Apr 13 2009

Alpha Centauri

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Most people — at least, science fiction fans — know that Alpha Centauri is our nearest stellar neighbor beyond the Sun. It’s actually a triple star system, comprising a sun-like G star, Alpha Centauri A, a slightly cooler K star, Alpha Centauri B, and orbiting both at a much greater distance, a red dwarf, Alpha Centauri C. Because the latter is currently at a place in its orbit that is much closer to Earth than the others, it is also known as Proxima Centauri. Another classical name for the system as a whole is Rigil Kentaurus.

Multiple star systems are problematic for the formation of planets stable enough to evolve life, because the complex gravitational interactions could tear a planet loose from one star, and to orbit both (unless the stars are very close together) the planet would be too far out to be warm enough. The scene in the first Star Wars movie, of Tatooine’s twin suns setting, shows the suns only a few diameters apart; this is very close as binary stars go, they’d be orbiting each other at perhaps a tenth of Mercury’s distance from the Sun. It turns out, though, that Alpha Centauri isn’t that bad.

Alpha Centauri B orbits A (actually they both orbit a common center of gravity, but that is much closer to A than to B) in an elliptical path whose distance varies from about 11 AU (astronomical units, the average distance between the Earth and the Sun) to about 36 AU, or roughly from Saturn’s orbit to Pluto’s orbit. It takes about 79.91 Earth years for a complete orbit. It turns out that orbits within the habitable zone of both A and B are far enough from the other star (B and A, respectively) to be stable even at closest approach. This means we can have habitable planets around both stars, with travel between them roughly equivalent to a voyage to Saturn, Pluto or points between, depending on where the stars are in their orbit. In my T-space stories, Sawyer’s World orbits Alpha Centauri A, and Kakaloa orbits B.

Proxima, at 12,000 to 13,000 AU (about 0.2 light years), is too small and far away to affect the planetary systems of A or B. Indeed, it’s possible that it isn’t properly part of the multiple system, and is just passing by.

It turns out that there’s something unusual about the Alpha Centauri system. Unlike many of our nearby stars, which orbit the galaxy more-or-less in concert with us, Alpha Centauri’s galactic orbit is highly inclined and it will not be in our neighborhood for more than a few hundred thousand years. This raises a problem in T-space: planets around the nearer stars have been terraformed, and initial estimates, based mainly on genetic analysis of their Earth-originated lifeforms, put that at 60 to 70 million years ago. Alpha Centauri was in a completely different part of the galaxy then. (Well, so were we — about 1/4 of a rotation around the galactic core — but Alpha Centauri would have been well out of the plane of the galaxy.) So, if Alpha Centauri has terraformed planets — with Earth-derived lifeforms (and it appears to), does that mean that the region of terraformed planets occupies much more of the galaxy than we currently (in T-space history) know, or is something else going on? There are profound implications either way.

I know, but as of the current crop of T-space stories, I’m not telling.

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One Response to “Alpha Centauri”

  1. […] promised earlier, I’ve added a page on Alpha Centauri to the “T-Space […]

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