[BRIEF NOTE] What the disappearing debris disk around TYC 8241 2652 1 might mean

[rfmcdonald]

The sudden disappearance of the debris disk orbiting the distant star TYC 8241 2652 1 reported in Deborah Zabarenko's Reuters article is fascinating, and has fascinating implications.

In a cosmic case of “now-you-see-it, now-you-don’t,” a brilliant disk of dust around a Sun-like star has suddenly vanished, and the scientists who observed the disappearance aren’t sure about what happened.

Typically, the kind of dusty haloes that circle stars have the makings of rocky planets like Earth, according to Ben Zuckerman, one of a team of researchers who reported the finding on Wednesday in the journal Nature.

Composed of warm dusty material, these disks can be seen by telescopes looking for infrared light. This one was first seen in 1983 by NASA’s Infrared Astronomical Satellite around the young star TYC 8241 2652. It glowed for a quarter-century before disappearing in a matter of 2-1/2 years.

An image taken May 1 by the Gemini observatory at La Serena, Chile, confirmed that the disk was gone.

Astronomers are accustomed to watching events that have unfolded over millions or billions of years, so seeing a bright ring depart from view in less than three years was an eye-blink in the astronomical context, Zuckerman said by telephone from the University of California-Los Angeles.

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“So much dust orbiting so close to a young star implies that rocky planets similar to the terrestrial planets of our own solar system were in the process of forming around this star,” he said. But all of a sudden, this potential planet-maker was absent.

“We don’t really know where the dust came from in detail, and we certainly don’t know what caused it to disappear so quickly,” Zuckerman said.

ScienceNOW's Ken Croswell suggests that the sudden disappearance of vast quantities of dust in the TYC 8241 2652 1 system, apparently 460 light-years away in the constellation Centaurus, has significant implications for the processes by which planets form. By all accounts, the system seems to have been typical insofar as very young stars and their systems go, making the disappearance all the more inexplicable.

Born about 10 million years ago, the TYC 8241 2652 1 system was chugging along just fine before 2009. Its so-called circumstellar disk glowed at the infrared wavelength of 10 microns, indicating it was warm and lay close to a star—in the same sort of region that, in our own sun's neighborhood, gave rise to the terrestrial planets Mercury, Venus, Earth, and Mars. The infrared data reveal that the dust was about 180°C and located as close to its star as Mercury is to the sun.

By January 2010, however, nearly all infrared light from the dusty disk had vanished. "We had never seen anything like this before," says astronomer Carl Melis of the University of California, San Diego. "We were all scratching our heads and wondering what the hell did we do wrong?" But subsequent observations with both infrared satellites and ground-based telescopes confirmed the surprising discovery, he says: "The disk was gone."

Melis and his colleagues report the mystery online today in Nature—but they don't know what caused it. "It's very bizarre," he says. "Nothing like this was ever predicted." He says there's no way something could eclipse the infrared-emitting disk for more than 2 years, because such an object would be immense. Furthermore, the star itself didn't fade.

All this could mean that planets form much more quickly than anyone had suspected. (Or, possibly, that they are much rarer than thought.)

The most commonly held theory of planet formation is that minute particles of dust left over after a star forms clump onto each other, first through weak electrostatic interactions and later through gravitational forces. The aggregated dust particles eventually grow to become pebble-sized and then car- to house-sized objects. Ultimately, they become planets. The timescale at which this accretion occurs has been theorized and modeled mathematically, and Song said it is commonly thought to occur over hundreds of thousands of years, a time period that spans civilizations on Earth but is an astronomical blink of an eye.

"If what we observed is related to runaway growth, then our finding suggests that planet formation is very fast and very efficient," [study co-author Inseok] Song said. "The implication is that if the conditions are right around a star, planet formation can be nearly instantaneous from astronomical perspective."

[. . .]

Song added that a slightly different version of the "runaway accretion" theory suggests that dust grains accrete onto the central star in a very short timescale, implying that the star effectively eliminates planet-building material. If such events occur frequently, planet formation is much less likely than previously thought.

Another explanation for the sudden disappearance of the dust is that it was expelled from the sun's orbit. Song explained that the particles are so small—a hundred times smaller than a grain of sand—that the constant stream of photons emanating from the sun could push them away and into each other, like pinballs, until they leave the suns' orbit.

Because large clouds of dust can be formed when orbiting planets crash into each other, astronomers have often viewed the presence of such clouds as indirect evidence of unseen planets. If clouds of dust are only fleeting, however, then many more stars than previously thought could harbor planets.

"People often calculate the percentage of stars that have a large amount of dust to get a reasonable estimate of the percentage of stars with planetary systems, but if the dust avalanche model is correct, we cannot do that anymore," Song said. "Many stars without any detectable dust may have mature planetary systems that are simply undetectable."'