habitable planets of binary stars
Artwork © Lynette Cook.
Two key questions arise when considering whether life could evolve in a binary or multiple star system. They are: (a) is it possible for planetary systems to form around such stars, and (b) if it is, can planets orbit so as to provide tolerable surface conditions for biological activity?
On the subject of planet formation, it has been suggested that the gravitational disturbance caused by another nearby star might prevent material from settling into a stable protoplanetary disk. However, evidence is now to hand of such a disk around one of the stars in a young binary system. Assuming then that planets do form, could they support life? Before 1960, it was generally thought not. Then the Chinese-American astronomer Su-Shu Huang showed that there existed various special orbits in which a planet could move in a two-star system and enjoy clement, stable temperatures.
Basically, there are three possibilities: a wide, roughly circular path around both stars, a figure-eight orbit around both stars, or a small orbit around just one star. The exact circuits required for habitability would depend on details such as the type of stars involved and their separation distance. Due to its proximity to the Sun, the Alpha Centauri system has been widely discussed in this regard.
In fiction, Brain Aldiss has speculated what effects the climatic swings of a planet circling around a binary star might have on the social structures of its inhabitants. In his Heliconia series, he looks at life and culture on a world orbiting in a complex planetary system of a binary star such that its seasons lasts hundreds of years.
1. Benest, D. "Stable Planetary Orbits Around One Component in Nearby
Binary Systems," Celestial Mechanics, 43, 47 (1988).
2. Benest, D. "Planetary Orbits in Double Star Systems," Bulletin of the American Astronomical Society, 24, 983 (1992).
3. Holman, M., and Wiegart, P. "Long-term Stability of Planets in Binary Systems," Bulletin of the American Astronomical Society, 28, 1113 (1996).
4. Huang, S. S. "Life-Supporting Regions in the Vicinity of Binary Systems," Publications of the Astronomical Society of the Pacific, 72, 106 (1960).
5. Whitmire, D., Matese, J., Criswell, L., and Mikkola, S. "Habitable Planet Formation in Binary Star Systems," Icarus, 132, 196 (1998).