Alpha Centauri System

Alpha Centauri is the nearest star system to the Sun, just over 4 light years away.

Alpha Centauri is a triple star system that contains a star a little bigger than the Sun, Alpha Centauri A, and a star a little smaller, Alpha Centauri B, in a close but eccentric orbit around a common centre of mass. The star system’s third member is a more distant and much smaller Red Dwarf companion, Proxima Centauri, orbiting the other two. The system has one Earth mass planet in a very close orbit around Alpha Centauri B.

On October the 16th 2012, astronomers announced to the world that an Earth mass planet was detected in orbit around Alpha Centauri B. The discovered Earth-like planet, with the designated name Alpha Centauri Bb, is slightly larger in mass than Earth with a minimum of 1.13 times the mass. It orbits extremely close to Alpha Centauri B at 0.04 au, much closer in than the star's habitable zone, and completes one orbit in just 3.236 days. Consequently, it is a tidally locked planet with a surface temperature, on the sun facing side, reaching in excess of 1000 degrees Celsius. For an Earth-like planet the perfect temperature location, adjusted to account for Alpha Centauri B's relatively high infra-red output when compared to the Sun's, would be around 0.75 au. The full extent of the habitable zone for a planet of Earth mass with an Earth similar atmosphere, again adjusted to account for the higher infra-red heating from Alpha Centauri B, is between 0.71 au and 1.05 au. The habitable zone is slightly further out for a smaller mass planet or one with a thicker atmosphere and slightly closer in for a larger mass planet or one with a thinner atmosphere. For a desert planet the habitable zone would be wider. To date, Alpha Centauri Bb is the closest known extrasolar planet, at 4.4 light years away. It is the lowest mass planet yet detected around a Sun-like star. It was also the first planet discovered orbiting one star in a close binary configuration where neither star is in orbit around the other. It was detected by scientists using the HARPS (High Accuracy Radial Velocity Planet Searcher) high precision echelle spectrograph on ESO's (European Southern Observatory) La Silla Observatory telescope, located in Chile. Alpha Centauri Bb has yet to be given a proper name. If it is given one, an Arabic name could well be chosen so as to be consistent with the name of its host star or perhaps in honour of the team that discovered it, led by Xavier Dumusque, it might be given a French, Portuguese or Swiss name. Only a very marginal radial velocity peak, at 5.6 days has been detected around the slightly closer Proxima Centauri, 4.2 light years away. This weaker signal was detected by the HARPS spectrograph, on the La Silla telescope. No promising radial velocity signals or any other hints of a planet have yet been discovered for Alpha Centauri A.

Initial Target for Proposed ExoplanetSat (Dec 2011)

 * http://www.thespacereview.com/article/1983/1