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.

posted almost 10 years ago

celtschk‭

341 reputation 24 102 42 10

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.

posted almost 10 years ago

HDE 226868‭

601 reputation 61 463 80 49