Chile’s ALMA telescope discovers “comet factory”

Groundbreaking discovery made by the Atacama Large Millimeter/submillimeter Array (ALMA) solves longstanding mystery of early planet formation.


A new discovery made by astronomers at the ALMA observatory in Chile’s Atacama desert has provided evidence that validates, for the first time, the “dust trap” theory of large particle formation.

ALMA has already generated images of planets in mid-formation, though the process by which dust particles gather and stick together to form pebble, rock, and comet sized formations has eluded scientists — until now.

Researchers using the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile studied the Oph-IRS 48 disc system and identified commonly found dust and gas rings orbiting the central star system. They then turned a number of ALMA’s highly sensitive radio antennae toward the disc and identified an anomaly.

“At first the shape of the dust in the image came as a complete surprise to us,” Nienke van der Marel, a Ph.D. student at Leiden Observatory and lead author of the article, said. “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 realised what we had found.”

The researchers had found a “dust trap,” something that had been theorized via computer generated models though never directly observed before. Within orbiting rings, dust particles can make contact and stick together, though progressively larger particles are just as easily smashed apart within such a high energy environment. Astronomers had theorized that safe havens with a more stable environment — dust traps — had to exist within disc systems if larger rocks were to have a chance to form.

“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,” van der Marel said. “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.”

Dust traps form as bigger dust particles move in the direction of regions of higher pressure. Computer modeling has shown that such a high pressure region can originate from the motions of the gas at the edge of a gas hole, like the one found in this disc.

“The combination of modeling work and high quality observations of ALMA makes this a unique project”, Cornelis Dullemond, team member and dust expert from the Institute for Theoretical Astrophysics in Heidelberg, said. “Around the time that these observations were obtained, we were working on models predicting exactly these kinds of structures: a very lucky coincidence.”

ALMA was officially inaugurated earlier this year, and already its technology has revolutionised the way astronomers conduct their studies. The telescope is not even close to operating at full capacity, and yet it is already so powerful that in a few hours during one earlier study it made as many discoveries of early galaxies as were made in the decade prior to its opening.

“These observations show that ALMA is capable of delivering transformational science, even with less than half of the full array in use,” Ewine van Dishoeck of, lead ALMA scientist from the Leiden Observatory, said. “The incredible jump in both sensitivity and image sharpness in Band 9 gives us the opportunity to study basic aspects of planet formation in ways that were simply not possible before.”