Disappearing Space Dust Cloud Challenges Theories Of How Planets Form

In a paper published in the journal Nature on Wednesday, an international group of scientists described how they observed the rapid disappearance of a cloud of dust surrounding an infant star in the Scorpius-Centaurus stellar nursery -- a hotbed of star formation around 400 light years away from our sun.

In 2008, the scientists examined a star known as TYC 8241 2652 1 and found that its dust cloud appeared to be around the same size as it was in a previous measurement taken in 1983. But in 2009, they looked at the star again and found that the dust cloud's infrared radiation signature had dropped by nearly two-thirds, and in 2010 the dust had just about disappeared.

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, lead author and UC San Diego researcher Carl Melis said in a statement on Wednesday.

Co-author Inseok Song, a University of Georgia professor of physics and astronomy, says scientists had previously thought it took hundreds of thousands to millions of years for that much cosmic dust to be swept away.

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, Song said.

There are two theories the team is putting forward for how all that dust disappeared. The first is runaway growth. Scientists think that planets form as dust left over from a star's birth clumps together, first through weak electrostatic interactions and then, as the clumps grow, thanks to gravity. But to date, it's been unclear exactly how long this process takes. In this instance, conditions around the star were such that planet formation was very fast and efficient.

Whether or not the fast-paced planet formation suggested by the scientists' observations is typical for all planet formation is unclear.

The other possibility is that the dust was pushed out of the star's orbit. The bits of dust are tiny enough to feel the impact of photons emitted by the star, thus getting shoved out unceremoniously by light. If this model is true, then there may be many more stars with planets than previously assumed.

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.

SOURCE: Melis et al. Rapid disappearance of a warm, dusty circumstellar disk. Nature 487, 74-76 (5 July 2012).