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Sunday, March 27, 2011


So far we've looked at all three of the Alpha Centauri stars individually for potential habitable planets. There are, however, other possible planetary orbits in a binary or trinary star system.

I remember the first time I read the opening words of the original Star Wars:

It was a vast, shining globe and it cast a light of lambent topaz into space -- but it was not a sun. Thus, the planet had fooled men for a long time. Not until entering close orbit around it did its discoverers realize that this was a world in a binary system and not a third sun itself.

Immediately I thought of Alpha Centauri, and the possibility that Proxima was really a planet like Tatooine (yes, I was that 12-year-old). Proxima isn't a planet. But it is possible for a planet to orbit both stars of a binary system.

Here is what we see most typically:

If the two stars in a binary system are less than three astronomical units apart, the planetary disk tends to form around both of the stars, like Tatooine. If the stars are separated by greater than fifty astronomical units then the planetary disks form around the individual stars. Between three and fifty AU a planetary disk is less likely to form.

The distance between Alphas Centauri A and B is between 11 and 36 AU, averaging 24 AU. This would seem to indicate that a planetary disk around either star is unlikely, however we believe there happen to be planetary disks around both A and B.

A "Tatooine-like" orbit around both stars would take the planet far outside of the habitable zone; it would be rather more like Hoth than Tatooine, except that humans truly could not live there.

All that said, a world around either of the two major stars of Alpha Centauri would, occasionally, be afforded really spectacular twin sunsets.

Stupid Star Wars trivia: The planet Tatooine is never actually named that at any point in the first movie (what eventually became Episode IV of the series). During the filming it was called Utapau, which name was ultimately given to one of the worlds in Episode III.

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