More than half of all main sequence stars occur in multiple star systems, and we’ve already found 19 planets in such systems (Tau Bootis and 55 Rho Cancri are examples). But most of our models of planetary formation have been based upon single stars. Are planets common in double star systems?

The answer has huge implications for the number of possible planets, and it’s a fascinating issue because the the nearest stars, the Alpha Centauri triple star system, are now considered capable of sustaining planets. Paul Wiegert and Matt Holman showed in 1997 that stable orbits can exist within 3 AU of Alpha Centauri A or B, and they calculated a habitable zone around Centauri A of 1.2 to 1.3 AU, with a zone around Centauri B of 0.73 to 0.74 AU. Planets at Jupiter-like distances seem to be ruled out around Centauri because of the disruptive effects of the two primary stars; after all, Centauri A and B sometimes close to within 10 AU, roughly the distance of Saturn from the Sun. The red dwarf Proxima Centauri, meanwhile, is far enough away from both (13,000 AU) so as not to affect these calculations significantly.

Jack J. Lissauer at NASA Ames and Elisa V. Quintana (NASA Ames and the University of Michigan), have been working with models of terrestrial planet formation in binary star systems for some time, as this online presentation from March 2004 makes clear. The two have been using mapping methods that have proven effective in studying the long-term behavior of binary systems, and applying them to ‘close binaries’ (stellar systems so close that planets would orbit both stars) and much more widely separated ‘wide binaries,’ where any planets would orbit one or the other of the two stars. The two scientists have simulated the late stages of terrestrial planet growth in such systems.

Applying their methods to Alpha Centauri, Lissauer and Quintana have shown that terrestrial planets may well have formed around both Alpha Centauri A and B, despite their proximity. But a key factor may be the inclination of the circumstellar disk (where any planets would orbit) to the orbital plane of the two stars. If the disk is inclined less than 45 degrees to the binary orbital plane, the simulations produce 3 to 5 terrestrial planets within 2 AU of the primary star in roughly circular orbits. If the disk is at a higher inclination than 45 degrees, most of its mass falls into the primary star.

Assuming planets did form around the Centauri stars, would they be habitable? Quoting Lissauer’s 2004 presentation:

“Probably not. Models for delivery of volatiles…suggest terrestrial planets receive volatiles primarily from the asteroid belt and beyond. In the Alpha Centauri system, orbits >3 AU from Alpha Centauri A or B are very unstable, and material would not form planetesimals in these regions… Alpha Centauri may thus have dry terrestrial planets, devoid of the [carbon and water-based] life which thrives on Earth.”

Findings from the above presentation, which Lissauer made to the Kavli Institute for Theoretical Physics conference on Planet Formation (March 2004) are well worth your time, and you should be aware that a QuickTime file of his talk is available online. See also this abstract from the Second Astrobiology Conference at NASA Ames Research Center (April, 2002).

Lissauer and Quintana have brought their study of close binary systems to the fall meeting of the American Geophysical Union, in a presentation called “Terrestrial Planet Formation Around Close Binary Star Systems.” Updates on this one as information becomes available.