A group of astronomers has stumbled on the closest, most detailed look at the cauldron of planet formation anyone is ever likely to have. The view is of a newborn star whose protoplanetary disk has fortuitously set up a monitor of its own innermost workings. By simply measuring the star's brightness, researchers are seeing how a protoplanetary disk works.
Clues to the formation of our solar system are hard to read (Science, 31 August 2001, p. 1581). And around other stars, disks of dust and gas where planets might be forming today are fuzzy patches of light in even the most powerful telescopes. But this much is known about planet birth: A star forms in the midst of a ball of dust and gas, the remainder of which can collapse into a spinning disk resembling the rings of Saturn. Planets could agglomerate in such disks, but the disks seen so far have been nearly featureless and constant, with any protoplanets invisibly small.
In 1995, astronomy students led by astronomer William Herbst of Wesleyan University in Middletown, Connecticut, caught something happening in the constellation Monoceros. They noticed that star KH 15D--just 3 million years old--faded dramatically every few weeks to 4% of its normal brightness. After a recent international observing campaign organized by graduate student Catrina Hamilton of Wesleyan and Herbst, "now we're sure we can predict what it's going to do," Herbst said last week at the Scientific Frontiers in Research on Extrasolar Planets meeting in Washington, D.C. Alternate dimmings progress slightly differently, suggesting that whatever obscures the star, there are two of them circling it every 96.7 days at a distance closer than Mercury orbits the sun.
The obscuring matter could be the long, low crests of two pinwheel-like waves of gas and dust. These seem to spiral outward from either side of the disk, viewed edge-on. It appears that the inner region of this protoplanetary disk, where rocky, Earth-like planets might be forming, is spiraling much as Saturn's rings do.
"It's fascinating," says astronomer Ray Jayawardhana of the University of California, Berkeley. "It's all pointing to a lot happening in the first few million years."