holes orbiting each other will have five Lagrange points, of which two are stable, L4 and L5. Those both sit at equal distance from both black holes, at opposite sides. If the black holes are both sufficiently more massive than the star (and of course also the planet), the configuration where the star sits in L4 and the planet sits in L5 is stable. While technically, the planet would not orbit both black holes (the points are only stable if one black hole is at least about 25 times as massive as the other, thus effectively everything orbits that black hole).

As seen from the planet, the black holes would be 60 degrees from each other, and the sun would be in the middle between the black holes, at about 1.7 times the distance (more exactly, $\sqrt{3}$ times the distance).

Let's assume a sun-like star and a distance planet‒star of 1 AU (which is per definition the mean distance between earth and sun). Then any other distance between those bodies is 0.58 AU.

# What would this look like from the planet?

Assuming a 24h day like the earth, the black holes would raise/set two hours before/after sunset, so I think that's more than enough time to see them without being completely hidden by the light of the star. Assuming the black holes are not active (that is, there's nothing falling into them), the only effect should be gravitational lensing. You can get an idea what gravitational lensing of a black hole looks like here.

If the black holes are large enough, the lensing could probably also generate secondary images of the sun, close to the black hole's position in the sky.

• Note that the black holes need to have a mass ratio greater than about 25 for L4 and L5 to be stable. – 2012rcampion Feb 14 '15 at 19:49
• @2012rcampion: Thanks, I was not aware of that. I'll update the answer accordingly. – celtschk Feb 14 '15 at 19:52

A black hole would act gravitationally towards an orbiting mass in the same way as a regular star would. Depending on the binary formation, quasi-stable orbits are possible around a binary system, even if one of the main centers of gravitational influence in the system is a black hole. The planet would have to be a survivor of the original super-large star (and the subsequent hyper-violent explosion) or a captured stray world. All heat would have to come from the live stellar companion, so Goldilocks calculations would have to be done in reference to that. Your current specification is too vague for me to be of more help.

This is blatantly stolen from Larry Niven, I take no credit.

The book is about a fictional "world" that is mostly just an atmospheric torus in a ring around a neutron star (think a donut). So there's no gravity - life has evolved in free-fall.

The reason I offer this is you could have this "world" a lot closer to the black hole than a traditional planet and have it be stable - there's no earthquakes in the sky, and tidal forces won't rip it apart. That might be close enough that Hawking Radiation (filtered by a super-powered ozone layer analog?) would support life. You might not get a lot of light, but you'd get radiation/heat, so there should be some way to make it work.

I'm not sure how the black holes would appear, but if I had to guess I'd go with some sort of distortion in the sky with two pinpricks of bright light where the radiation is the strongest.

• Hawking radiation from a decent-size black hole is actually colder than the cosmic microwave background, so you probably won't need to worry about it. – 2012rcampion Feb 3 '15 at 3:58
• Interesting. In that case to support life you would need an external source of material funneled in, that would form a light-giving accretion disk? – Dan Smolinske Feb 3 '15 at 4:00
• Yes, an accretion disk will generate a significant amount of radiation as the matter is compressed falling into the black hole. Maybe you have one BH, and one companion star feeding the accretion disk? – 2012rcampion Feb 3 '15 at 4:05
• The thing is if you have a star do you really need the accretion disk for light? Unless it's a brown dwarf or something like that that would normally be too dim. – Dan Smolinske Feb 3 '15 at 5:22
• The Smoke Ring is lit by a separate star and has a neutron star in the center. No black holes involved. – Tim B Feb 3 '15 at 14:50

While stability would be a BIG problem you could in theory have a habitable zone around a black hole or a binary black hole.

As HDE 226868 shows, the black hole won't provide meaningful energy. You could have energy from an accretion disk, though. IF the flow of matter into the disk is stable enough you could have a habitable zone (although it would be far closer in than we are.) A stable source of inflow is quite problematic, though.