Study in Nature Sheds New Light on Planet Formation
Athens, Ga. - A study published in the July 5 edition of the journal Nature is challenging scientists' understanding of planet formation, suggesting that planets might form much faster than previously thought or, alternatively, that stars harboring planets could be far more numerous.
The study—a collaboration between scientists at the University of Georgia; the University of California, San Diego; the University of California, Los Angeles; California State Polytechnic University and the Australian National University—began with a curious and unexpected finding: Within three years, the cloud of dust circling a young star in the Scorpius-Centaurus stellar nursery simply disappeared.
"The most commonly accepted time scale for the removal of this much dust is in the hundreds of thousands of years, sometimes millions," said study co-author Inseok Song, assistant professor of physics and astronomy in the UGA Franklin College of Arts and Sciences. "What we saw was far more rapid and has never been observed or even predicted. It tells us that we have a lot more to learn about planet formation."
Lead author Carl Melis, a postdoctoral fellow at UC San Diego, said, "It's like the classic magician's trick: Now you see it, now you don't. Only in this case we're talking about enough dust to fill an inner solar system, and it really is gone."
The scientists first identified their star of interest by examining data from the Infrared Astronomical Satellite, or IRAS, which surveyed more than 96 percent of the sky in 1983. The star, known as TYC 8241 2652 1, was surrounded by a cloud of dust that was identifiable by its distinctive radiation of infrared energy. Like a skillet absorbing heat and then radiating it, the dust cloud was absorbing energy from the central star and radiating it in the infrared range. This warm dust is thought to be the raw material from which planets form, but scientists don't have a clear understanding of how long the process takes.
The scientists observed the same star in 2008 using a mid-infrared imager at the Gemini South Observatory in Chile and again with the same ground-based telescope in 2009. The 2008 observation revealed an infrared emission pattern similar to the 1983 measurement, but something surprising happened in 2009: The infrared emission dropped by nearly two-thirds. NASA's Wide-field Infrared Survey Explorer, or WISE, looked at the same star again in 2010, and the scientists found that the dust had mostly disappeared. The scientists confirmed their findings using two additional telescopes, the Japanese AKARI telescope and the European Space Agency's Photodetector Array Camera and Spectrometer, or PACS, at the Herschel Space Observatory, and the pattern held.
"It's as if you took a conventional picture of the planet Saturn today and then came back two years later and found that its rings had disappeared," said co-author Ben Zuckerman of UC Los Angeles, who has been investigating circumstellar disks since the 1980s.
The researchers explored several different explanations for how such a large quantity of dust could disappear so rapidly, and each of their explanations challenges conventional thinking about planet formation.