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Planetary smashup

Astronomers found compelling evidence for a type of giant collision never directly observed.

December 17, 2021
illustration of collision in space
At least 200,000 years ago in the nearby HD 172555 star system, an Earth-size planet collided with a smaller body so forcefully that part of its atmosphere was stripped away.Mark A Garlick

Scientists believe that young planetary systems experience extreme growing pains as protoplanets collide and fuse—the process thought to have produced the Earth and moon. But such smashups have been difficult to observe in other solar systems.

Now astronomers at MIT, the National University of Ireland Galway, Cambridge University, and elsewhere have discovered evidence of a giant impact around the 23-million-year-old star HD 172555, 95 light-years away. It appears that at least 200,000 years ago, a roughly Earth-size body and a smaller one collided at more than 22,000 miles per hour—a crash so powerful that part of the larger body’s atmosphere was blown away.

The researchers examined data taken by the Atacama Large Millimeter Array in Chile to analyze the dust around HD 172555 and found carbon monoxide circling in large amounts, surprisingly close to the star; a star’s light normally causes this gas to break down at such close range. Their conclusion was that it must be the remnant of an atmosphere-stripping collision. “The only plausible process that could produce carbon monoxide in this system in this context is a giant impact,” says the study’s lead author, Tajana Schneiderman, a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences.

“This is the first time we’ve detected this phenomenon of a stripped protoplanetary atmosphere in a giant impact,” says Schneiderman. “Everyone is interested in observing a giant impact because we expect them to be common, but we don’t have evidence in a lot of systems for it. Now we have additional insight into these dynamics.”

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