Two Suns Could Boost Odds of Habitable 'Exomoons'

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Moons in close binary solar systems have a better chance of hosting life than those in single-star systems, new research has shown.

Binary stars dampen each other's solar radiation and stellar winds, thereby creating a more hospitable environment for life and increasing the habitable zone around such solar systems, according to research presented at the 223rd American Astronomical Society meeting in January.

"The two stars calm each other down in terms of activity," said Paul Mason, an astrophysicist at the University of Texas at El Paso in an interview with Astrobiology Magazine. [13 Ways to Hunt Intelligent Alien Life]

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Mason presented the results of a study, which used data collected by NASA's Kepler spacecraft mission to discover potentially habitable exoplanets in our region of the Milky Way galaxy.

Stretching the habitable zone

Although more than a thousand planets have been found outside of the solar system, as well as a host of candidates waiting for follow-up observations, no moons have yet been confirmed. Scientists like Mason are performing theoretical calculations to determine which solar systems might be better for hosting potentially habitable moons.

Violent and active young stars spin rapidly, emitting radiation and stellar winds that could interfere with the habitability of planets and moons nearby. A close binary system of stars can help to dampen these effects, as the two stars synchronize their spins.

Binary stars exist in a range of configurations. Some are widely separated, so that a planet in orbit around one functions much like a planet around a single sun, while the companion is so distant that it appears as point-like as any other star. Others may be extremely close together, synching together to keep each other rapidly spinning for billions of years.

Mason's research focuses on pairs of stars that orbit each other between 10 and 60 Earth-days, with a planet in orbit around both suns. These are known as circumbinary systems. The paired stars exert tidal forces on one another that cause a slowdown in spin, weakening the radiation and stellar wind of the pair faster than they would suffer as single stars. Fast-moving stellar winds can strip a moon or planet of its atmosphere, leaving it open to heavy radiation bombardment that can interfere with the development of life.

At the same time, the combined light from the duo pushes the edge of the region where water can exist, commonly termed the "habitable zone," farther back than it would lie around a single star. Moving the entire zone a greater distance from its sun further reduces the negative effects from the stars. [The Strangest Alien Planets Ever (Gallery)]

"The habitable zone in a binary system is a little bit farther away, simply because you have the light from two stars rather than the light from one," Mason said.

This distance is important because, if a planet orbits too close to its parent star, its moon can be stripped away completely.

"The closer a planet is to the star, the smaller its gravitational sphere of influence," said David Kipping and astronomer at the Harvard-Smithsonian Center for Astrophysics in an interview with Astrobiology Magazine.

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"Essentially, the star will rip off the moon if it gets too close," he said.

Kipping, who was not involved in the research, searches for exomoons and is the principle investigator of The Hunt for Exomoons with Kepler project.

Pushing exomoons farther away also has ramifications for red dwarfs, the most populous stellar type in the galaxy. The habitable zone around these smaller, long-lived stars is so close to its parent star that stellar activity made many astronomers consider habitable planets around them unlikely to even exist, though recent research has increased the potential. In a binary system, the pushed-back habitable zone would decrease many of the negative effects that limit habitability around the plentiful stars.

According to Mason, if the sun had a companion star, the makeup of the solar system would change significantly. The water stripped from the atmosphere of Venus would likely still be present, making it potentially habitable. Earth itself could have been a very different environment.

"Earth would be a wetter planet if we were orbiting a binary star," Mason said.

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