A disk of dust surrounding a nearby star contains lumps and other signs of rocky planets coming together. The second of its kind, the new observation suggests that many young stars are surrounded by colliding boulders.
Based on a handful of observations, astronomers have a rough idea of how a new solar system forms. First a star coalesces from a swirling disk of dust and gas. Then, in less than 10 million years, gaseous planets similar to Jupiter and Saturn come together, gobbling up the leftover gas. After that, rocks, boulders, and "planetesimals" begin to collide, kicking up a whole new disk of dust and, over many millions of years, creating rocky planets similar to Earth and Mars.
Or so the scenario goes. In fact, researchers have spotted only a few young stars with dusty disks. Of these, the most revealing has been b Pictoris (b Pic), a star less than 20 million years old, twice as massive as the sun, and only 60 light-years away. Intriguingly, its dust disk appears to have been warped and sculpted by the gravity of at least one planet.
Now, a star born in the same stellar rookery as b Pic provides an even better look at a dust disk, astronomer Michael Liu of the University of Hawaii, Honolulu, reports online today in Science. Known as AU Microscopii (AU Mic), the star is half as massive as the sun and lies a mere 33 light-years from Earth. Astronomers discovered the dust disk around the star last year, and for two nights in June, Liu examined it with the 10-meter Keck II telescope at Mauna Kea, in Hawaii. Seen edge-on, the disk shows bright and dark spots that could be nascent planets or evidence of their gravitational pull, Liu says. And like the disk around b Pic, AU Mic's disk appears to have a denser core that contains the larger colliding bodies that generate the dust and might glom onto protoplanets.
"It's the most detailed observation we have yet of a disk in the process of planet formation," says Eugene Chiang, an astronomer at the University of California, Berkeley. The sighting proves that b Pic was no mere fluke, and it suggests that dust disks are relatively common, Chiang says. Marc Kuchner, an astronomer at Princeton University in New Jersey, says researchers will hunt for more dust disks now that their significance is becoming clearer.