View Full Version : Binary stars and orbiting planets

04-08-2009, 11:07 PM
Maybe someone can help explain this to me.

Given that two stars in a binary pair orbit around a common center of mass... (Let's ignore trinary and more stars.)

I have read that binary stars are unlikely to have orbiting planets. Let's ignore that for a moment.

When I think about it, there are a few possible situations.

1. The stars are very close. They might even exchange material, and (if you had the right situation to make a planet) from a planet's point of view, it's like having one star--it orbits around the center of mass, which is well inside the planet's orbit. I don't know what the limit for distance is--the phrase "Hill radius" occurs to me, but it might not apply. (I assume there's a different limit than the Roche limit.) These are probably unstable and short-lived, which is why they probably don't have (habitable) planets. I can imagine a pair of dwarfs that do this, though habitable planets would be tough.

2. The stars are close but not that close. I don't actually see habitable planets developing here, because the center of mass is so close to where an orbit is that the original dust cloud gets swept clean by one of the stars. Stars are pretty massive, so really, this is up to, oh, maybe Jupiter's orbit (if we were talking about G type stars). (And I am just guessing on that distance; real math is always preferable.)

3. The stars are actually pretty far apart; one star might actually have planets and the other star just looks like a really bright star. Gravitational effects don't affect (much) the inner planets of the dust cloud.

(I'm assuming the planets are from the original dust cloud; you can have lots of other situations if you have a captured rogue planet.)

Have I summarized the situation reasonably well, or are there other situations or things I have overlooked?

04-08-2009, 11:59 PM
Have I summarized the situation reasonably well, or are there other situations or things I have overlooked?Difference in mass of the stars is also an important factor.

04-09-2009, 01:02 AM
I was going to say the same thing. If you've got a white dwarf orbiting a larger star, essentially it wouldn't really change anything from that dwarf being a large planet.