Dust May Be the Stuff of Planets

WASHINGTON -- A sun-like star just out of infancy has winked at astronomers, indicating its eclipse by cosmic dust and rocks, the stuff of which planets like Earth could possibly form, scientists reported Wednesday.

The star, located in the constellation Monoceros, is 2,400 light-years from Earth. It disappeared from view for regular periods of about 48 days over the past six years, astronomers said. Its disappearance suggested an eclipse, but not a typical one caused by an intervening planet, star or moon.

Astronomers said only a collection of smaller objects, such as dust and rocks, could cause the long eclipses they saw.

Known as KH 15D, the star is about 3 million years old--young by stellar standards, and a prime age for monitoring by astronomers investigating our solar system’s past by exploring analogous regions of the universe visible today.

“We’ve monitored thousands of these stars over the years, and this is the only one that behaves this way,” said astronomer William Herbst of Wesleyan University, in Connecticut.

“Essentially the star winks at us,” Herbst said at a meeting on extrasolar planets at the Carnegie Institution in Washington.

The dust that caused the “wink” is different from the fine interstellar dust that is distributed throughout the cosmos, Herbst said. Its particles are bigger, indicating that it is clumping into a proto-planetary disk--from which planets can form.

“Is there a mass in here that is somehow sculpting the obscuring clouds so that it’s producing these rings of material which then circle around the star and alternately block the object? We think that’s very possible,” Herbst said.

There could be two blobs circling the star, or just one, but there is no confirmation yet of what could be causing this kind of disk to form, said Herbst’s colleague Catrina Hamilton.

At 3 million years old, KH 15D is much younger than the star at the center of our solar system--the sun, which is about 4.5 billion years old. It is generally established that the solar system’s planets formed when the sun was a few million years old.

The disk in Monoceros is forming quite close to KH 15D, closer than Mercury is to the sun.

“The star is ... like the sun was when it was 3 million years old, so the processes that are going on in this inner disk region, where terrestrial planets would be forming ... could be analogous to what was going on with the formation of Earth,” Herbst said.

The discovery was made with a relatively small, 24-inch telescope, so it could well be the subject of scrutiny with larger instruments.

“I think this gives us a whole new window into how planets form, and it’s one that a lot of astronomers will be trying to exploit with their telescopes in the next few years,” said NASA astronomer Steve Maran.

“I think this object might turn out to be something of a Rosetta Stone for deciphering some of the mysteries of planet formation,” said Alan Boss of the Carnegie Institution.