The discovery of carbon monoxide around a nearby star has surprised astronomers, with the most likely explanation a demolition derby of cometary collisions at the astonishing rate of one every five minutes.
Directly observing comets around other stars, even one a mere 63 light years away such as Beta Pictorus, is well beyond the capacity of existing telescopes. However, we can detect regions of gas around the nearer stars, with absorption lines revealing the content of the gas.
Using the Atacama Large Millimeter/submillimeter Array (ALMA) a team led by the European Space Agency's Dr Bill Dent found a two patches of carbon monoxide 13 billion kilometers from Beta Pictoris, about three times the distance of Neptune from the sun. The patches (which may in fact be a single united system) weigh around 1017 tonnes, or about a thousandth of the mass of the moon. This makes up a third of all the carbon monoxide circling the star.
“Molecules of CO can only survive around a star for a brief time, about 100 years, before being destroyed by UV radiation,” says Dent. “So unless we are observing Beta Pictoris at a very unusual time then the carbon monoxide we observed must be continuously replenished.”
The question then becomes where the carbon monoxide could be coming from. Comets are known to contain lots of CO, and release it as they melt on close approaches to the sun. However, even with a star nine times as bright as the sun, such as Beta Pictoris, melting would not occur out past the orbit of Neptune.
Dent's conclusion is that comets are smashing into each other, releasing their carbon monoxide in the process. This raises the question of why, in such vast areas of empty space, comets should be colliding frequently enough to keep the CO patch maintained. It is already known that Beta Pictoris has a planet orbiting at a distance of 1.2 billion kilometers (a little less than Saturn), but this wouldn't be expected to influence collisions further out.
In Science, Dent's team propose two explanations. In one, objects of a mass similar to Mars happened to collide recently enough for the carbon monoxide to still be visible. Alternatively, they suggest a large number of comets exist in the same location and regular collisions sustain the cloud.
The presence of so many comets so close together might seem puzzling, but Dent suggests these have been shepherded together by an as yet undetected planet.
Saturn's rings provide a model along these lines, with clumping of particles in the rings driven by the gravity or nearby moons. One paper proposes that the relationship between the the moon and the objects in the rings is less like a sheepdog and sheep, and more like foxes and hares.
In this case team member Dr Mark Wyatt of the Cambridge Institute of Astronomy says, "We think the Beta Pictoris comet swarms formed when the hypothetical planet migrated outward, sweeping icy bodies into resonant orbits."
"To get the amount of CO we observed—which is equal to about one-sixth the mass of Earth's oceans—the rate of collisions would be truly startling, with the complete destruction of a large comet once every five minutes," notes NASA's Dr Aki Roberge, a coauthor of the paper. "To get this number of collisions, this would have to be a very tight, massive swarm."
So far the giant planet required to keep so many comets in place has yet to be observed. Finding it will not be easy, since at such a great distance the gravitational effects on the parent star will take decades to show up and the chance of it transiting across Beta Pictoris is small. However, astronomers are increasingly coming to the conclusion that the presence of some planets indicates more are likely, and are encouraged to step up the search for planets further in. Beta Pictoris, is a very young star, just 20 million years old, and provides us with a rare opportunity to watch the formation of a planetary system at such a close distance, astronomically speaking.
“Carbon monoxide is just the beginning, there may be other more complex pre-organic molecules released from these icy bodies,” says Roberge.