Alpha Centauri A and B have a mean separation of 23 AU. In Solar System terms, that gives you a spacing a bit further from the Sun than the orbit of Uranus. But with the two stars moving around a common center of mass, the distance between them varies over time. Centauri B is sometimes as far from Centauri A as Pluto is from Sol, while at other times it closes as close as Saturn. From a planet around Centauri B, Centauri A would sometimes shine with the light of 5000 full moons, creating day and night sky scenarios that would be, to say the least, striking.
Recent research is making it clear that planets can form in such systems, but binaries are tricky, and we still have much to learn about how such planets would form and where, and under what conditions certain kinds of objects are more likely to occur. Twenty percent of the exoplanet systems thus far found are binary, with the majority of these being wide binaries (separated by 250 to 6500 AU, a far cry from our Centauri stars). But three exoplanetary binary systems — GL 86, Gamma Cephei and HD 41004) — with much closer separations are known to harbor gas giants.
And look at these separations. The Gamma Cephei stars are 18.5 AU apart, while GL 86 shows a separation of 21 AU and HD 41004 a separation of 23 AU. Given the presence of gas giants in these circumstances, can binary systems with separations of 50 AU or less also form terrestrial worlds in the habitable zone? And how would the presence of the secondary star and the gas giant afffect the delivery of volatiles to the inner planets of the binary system in this scenario?
A new paper by Nader Haghighipour (NASA Astrobiology Institute, University of Hawaii-Manoa) looks at the question by simulating the late stage of habitable planet formation, using the collision and growth of a little more than a hundred objects of Moon to Mars-size. Those ’embryos’ inside 2 AU were assumed to be dry, while those between 2 and 2.5 AU were considered to have 1 percent water, and those beyond 2.5 AU to have a water to mass ratio of five percent.
Running the simulations over a 100 million year period and varying the distance, orbital eccentricity and mass of the secondary star, the team found that terrestrial class planets with substantial water reserves can form in the habitable zones of the primary star. From the paper:
Since at the beginning of each simulation, the orbit of the giant planet was considered to be circular, a non-zero eccentricity is indicative of the interaction of this body with the secondary star. As shown here, Earth-like objects are formed in systems where the average eccentricity of the giant planet is small. That is, in systems where the interaction between the giant planet and the secondary star has been weak. That implies, habitable planet formation is more favorable in binaries with moderate to large perihelia, and with giant planets on low eccentricity orbits.
Thus we firm up the picture on binary systems that may prove of astrobiological interest. The paper is “Habitability of Planets in Binaries,” slated to appear in Extreme Solar Systems, ASP Conference Series, ed. Debra Fischer, Fred Rasio, Steve Thorsett and Alex Wolszczan, and now available online. Finding gas giants in systems like the three mentioned above is another indication that planets tend to form wherever they can, even where binary separations are relatively small. But the gas giants around GL 86, Gamma Cephei and HD 41004 make the formation of habitable planets in their systems unlikely.