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Pulverized planet dust might lie around double stars

By John Barrat

This artist’s concept illustrates an imminent planetary collision around a pair of double stars. The stars are similar to the sun in age and mass, but they orbit tightly around each other. With time, they are thought to get closer and closer, until their gravitational influences change, throwing the orbits of planetary bodies circling around them out of whack and leading to collisions.

Tight double-star systems might not be the best places for life to spring up, according to a new study using data from NASA’s Spitzer Space Telescope. The infrared observatory spotted a surprisingly large amount of dust around three mature, close-orbiting star pairs. Where did the dust come from? Astronomers say it might be the aftermath of tremendous planetary collisions.

“This is real-life science fiction,” says Jeremy Drake of the Smithsonian Astrophysical Observatory. “Our data tell us that planets in these systems might not be so lucky — collisions could be common. It’s theoretically possible that habitable planets could exist around these types of stars, so if there happened to be any life there, it could be doomed.”

The particular class of binary, or double, stars in the study are about as snug as stars get. They are separated by only about two million miles (3.2 million kilometers), or one-fiftieth the distance between Earth and our Sun. The stellar pairs orbit around each other every few days, with one face on each star perpetually locked and pointed toward the other.

The close-knit stars are similar to the Sun in size and are probably about a billion to a few billion years old. But these stars spin much faster and, as a result, have powerful magnetic fields and giant, dark spots. The magnetic activity drives strong stellar winds — gale-force versions of the solar wind — that slow the stars down, pulling the twirling duos closer over time. And this is where the planetary chaos begins.

This artist’s concept illustrates a tight pair of stars and a surrounding disk of dust–most likely the shattered remains of planetary smashups. Using NASA’s Spitzer Space Telescope, scientists found dusty evidence for such collisions around three sets of stellar twins. The stars are similar to our sun in mass and age and orbit very closely around each other. (NASA/JPL-Caltech images)

As the stars cozy up to each other, their gravitational influences change, and this could disturb planetary bodies orbiting around both stars. Comets and any planets that might exist in the systems would start jostling about and sometimes banging into each other. This includes planets that could theoretically be circling in the double stars’ habitable zone — a region where temperatures would allow liquid water to exist.

“These kinds of systems paint a picture of the late stages in the lives of planetary systems,” says Marc Kuchner, a scientist at NASA Goddard Space Flight Center in Greenbelt, Md. “And it’s a future that’s messy and violent.”

The Spitzer telescope spotted the infrared glow of hot dusty disks, about the temperature of molten lava, around three such tight binary systems. The team says that dust normally would have dissipated and blown away from the stars by this mature stage in their lives. They conclude that something — most likely planetary collisions — must therefore be kicking up the fresh dust.

If any life forms did exist in these star systems, and they could look up at the sky, they would have quite a view. Marco Matranga, from the Harvard-Smithsonian Center for Astrophysics and now a visiting astronomer at the Palermo Astronomical Observatory in Sicily, says, “The skies there would have two huge suns, like the ones above the planet Tatooine in ‘Star Wars.’”


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