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Re: Speculation Re Planet Orbiting Alpha Centauri 1

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Posted by Adrian James/">Adrian James on February 15, 1999 04:51:06 UTC

: : : : : : I'm a complete novice to astronomy but am interested in some "expert" opinions for some fiction I am considering writing. What I'd like to know is whether its possible that an inhabitable (by humans) planet might orbit Alpha Centauri 1 in such a way that it actually passes between Alpha Cent. 1 & Alpha Cent. 2, during its year. Is that possible and if it is, what might conditions be like on such a planet? I do not know how close together the stars of the Alpha Centauri system are to one another but I assume there is plenty of room for a planet to orbit one of them? I'm looking for feasible stuff for the imagination here. : : : : Thanks.

: : : No way can a planet survive in the Centauri system, : : : there are three stars about couple hundred AU's from : : : each other, their gravitational pulls would neither : : : make the planet collide with a star or gain speed : : : and escape into space. If there is a planet, it could : : : be a gas giant like Jupiter. I suggest you use stars : : : like 51 Pegasi or 47 Ursae Majoris, they are stars that : : : have been proven to have planets.

: : Thanks. I'll do some more research. : ************************************* : ************************************* : Although there has been no discovery of a planet, : recent research projects have shown that planets could : indeed be orbiting either Alpha Centauri A or B. A standard rule of thumb is that stable planetary orbits can be found in multiple systems if those orbits are no more than 1/5 the distance to the nearest neighbor. In the case of Alpha Centauri, this works out to about 5 AU for both stars. There are other matters to take into account, but in general, this works quite well for this system. Proxima Centauri, liekly to not even be physically associated with this system, is so far away from the primary members that it could have a large system of planets indeed. Of course, it is too dim to allow for life, as far as we know....

The Gravitaional pull of each star should not cause too much of a problem as long as the planets are considerably closer to one star than the other. However, you will have some interference and the formation of Kirkwood Gaps. The planet can not orbit at a distance that is a tidy fraction of the distance between the stars, ie one third or three sevenths. In this case it would gain a gravitational boost at the same points in it's orbit and that orbit would rapidly become non circular, resulting in the planet's hasty departure from the system. The effect can be seen in our asteroid belt with Jupiter's influence. In a non fractional orbit such problems would probably not occur. The star's luminiosity declines with the inverse square of the distance, ie Starlight=1/distance^2. You can calculate what effect the secondary star will have on the world usin this calculation, however if the secondary is considerably further away than the primary, as it must be, the light received rapidly diminishes to almost nothing. As an afterthought, the gravity of two main sequence stars in close (?) proximity will produce quite marked Lagrange points. The stars will orbit each other with L1 and L2 points at 60 degrees ahead and behind in thier orbits. You also get the L1 point at the mid point between the two. These points are areas of equalised gravity and will collect a lot of debris such as asteroids. In Alpha Centauri, I would be possible for planets thrown out of the Kirkwood Gaps to end up in one or more of these points, following ahead of or behind the stars in their orbits. The L1 planet would be especially strange, permanently having two suns in the sky exactly opposite one another. No total darkness ever, but at that distance not a lot of heat or light either.

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