Stars wrapped in warm, dusty disks sound cozy, and in cosmic terms, they are. They are the incubators in which planets like our own rocky, and fairly close to their parent sun are most likely to form. In these disks, dust is coalescing and tiny chunks of rock are colliding to become larger masses of matter and, over the course of millions of years, planets. The theory is far from perfect, though, and computer models meant to simulate the process raise as many questions as they answer. Astronomers have lately tried to remedy that, watching dust-shrouded stars for any small changes maybe a tiny shift in the amount of starlight making its way to Earth that they can plug into the models to help make them fit reality.
Carl Melis, a postdoctoral fellow at the University of California, San Diego, and his collaborators chose one of the dustiest stars ever seen one that goes by the prosaic name TYC 8241 2652 and planned to watch it for a good long time. When the team took a look at it in May 2008, through one of the Gemini Observatory telescopes in Chile, the star appeared much as it had when it was first observed in 1983. In January 2009, they took another look. What they saw was astounding.
Nearly all the dust had disappeared.
Their first thought was that there must be some mistake. "Things in space don't usually evolve very quickly," Melis says. "We refer to things as having astronomical timescales for a reason. To have this sort of rapid evolution, especially something that happens in two years or less, on the solar system spatial scale..." Well, it was unbelievable.
A cosmic chase scene followed reported in the current issue of Nature, as the team wangled six looks through four separate telescopes over the course of the next few years. January 2009, from NASA's WISE satellite: "The disc was just ... gone," Melis says. July 2010 from WISE: "Still gone." April 2011 from Mauna Kea in Hawaii: "The disc appeared to be gone." Then the European Space Agency's orbiting Herschel telescope: "Gone." And finally, through Gemini again in May 2012: "Really gone."
So where did the dust go? And why was it in such a hurry?
After poring over the literature on dusty disks, Melis and his collaborators identified two processes that can explain what they observed. One occurs when two giant objects in a disk crash into each other and release metal-rich gas into the dust cloud. The gas slows orbiting dust particles so much that they lose momentum, plummet out of orbit, and fall into the star, clearing out the disk quickly. Metal-rich gasses, though, are difficult to see, so the team isn't sure whether there are any around TYC 8241 2652.
Another plausible explanation involves something called a collisional avalanche, a kind of giant, dusty domino effect. In this scenario, a collision between two large objects sends out a spray of tiny dust particles. These particles are so small that the gentle push of the electromagnetic radiation streaming out from the star can blow them out into space. (This "radiation pressure" is the effect that powers solar sail craft like Japan's IKAROS.)
If the disk is tightly packed, the dust particles will smack into other objects on their way out, breaking off other dust particles that, in turn, are small enough to be pushed out by the star's radiation. This happens over and over and over again, ending with much of the dust in the disk dispersing into space. Such collisional avalanches can happen very quickly consistent with the time scales observed at TYC 8241 2652. This model isn't perfect either, says Inseok Song of University of Georgia, one of Melis' coauthors. He believes there wasn't quite enough dust around the star for the clearing-out process to make total sense.
Still, the collisional avalanche remains the most likely explanation, at least so far, says Zoe Leinhardt, a research fellow at the University of Bristol who studies planet formation. She says that the uncertain science of measuring dust around stars means that there may have been more present at TYC 8241 2652 than Song believes.
The larger point, Leinhardt says, is that both of these scenarios start with a collision between large objects. That implies that planet formation in that disk was fairly far along, in the stage that involves rocks at least the size of asteroids. "This could be an observation of a phase in planet formation that we have up to this point effectively been blind to," she says. And it means that in the future, swiftly disappearing dust could be a sign astronomers could use to tell from far away that rocky, Earthlike planets are well on their way to forming.
Melis, Song, and their collaborators are hoping that other astronomers, will weigh in and help them solve the mystery of the disappearing dust. In the meantime, they have already begun to pick out other shrouded stars to keep an eye on, just in case their dust makes a speedy getaway, too. If they can catch those stars in the act, they will be that much closer to cracking the case.
Greenwood's stories are also available at discovermagazine.com
An earlier version of this story reported that the astronomers used the Gemini Telescope in Hawaii in their work. They in fact used the one in Chile.