Exoplanet hunters look at Alpha Centauri

Exoplanet hunters have found over 280 star systems in our neighborhood with at least one planet. On that list of star systems, with all their worlds – hot Jupiters, super-Earths, ice giants and at least one giant ball of metal – one will not find the closest star system to Sol…. the Alpha Centauri system. Only 4.4 light years away, Alpha Centauri is tantalizingly close on a galactic scale. It’s also fairly weird as star systems go. No one has ever seriously examined the AC system for exoplanets, despite its proximity, but now a newly funded project will do just that. Javiera Guedes, a graduate student at the University of California, Santa Cruz, and her advisor Greg Laughlin are beginning a NASA-funded project to examine the AC system and look for worlds.

Alpha Centauri is an apparent trinary. Two suns – Alpha Centauri A and B – form the core of the star system, with AC-A the larger of the two and slightly hotter and more massive than our Sun. The primary AC-A is orbited by AC-B (slightly cooler and oranger than Sol) once every 79.91 Solar years. Every 80 Solar years, AC-B swings close (11.2 AU) to AC-A, but spends most of its time coming and going from its furthest point of 35.6 AU. Put in terms of our own Solar system with AC-A as the Sun, AC-B would approach Earth not quite as close as Saturn is from us, and would swing out beyond the orbit of Neptune at each aphelion.

The third member of Alpha Centuri’s trinary is a tiny red dwarf, Proxima Centauri, which may not actually be a permanent member of the group. Proxima is a bit over 12,000 AU from the AC-A/AC-B pair, which means that from a terrestrial planet orbiting AC-A tiny Proxima would be very faint if it was visible at all. If Proxima is a permanent part of the triplet it’s orbit might be up to half a million Solar years long, but it might be only a passing ship in the night that will swing out and away from the double star system in a few million years.

The upshot of this layout is that terrestrial planets could plausibly exist in the Alpha Centauri system. Worlds within AC-A’s habitable zone would sit somewhere from ~1 AU to maybe ~2 AU at the outer HZ edge, depending on planetary characteristics like size and atmosphere. The B companion star never swings close enough to disturb the orbit of an inner terrestrial world  or a terrestrial moon of a gas giant in AC-A’s HZ. The same thing is true, to a slightly lesser extent, for AC-B… it might have its own habitable inner planets. Or they both might. For that matter so might Proxima have planets.

Climate on any terrestrial world in the Alpha Centauri system would be strange. A world orbiting AC-A or AC-B would experience moderately strong climatic swings every 80 years, from normal to briefly very hot then back to normal again. Or from cool to mild, then back to cool. With another sun as far as Saturn, Earth would receive a bit more solar insolation… probably enough to turn winter into summer, or summer into a severe heat wave. I haven’t done the calculations, but someone out there probably has and can tell me exactly. Depending on how much water any terrestrial planet there has, those climate shifts may or may not be a major inhibitor of biosphere development. A watery world would have a larger heat buffer to soak up extra warmth, and would keep world climate more stable… but a dry world would see much more dramatic swings in temperature. Water has a much higher heat capacity than silicate rock… which is why maritime regions on Earth are generally milder than continental interior biomes.

It could be argued that Proxima might pose a problem for life in the AC-A/AC-B system, because its presence in the outer reaches of the system might cause comets to destabilize and tumble in where they could pose an impact threat. But if Proxima is a permanent companion any comets vulnerable to such disturbance would probably have been destroyed long ago… perhaps improving long term biosphere survivability on any AC-A/AC-B worlds. Proxima would have tossed all its ammunition in the early history of the system, leaving plenty of post-bombardment time for virtually impact-free biological evolution. Or maybe not. Again, some calculations would be needed to predict either way.

Obviously it remains to be seen whether any planets turn up in Guedes’ and Laughlin’s search. I hope some are found. Alpha Centauri is close enough to consider for a robotic mission at some point down the road, if any potentially habitable worlds are discovered there. An ion drive probe could make the trip within a human lifetime, and send back closeup pictures. Hopefully of at least one pale blue dot.


~ by Planetologist on January 17, 2009.

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