I’m thinking specifically of the Alpha Centauri system and its planet.
For a stable orbit, the key is that the orbits can’t be too close to each other. So the planet could orbit close to one of the stars, while two stars orbit each other at a much greater distance. Or, the two stars could orbit close to each other, while the planet orbits much further away. Otherwise the orbits will be unstable and the smaller body (the planet) would probably get ejected from the system.
I’m not sure what the particular case is with Alpha Centauri. Perhaps someone else can answer that.
February 18th, 2010 at 5:43 pm
Very tricky.
There are very few families of stable orbits in a multiple star system (and the details probably change from one system to the next).
1. The far away planet:
If the stars are relatively close together and the planet is very far from the stars, then it’s orbit will behave (more or less) as if the stars were a unique mass. However, this usually means that the planet is way too far to get any decent heat from the stars.
2. The planet orbit the more massive star.
If the stars have good separation and if the planet is close enough to the more massive of the two stars, it could enjoy a relatively stable orbit. You could imagine, for example, our solar system with a small red dwarf star where Neptune is. Such an arrangement would still allow a planet like Earth to exist at a decent distance from our Sun.
3. The very unstable equilibrium.
It is possible to imagine orbits where a planet swings by one star, then the other (the figure 8 orbit was popular in some science-fiction stories). In theory it is even possible to have a planet "orbit" a point exactly halfway between the stars or even remain at the exact centre of mass of the two stars. However, such orbits would be very unstable. The least disruption (a tidal effect from a third star passing by) would be enough to disrupt the orbit and eventually cause the planet to crash on one of the two stars.
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February 18th, 2010 at 5:53 pm
For a stable orbit, the key is that the orbits can’t be too close to each other. So the planet could orbit close to one of the stars, while two stars orbit each other at a much greater distance. Or, the two stars could orbit close to each other, while the planet orbits much further away. Otherwise the orbits will be unstable and the smaller body (the planet) would probably get ejected from the system.
I’m not sure what the particular case is with Alpha Centauri. Perhaps someone else can answer that.
References :
February 18th, 2010 at 6:23 pm
It would be very tricky.
For the orbit to be stable the planet has to be very far from the 2 binary stars, far enough so that the gravity of the 2 stars act as one for the planet. But this is bad too, because if the planet is very far away, it wont receive much sunlight and heat, and the gravitational pull will be weaker.
Another way for an orbit around a binary pair to be stable is for the 2 stars to be very far from each other. And the planet is orbiting one of the stars, and the two stars are far enough away for the planet orbiting one of the stars to be unaffected by the other stars gravity. The planet will probably orbit the more massive star because it will have a stronger gravitational pull.
Sometimes the orbit is very weird, the planet orbits both stars in an 8 formation, kind of like an infinity sign, it orbits one stars and swings to the next and orbits that star and swings back to the first one. This is not a good orbit, its very unstable and can cause the planet to be ejected from the binary pair by the gravitational effects of both stars on it. This was seen happening once before.
As for the Alpha Centauri System, that’s a triple star system, its even harder to get a stable orbit on a triple star system but it is possible. Unfortunately the Alpha Centauri system does not have any planets orbiting it, but there have been predictions and theories saying that there might be some terrestrial planets orbiting the system.
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