Comet factory discovered - It's a cashew!

A comet factory around the star Oph-IRS 48 in the constellation Ophiuchus has been imaged by astronomers using the Atacama Large Milimeter/submillimeter Array (ALMA). This observation resolves a long-standing debate about the formation of planets.

Astronomers have long theorized that comets, planets and other bodies around a star form from a disk of dust and gas that clumps together to form the structures. This image is the first time such a disk has ever been observed and modeled, however. This one sits 390 light years from the Earth.

When dust and small particles collide as the travel around a star, they sometimes stick together, forming a larger body. But, when thse aggregate particles collide with another piece of dust or pebble, they can break apart, ending that cycle of building. Other particles are slowed by friction from the dust cloud, and go crashing into the local star. So, astronomers were puzzled how the little bits of debris could accumulate into pieces big enough to form asteroids, comets and planets.

One theory which was proposed to give these dust bunnies a chance to build up was the existence of dust traps.

The Very Large Telescope in Chile had earlier found a disk of dust surrounding the star, featuring a gap that is likely the result of a companion star or large planet.

When Nienke van der Marel, a PhD student at Leiden Observatory in the Netherlands, looked at Oph-IRS 48 using ALMA, she uncovered something so radically different than the earlier observations that she thought it was a mistake.

"At first the shape of the dust in the image came as a complete surprise to us. Instead of the ring we had expected to see, we found a very clear cashew-nut shape! We had to convince ourselves that this feature was real, but the strong signal and sharpness of the ALMA observations left no doubt about the structure. Then we realized what we had found," van der Marel said.

The gaps form from the presence of a large planet creating high-pressure systems in the gas, which last for a period of time from few hundred thousand to a few million years. Although this is not much time on an astronomical scale, it is enough to allow particle accumulations to grow into larger pieces.

"It's likely that we are looking at a kind of comet factory, as the conditions are right for the particles to grow from millimeter to comet size. The dust is not likely to form full-sized planets at this distance from the star. But in the near future, ALMA will be able to observe dust traps closer to their parent stars, where the same mechanisms are at work. Such dust traps really would be the cradles for new-born planets," van der Marel said.

An article publicizing the results of the observations was published in the journal Science on June 7. 

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