Astronomers discover a comet factory
Sen—Astronomers have imaged a region around a young star where dust particles are clumping together to form the building blocks of comets and rocky bodies.
The findings shed light on how dust particles in discs around stars can grow and eventually form planets.
The young star system, designated Oph-IRS 48, is located in the constellation of Ophiuchus (The Serpent Bearer), about 400 light years from Earth.
This is the first time images have been captured of a "dust trap", an area in the disc of dust orbiting the star where particles clump together to form larger rocky bodies.
Computer modelling of planet formation suggests that dust particles orbiting a star grow when they collide and stick together but disintegrate again when the bigger grains collide at high speed, sending the process back to square one. Astronomers theorised that there must be a safe haven where dust particles clump together until big enough to survive on their own, growing to form rocky bodies and planets. Such a safe haven had been theorised and called a "dust trap" - but until now had not been observed.
The observations showed a hole in the ring of gas around the star. Astronomers believe the dust trap forms as bigger dust particles move into regions of higher pressure which can originate from the motions of the gas around the edge of a gas hole such as the one observed.
Nienke van der Marel, a PhD student at Leiden Observatory, who led the study, said: "At first the shape in 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!
“It’s likely that we are looking at a kind of comet factory as the conditions are right for the particles to grow from millimetre 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."
ALMA's image of the dust trap in the disc that surrounds the system Oph-IRS 48. The dust trap provides a safe haven for tiny particles in the disc, allowing them to clump together and grow to sizes that allow them to survive on their own. Credit: ALMA (ESO/NAOJ/NRAO)/Nienke van der Marel.
ALMA, which was officially opened in March, is located 5,000 meters above sea level in the Atacama desert in Chile and is operated by the European Southern Observatory and other international partners. The findings were based on observations made whilst ALMA was still under construction.
With a growing number of exoplanets being discovered, astronomers are keen to understand the process by which clouds of dust circling young stars eventually turn into planetary systems like our own and those being found around other stars.
ALMA's observations, which will be published in the journal Science on June 7, 2013, provide a clearer understanding of the planetary formation process.