Campus News

When the dust settles

Song
UGA assistant professor of astronomy Inseok Song stands with the 24-inch reflecting telescope on the roof of the physics and astronomy building on UGA's Athens

Collaborative study sheds light on planet formation

A study published in 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 UGA; 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 UGA’s 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 what we’re talking about is 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 at the Herschel Space Observatory. 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.

Like many important discoveries, the scientists’ finding raises more questions than it answers. Song said his colleagues now aim to compare measurements from 1983 with data from modern telescopes to systematically search for other stars that have either rapidly depleted or replenished their dust clouds. The goal is to understand the frequency with which these processes occur and, ultimately, to advance understanding of how planets form.

“Many astronomers may feel uncomfortable with the suggested explanations for the disappearance of the dust because each of them has non-traditional implications,” Song said, “but my hope that this line of research can bring us closer to a true understanding of how planets form.”