Binary system Kepler 47 contains at least one planet in its habitable zone. The two stars have different masses, however, and so the habitability of such planets is limited by the shorter lifetime of the larger and more massive star. Image: NASA/JPL-Caltech/T. Pyle
Planets orbiting binary star systems have to deal with the stresses of more than one star. But new research reveals that close binaries could be as good as singles when it comes to hosting habitable planets. Low-mass twins could make the best hosts, because their combined energy extends the habitable region farther away than would exist around a single star.
After modeling a variety of binary systems, two astronomers determined that stars 80 percent as massive as the Sun, if close enough together, could allow for conditions that would be ideal for hosting habitable planets.
"Potentially, life could exist even more in binary systems than it does in single systems," Joni Clark, an undergraduate at New Mexico State University, told Astrobiology Magazine. Clark worked with astrophysicist Paul Mason of the University of Texas at El Paso.
Pushing the boundaries
Low-mass stars are two to three times more common than the Sun. Their sheer numbers may give them greater odds forhosting planets. But their smaller size also means they have more ultraviolet radiation early in the life of the star and dangerous solar winds in the habitable zone, both important when it comes to maintaining a niche for life to exist. Planets must lie extremely close to small single stars to reap the benefits, a position that brings a number of challenges. Such planets are more prone to be tidally locked, with one face permanently turned toward its sun, and to receive the brunt of any stellar activity. [9 Exoplanets That Could Host Alien Life]
But when two such stars are closely paired, their combined energy extends the habitable region farther away and makes it larger, minimizing some of the threats faced by planets orbiting low-mass stars.
"We have much more room here for planets to hang out," Clark said.
Not just any binary system will work, however. Habitable zones receive the best effect when the low-mass stars are close together, circling each other every ten days or less. Radiation of all types coming from two such closely bound stars would be more consistent, and the planets orbiting them would resemble that of a planet orbiting a single star.
But when the stars are farther apart, the planet's orbit is more likely to be unstable as it feels the tug of gravity stronger from first one star and then the other. When stars are spread out over a distance, orbiting planets would experience significant changes in temperature. With a large enough gap, planets would travel around only one star, with the possibility of occasionally entering the danger zone of the other.
"There are many regions around binary star systems where having a stable orbit simply isn't possible," said Stephen Kane, of the California Institute of Technology. Kane, who studies the habitable zones of planets orbiting binary stars, was not involved in Clark and Mason's research.
Living conditions on the planets would vary based on cloud cover, which could help to both insulate the planet and shelter it from ultraviolet radiation. Such cloud cover could help to protect the planet from the changes it would encounter as it orbits closer first to one star and then to the other.