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stability requires one BH to be much larger than the other

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edited Feb 14, 2015 at 19:58

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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 (ifthe points are only stable if one black hole is much moreat least about 25 times as massive thanas the other, thus effectively everything orbits that black hole; if both black holes are of the same mass, everything orbits the center of that systemhole).

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.

How could it be possible?

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 (if one black hole is much more massive than the other, everything orbits that black hole; if both black holes are of the same mass, everything orbits the center of that system).

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?

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.

How could it be possible?

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).

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?

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.

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answered Feb 14, 2015 at 19:17

celtschk

31.8k
13
95
155

How could it be possible?

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 (if one black hole is much more massive than the other, everything orbits that black hole; if both black holes are of the same mass, everything orbits the center of that system).

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?

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.