Roughly twenty percent of all detected exoplanets are in binary systems, intensifying our interest in Alpha Centauri. Recent work, however, has been less than encouraging to those hoping to find one or more terrestrial worlds around these stars. Indeed, Philippen Thébault (Stockholm Observatory), Francesco Marzari (University of Padova) and Hans Scholl (Observatoire de la Côte d’Azur) have shown that in the case of Centauri A, the zone beyond 0.5 AU is hostile to the accretion processes that allow planets to form. Any terrestrial-class world that close to Centauri A would be excluded from the habitable zone, a region thought to extend from 1.0 to 1.3 AU around the star.
The same team now goes to work on Centauri B, having pointed out in the earlier paper that the mathematical modeling it used there was unique to Centauri A and could not be applied indiscriminately to other systems, not even to the second star of the Centauri binary. The authors are targeting the phase of planetary formation when kilometer-sized planetesimals accrete, and examining the effect that perturbations and gas drag have on impact velocities within a test population of such planetesimals. As with Centauri A, the results show that this early formation stage can take place only in a region within 0.5 AU.
That just might allow the needed accretion and hence planet formation to occur within the innermost part of Centauri B’s habitable zone, which is estimated to extend from 0.5 to 0.9 AU. But even here the limits are tight, and the authors believe that because planetesimals in this region would have such high relative velocities (as opposed to a system unperturbed by the close binary Centauri A), planet growth would tend to occur much more slowly.
But let me quote the paper directly on this crucial matter:
Planetesimal accretion is marginally possible in the innermost parts, ∼0.5 AU, of the estimated habitable zone. Beyond this point, high collision velocities, induced by the coupling between gas friction and secular perturbations, lead to destructive impacts. Moreover, even in the ∼ 0.5 AU region, ∆v are increased compared to an unperturbed case. Thus, ”classical”, single-star like runaway accretion seems to be ruled out.
The case is strong — in order to arrive at the subsequent planet, you first have to let accretion go to work to build it, whether in or out of the habitable zone. This work does not, then, contradict other findings that planets are feasible within the habitable zones of the Centauri stars, but does insist that they are highly unlikely to form there. So are there other ways we could wind up with planets in the habitable zone?
Two possibilities remain: Changes in planetesimal orbits as gas within the early system is dispersed, and greater separation between the two stars early in their history, which would make perturbations less pronounced. Can either leave us hope for such worlds? The first seems dubious:
…we ﬁnd that later progressive gas dispersal reduces all ∆v to values that might allow accreting impacts. However, we ﬁnd that the system has ﬁrst to undergo a long accretion-hostile transition period during which most of the smaller planetesimals are removed by inward drift and most bigger objects are probably fragmented into small debris. Thus, the positive effect on planetesimal accretion is probably limited.
As to a wider initial separation of the Centauri stars reducing the perturbation effects, much work remains to be done. The minimum separation for accretion processes to prove favorable to the formation of a planet in the habitable zone seems to be 37 AU. Could the Centauri stars have once been separated by this amount or greater? Finding the answer to that question depends upon working out the likelihood of orbital changes in early open clusters. In short, we don’t know.
Thébault, Marzari and Scholl are doing significant work, examining as they do the crucial early stages of planet formation, and the authors point out that the even earlier phases, when the kilometer-sized planetesimals are themselves forming, have not yet been modeled. Right now we can hold out hope for the region 0.5 AU or so around Centauri B, but it is chastening to reflect that this narrow region may offer the only realistic prospect for a habitable world in this nearby system.
The paper is Thébault et al., “Planet formation in the habitable zone of alpha Centauri B,” accepted for publication in Monthly Notices of the Royal Astronomical Society and available online. Thanks to andy for the pointer.