Building a planet seems straightforward enough: Just take a disk of dust swirling around a young star, and let it sit. Micron-sized dust particles will collide and clump together, growing in size, until eventually, voilà! One planet. There's a problem, however: Astrophysicists have calculated that once clumps have reached a certain size, collisions with other clumps should smash them to smithereens. Or they should start drifting inward toward the star, where they can't grow further. How then can planets even form?
Astronomers who've imaged the dust disk spinning around a young star 390 light-years from Earth may have found an answer. Viewing the star using the newly unveiled Atacama Large Millimeter/submillimeter Array (ALMA) in northern Chile, the researchers observed a crescent-shaped vortex of gas in the disk that appears to provide a safe haven in which a planetary seed can continue to grow . The vortex essentially creates a dust trap within which large clumps can get larger while avoiding drifting inward long enough to form the rocky core of a planet.
Such dust traps have been theorized before, but this is the first empirical observation that appears to confirm the hypothesis. The vortex in the disk around Oph IRS 48 was probably created when some companion object orbiting the star—another star or a preexisting planet—rammed through and punched a hole in the disk. The astronomers found that the concentration of dust within the disk was startlingly lopsided, 130 times denser on one side than on the other.
The observation "led us to the interpretation that the large dust particles must be trapped in a vortex in the gas distribution," says Nienke van der Marel, a researcher at Leiden University in the Netherlands and lead author of the paper describing the findings, published online today in Science. "Trapping the large dust particles prevents the radial inward drift and therefore allows the particles to grow to much larger sizes, up to rocks as wide as a kilometer or more." Van der Marel adds that "the existence of dust traps in disks around young stars provides a crucial step in the start of planet formation by dust coagulation."
The finding fills an important gap in astronomers' understanding of how planets are born, says Philip Armitage, an astronomer at the University of Colorado, Boulder, and author of an accompanying Perspective  in the same issue of Science. But it also raises the question of how gas vortices that harbor dust traps get created in the first place. And, Armitage says, astronomers will now want to know "whether large scale structures in the dust, such as the one imaged for IRS 48, are the norm in protoplanetary disks."