ALMA finds a cashew-nut-shaped comet factory
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Through a new discovery by ALMA, astronomers have solved a riddle about the birth of planets.
Planets are born in flat, rotating disks of gas and dust around young stars. Small dust particles in the disk stick together to form larger dust grains. Dust grains then coalesce into pebbles, rocks and boulders. Eventually, the biggest chunks attract each other through their gravity, and a planet is born.
But there’s one problem. No one really understands how dust grains of about a millimetre across are able to survive and grow into larger pieces. When they collide, they easily break up into smaller dust particles. They are also slowed down by friction with the gas in the disk. As a result, you would expect them to spiral inward, and to fall into the star.
Using ALMA, astronomers have now found a ‘dust trap’ in the disk of a young star. The disk has a central hole, so it is actually a ring. The smallest dust particles are everywhere in the ring. But ALMA discovered that the larger grains are not spread out so evenly. They are concentrated in one part of the ring, in a region shaped like a cashew nut.
Because the larger dust grains are concentrated in one region, they are more likely to stick together. So the dust trap enables the formation of larger and larger chunks of matter, maybe well up until a kilometre across.
No one knows exactly how the dust trap forms. It’s probably the result of a vortex in the ring, maybe due to the gravity of a large planet close to the star. But the most important thing about the new discovery is that dust traps do indeed exist.
Astronomers have described the dust trap as a ‘comet factory’. Why? Because the largest objects that will form here are probably comets – icy bodies of a few kilometres across. But other young stars may also have dust traps. And in some cases, these other dust traps could provide the right conditions for the formation of full-fledged planets.
The ‘dust trap’ was discovered in the ring of gas and dust that encircles a young star known as Oph-IRS 48. This star is at a distance of some 400 light-years, in the constellation Ophicuhus, the Serpent Bearer. Earlier observations with other telescopes had already revealed the dust ring, but ALMA was the first to discover that the larger grains in the ring are concentrated in a cashew-nut-shaped region.
This study was led by Nienke van der Marel of Leiden Observatory in the Netherlands. Nienke worked together with colleagues from Chile, China, Germany, Ireland and the Netherlands. The results were published in the weekly journal Science on 7 June 2013.
Credit: Universe Awareness (unawe.org)