Call it a planetary pile up. Astronomers have discovered the remains of a collision of two planet-sized bodies in a well-established solar system in the Milky Way. They think this kind of event is rare, but the findings suggest that there’s no such thing as a safe solar neighborhood.
Back when our own solar system was still forming, collisions between young planets were commonplace. In fact, astronomers think our moon is the product of an encounter between Earth and a Mars-sized body. But other than occasional and relatively small-scale smash-ups, such as Comet Shoemaker-Levy's 21 pieces pelting Jupiter in 1994, no nearby worlds have been destroyed for billions of years.
Not so in a binary star system called BD+20 307, located about 300 light-years away in the constellation Aries. In 2004, a team of astronomers discovered a huge cloud of dust encircling what they thought was a young star. Now measurements using NASA's orbiting Chandra X-ray Observatory and Tennessee State University's automated ground-based instrument have revealed two old stars, each about the same age as the sun, locked in close orbit. That means the dust must have come from a collision between two planetary bodies, a collision that must have happened within the past 100,000 years or so--or even more recently, says astronomer Benjamin Zuckerman of the University of California, Los Angeles, a member of the 2004 team who led the new study.
Zuckerman and colleagues reached this conclusion because the dust is hanging so close to the twin stars, and there was no other reason for it to be there. Over time, dust particles will either spiral into a star or be blown away by stellar winds. Based on the mass of the dust clouds--which is a million times heavier than the dust hanging in our own solar system--Zuckerman guesses that the crash involved "planets with at least the mass of our moon or Mercury" and possibly even as large as Earth. Zuckerman's team will report its findings in December in The Astrophysical Journal.
Zuckerman doesn’t know how the orbiting planets became destabilized and crashed into each other. One possibility is that the two stars--which circle each other roughly every 4 days--produced gravitational irregularities that eventually destabilized the planets' orbits. There also might be another star in a much larger orbit that could have disrupted the otherwise stable system. "We've been searching for a third star," he says, "but so far we've seen nothing."
A large collision in an older system is surprising, says astronomer Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. But perhaps even more important is the idea that rocky planets are common in the galaxy. "We don't know if Earths are common," he says, "but we're learning that most stars form planets--that's pretty exciting."