The findings about possible terrestrial worlds around the Alpha Centauri stars have become more encouraging than ever. Key work in this regard has been performed by Elisa Quintana and collaborators, who have shown in their simulations that, depending on initial disk inclinations, 3-5 such planets might form around Centauri A and 2-5 around Centauri B.
We’ve already discussed that research and I don’t want to linger on Quintana’s 2002 paper (reference below) other than to note one interesting comparison. When the same initial disk parameters are placed around a single star like the Sun, the accretion of the planetary disk occurs over a much larger expanse of time. Evidently a stellar companion hastens the process of planetary formation, one billion years in the case of the Sun vs. perhaps 200 million years in the Centauri scenario.
Quintana, Jack Lissauer (both at NASA Ames) and team went on from that study to look at planet formation around close binaries. And they’ve now turned to the factors influencing terrestrial worlds around individual stars in binary systems. That includes, of course, binaries like Alpha Centauri, but its significance goes well beyond our closest stellar neighbor given that the majority of solar-like stars are found in binary systems. Using numerical simulations to model planet formation and stability in these circumstances, their new paper gives us important information on where we might look to find planets like ours elsewhere in the galaxy.
Much depends on how close the two binary stars are, the most important factor being the periastron value — the distance between the two stars at the closest point in their orbits. The periastron value for Centauri A and B is 11.2 AU, a perfectly acceptable figure for terrestrial planet formation. For as these studies show, a periastron greater than 10 AU allows planets to develop unperturbed. From the paper:
When the periastron of the binary is larger than about qB = 10 AU, even for the case of equal mass stars, terrestrial planets can form over essentially the entire range of orbits allowed for single stars (out to the edge of the initial planetessimal disk at 2 AU). When periastron qB < 10 AU, however, the distributions of planetary orbital parameters are strongly affected by the presence of the binary companion."
So the good news for terrestrial planet hunters is that 40 to 50 percent of binary stars are wide enough to allow Earth-like planets to form and remain stable in orbits circling one of the two stars. And interestingly enough, Quintana’s work also shows that about 10 percent of main sequence binaries are close enough to allow the formation and stability of such planets in orbits that circle both stars.
What a finish: “Given that the galaxy contains more than 100 billion star systems, and that roughly half remain viable for the formation and maintenance of Earth-like planets, a large number of systems remain habitable based on the dynamic considerations of this research.”
The paper is Quintana et al., “Terrestrial Planet Formation Around Individual Stars Within Binary Star Systems,” accepted by The Astrophysical Journal and available as a preprint. The 2002 paper, available here, is “Terrestrial Planet Formation in the α Centauri System,” The Astrophysical Journal 576:982-996 (September 10 2002).