Comets collide around young star

Carbon monoxide clumps released from comet debris around Beta Pictoris

Unseen planets may be smashing comets together around a nearby star. Astronomers have found a massive blob of carbon monoxide floating around the star Beta Pictoris, 51 light-years from Earth. The gas, most likely released from comet collisions, may help reveal the chemical makeup of other solar systems and find hidden worlds.

Beta Pictoris is already a star of firsts. Thirty years ago, it gave astronomers their first picture of a debris disk around a young star. More recently, Beta Pictoris became one of the first stars around which a planet was directly seen (SN Online: 6/10/10). And now it’s the first star to expose the insides of exocomets.

As remnants of the raw material that built the planets, the comets in our solar system are time capsules — they’re the closest astronomers can get to reaching back 4.6 billion years and getting a sample of what made Earth and our sister planets. Beta Pictoris could let researchers pick at these fossils in other solar systems as well.

In the disk surrounding Beta Pictoris, astronomers have found carbon monoxide clumped mostly on one side of Beta Pictoris, as seen in this image based on data from ALMA. The CO is evidence of a comet collision. The colors indicate the intensity of light (red is higher, blue is lower), which shows how much gas is released. The dashed circles mark the inner and outer edges of the star’s debris disk. Science/AAAS
William Dent, an astronomer at the Chilean Atacama Large Millimeter/submillimeter Array , or ALMA, discovered a CO clump sitting within the disk surrounding the star, which is in the faint southern constellation Pictor.  “Naively, you wouldn’t expect it to clump,” says Dent. “That was a surprise. When I first saw it I thought, ‘Whoa, this can’t be right.’ ”

Carbon monoxide doesn’t survive longer than about 120 years near a young star. The star’s intense ultraviolet radiation rapidly destroys the molecules. The only way to get a massive concentration of CO, Dent and his colleagues report March 6 in Science, is from ongoing collisions of icy bodies, like comets or small planets. As the comets grind together, they release gas. “We normally never see these comets,” Dent says. “This is the only way we get to see what’s inside them.”

Dent argues that there are two possible ways to make the CO cloud. Either an unseen planet is shepherding comets around the star, causing them to congregate in one spot, or his team has witnessed the aftermath of a spectacular collision between two Mars-sized bodies that occurred within the last 500,000 years.

To figure out which of the scenarios is correct, Dent plans to watch Beta Pictoris with ALMA for the next couple of years. If a planet is shoving comets around, he says, the CO clump will move with the planet. If the puff of gas is instead from a single collision between worlds, it will stay put. He also wants to look for other substances that are known to be in comets in our solar system, like methane and organic compounds.

Alexis Brandeker, an astronomer at Stockholm University who has spent a lot of time studying Beta Pictoris, is excited by the new results. He points out that any planet capable of herding comets to this one spot, most likely a smaller version of Neptune, would be too small and too far from the star for current planet-finding techniques to see. ALMA’s ability to see very fine detail around a star, he says, allows astronomers to not only infer a whole new class of planets that other methods can’t detect but also figure out what these worlds are made of. “It’s very difficult to find that in any other way without actually going there,” he adds.

The ALMA telescope, an array of 66 radio dishes, is only half complete. As more dishes are added, Dent plans to point the telescope array to other, fainter stars with disks and compare Beta Pictoris to other planetary nurseries. The current observations are laying the groundwork for an era when astronomers can gather clues scattered around the galaxy about how solar systems are built.

Christopher Crockett is an Associate News Editor. He was formerly the astronomy writer from 2014 to 2017, and he has a Ph.D. in astronomy from the University of California, Los Angeles.

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