Around 20% of all detected extrasolar planets inhabit binary or multiple star systems. Although most of these binaries have very large separations, a few planets have been detected in close binaries with separations as small as about 20 AU (1 AU is the mean distance between the Earth and the Sun). How planets can form in such an environment is a crucial issue, since the companion star's perturbations should strongly affect planet formation around the primary.

An international team lead by the Department of Astronomy at Stockholm University has investigated habitable planet formation in the most famous binary system, where no planet has been detected so far, our closest neighbour alpha Centauri. The system lies about 4 light years away and has a separation of 23 AU. In an article by Philippe Thebault and coauthors they find that planet formation is very difficult at a distance of more than 0.5 AU from each star in the system. For the smaller star alpha Cen B, however, this corresponds to the innermost part of its habitable zone (where liquid water can exist). Thus, this 0.5 AU region is probably the best place to look for possible terrestrial planets in future observational studies. Nevertheless, the presence of planets further out cannot be ruled out, and could be possible if for instance the binary had initially a wider separation than today.

Research article:
Planet formation in the habitable zone of alpha Centauri B Contact:

Philippe Thebault, Tel: 08-5537 8559, thebault@astro.su.se