It was not all that long ago that binary star systems were thought to preclude planets, and I can remember reading as a boy that stars like Tau Ceti and Epsilon Eridani were far more likely to have planetary systems than the close binaries Centauri A and B. Now, of course, all that has changed, and we know from theoretical work that stable planetary orbits are possible around both Centauri A and B, though naturally constrained to orbits in the inner systems of both (which includes, satisfyingly enough, their habitable zones).
But what can we make out not just from theory but observation? A new study of the 131 planetary systems detected by radial-velocity measurements (as of July 1, 2005) has come up with interesting results. 23 percent of these exoplanetary systems have stellar companions. Many of these had been recognized before as binary systems, but the international team behind this work, led by Deepak Raghavan and Todd Henry (Georgia State) also found six stars in five systems that are identified for the first time as companions to exoplanet host stars.
And this is intriguing: There are three systems (GJ 86, HD 41004, and γ Cep) that have close-in stellar companions. In all three, planets ranging from Mercury to Mars-distance from their primary star occur in systems where there is second star at roughly 20 AU. That’s the distance between Uranus and the Sun, and it’s also close to the mean distance — 23 AU — that separates Centauri A and B. We still don’t know whether the Centauri stars have planets, but how interesting to know of stars with similar separation that do.
Of course, Alpha Centauri is a triple-star system, but Proxima is a whopping 10,000 AU from Centauri A and B and thus unlikely to create problems for stable orbits around the two primaries. But the Georgia State study weighs in here as well; it identifies three and possibly five exoplanetary systems that occur in triple-star settings. And an added bonus: two exoplanet systems contain white dwarf companions, which adds to our knowledge of planetary systems around stars that have left the main sequence (we looked at this just yesterday with evidence of planetary formation around pulsars). From the study’s conclusion:
This comprehensive assessment of exoplanet systems indicates that solar systems are found in a variety of stellar multiplicity environments – singles, binaries, and triples; and that planets survive the post-main-sequence evolution of companion stars.
Centauri Dreams‘ take: The more we understand how how exoplanetary systems are formed, the better we can assess our own Solar System and whether or not its structure is unique. So this is significant work particularly insofar as it helps us see that planets can form in a wide range of environments — we would be foolish indeed, for example, to rule out double and triple star systems as potential homes for terrestrial worlds. In the same way, we are also coming to see that M-class red dwarfs may be orbited by such worlds.
Note that this study used planets found by the radial-velocity method only. The reasons for the restriction are clear: planets discovered by gravitational lensing and, so far, those discovered through transits have been distant enough to make searching for stellar companions problematic. We have, as the authors note, poor or no parallax and magnitude information for the primaries. But we still wind up with a hefty sample of some 155 planets in 131 systems.
On the question of stable orbits around the Centauri stars, see in particular Wiegert and Holman, “The Stability of Planets in the Alpha Centauri System,” Astronomical Journal 113 (1997): pp. 1445-50.
