Over twenty percent of the planets we’ve found around other stars inhabit binary systems. It’s intriguing to take a close look at these. Most of the planet-bearing binaries are what is known as ‘wide S-types,’ meaning that the companion star orbits the inner star/planet system at a distance of over 100 AU. But take a good look at GJ86b, γ Cephei b and HD41004b. Here we’re looking at three planets in close binary systems with a separation between the component stars of 20 AU or less. That separation raises the eyebrows, for Alpha Centauri A and B form a close binary with a semimajor axis of 23.4 AU.

We have three ongoing planet hunts around the Centauri stars, Debra Fischer’s work being matched by Michel Mayor’s team at La Silla and both complemented by a new search based at Mt. John Observatory in New Zealand. So it may not be long — months, possibly — before we have some word about planets around these stars. Informing all these searches, though, is the issue of gravitational perturbations caused by the proximity of the two stars. Various simulations have shown that planets of Earth mass can exist around stars like this, but recent work has questioned whether planetary embryos can form and remain stable in the first place, a question tackled in a new paper.

Astronomers Ji-Wei Xie, Jian Ge (University of Florida) and Ji-Lin Zhou (Nanjing University, China) have created a series of simulations to estimate how long it takes planetesimals embedded in a protoplanetary disk to accrete into planetary embryos. This is rapidly becoming the key issue. What we know so far is that the mass upper limit of a planet in this system is 2.5 Jupiter masses around Centauri A and 3.5 Jupiter masses around Centauri B. That leaves us with the possibility of lower-mass planets if this system will allow planetary embryos to form. The problem is that most simulations have assumed a disk of such embryos as the starting point and have followed planet development from that point.

From the paper:

…the remaining question is whether these embryos can form. Thébault at al. (2009) recently addressed this problem by analyzing the conditions for planetesimal accretion. They conclude planetary embryos formation through planetesimal accretion seems impossible around α CenB, unless the binary separation was wider in its initial stages. However, their conclusions are limited in the absolutely coplanar case, where the inclination between the gas disk and binary stellar orbit is exactly equal to zero…

Ji-Wei Xie and team reopen the case by extending Thébault’s work to include the effect of binary inclinations. They develop a new model tested through simulation to study whether the zone from 0.5 to 2.5 AU around Centauri B may allow planets to form. The simulations vary the gas-disk density as well as the binary inclination and work with variables like planetesimal mass distribution, impact rate and the fraction of accreting collisions to come up with a conclusion: Planetesimal accretion into planetary embryos takes significantly longer in this binary environment than around single stars, which does not favor the formation of gas giant planets, but the formation of smaller, terrestrial worlds is possible.

Let me quote the concluding paragraph on this:

Our results support recent work by Guedes et al. (2008), which has shown Earth-mass planets can be formed near the habitable zone (0.5-0.9 AU) of α CenB if the disk is initially composed of lunar-mass planetary embryos. The possible accretion zone shown in this paper is roughly between 1-2 AU, which matches well with their planet formation zone (~0.5-2.0 AU)… In addition, at the time of writing this paper, we note a promising result from Payne et al. (2009) that Earth-like planets can also form in the habitable zone of α CenB-like binary systems through outward migration from the inner accretion-unperturbed zone (within ~ 0.7 AU). Therefore, by combining these studies…and our simulations, it is quite possible that a habitable Earth-like planet may be hidden around α CenB.

That’s good news for planet hunters, of course, and the happy fact is that we should be getting solid data to measure these simulations against in the not distant future. The paper is Ji-Wei Xie et al., “Planetesimal Accretion in Binary Systems: Could Planets Form Around α Centauri B?” Astrophysical Journal 708, pp. 1566-1578 (abstract / preprint). For a discussion of Thébault’s work, see this earlier Centauri Dreams story.