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Tuesday, March 22, 2011

A, B, or both A and B

Our next candidate in the Alpha Centauri system is Alpha Centauri A, a G2V star like our own sun. It is about 10% more massive than our sun, but is otherwise the most "sunlike" of any star on our list. It is locked into a binary orbit with the slightly smaller and dimmer Alpha Centauri B, which will be the topic of tomorrow's post.

Alpha Centauri A and B orbit each other in a period of about 80 earth years, during which time they come as close to each other as the sun and Saturn, to as far away from each other as the sun and Pluto. Each has a habitable zone far enough away from the other that it is not seriously affected by the other star. So, theoretically, there may be planets within the habitable zones of both A and B.

Our searches so far have not detected planets around either A or B, which eliminates the possibility of planets any larger than five times as massive as the earth around either star. There is, however, some reason to believe that earthlike planets may have formed (or may have been gravitationally captured) into the habitable zones of these stars. Planets have already been detected orbiting similar binary star systems, such as Gamma Cephei.

For a planet to be a good candidate for human habitability around Alpha Centauri A it would need to be about 1.25 times the earth's distance from the sun. It is possible, based on current modeling, that an earth-sized rocky planet may have formed there. However, it is also possible that no planets have formed around Alpha Centauri A at all. There is also some concern that the absence of a gas giant planet such as Jupiter or Saturn may indicate a lack of cometary activity in the inner star system, which would mean that the planets of both Alphas Centauri could be bone dry. However, the very fact of the binary nature of the system, plus distant Proxima, means that it is likely that either Kuiper Belt comets or Oort Cloud comets are diverted into the inner star system.

As we will see tomorrow, Alpha Centauri B is a much better candidate for a human habitable planet. But the combination of both stars, plus Proxima, makes Alpha Centauri singularly intriguing, even if it didn't happen to be our closest neighbor.


  1. Are you going to talk about SIM?

    Have you considered compiling this series into a book? You've clearly done your research here, and your perspective on this as an actual ship captain is very interesting and unique.

  2. Yes, I'm definitely going to talk about SIM and the consequences of losing it. Had planned to wait for a soon upcoming post on the terrestrial search for exoplanets, but maybe this discussion of Alpha Centauri is as good a time as any.

    No, I don't have any plans to turn this into a book. None of the information here is my own, just lots of different people's research crammed into one place. Often without citation, although I do try to provide links whenever possible.

    But, once I'm done with this series, I might try to compile it into an ebook and have it available here for anyone who wants it, just to have all of the material consolidated into one place.

    Oh, for the record, I'm an "actual boat captain". I don't currently work on anything larger than 500 tons, and I really have no ambitions to. Been there, done that.