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A young star far away in the galaxy is winking in a regular pattern that astronomers say is unlike anything seen before. If scientists learn to read these signals, they may deliver a message about the early stages of planet formation.

Sometimes the star shines in full brightness. Other times it fades to a tiny fraction of full luminosity, remaining that way for about 18 days — too long for the cause to be an ordinary eclipse caused by an intervening star, planet or moon blocking the star's light.

After six years of studying the phenomenon, a research team led by astronomers from Wesleyan University reported last week that the light is being turned on and off by waves of dust grains, rocks and possibly larger objects that swirl about the sunlike star in a broad disk. This is the stuff from which planets are made, and these eclipses could reveal the dynamics of planet evolution on time scales of weeks and months, not millions of years.

Embedded deep in the revolving disk may even be a newborn planet.

William Herbst of Wesleyan and Catrina Hamilton, a graduate student, said this was the first time astronomers had been able to study in detail the structure of early protoplanetary disks around stars.

"The processes that are going on in this inner disk region could be analogous to what was going on in the formation of the Earth," said Herbst. "It could shed light on our origins by helping us to understand how our Earth and the planets in the solar system came to be."

JUST AN 'INFANT'

The star with the peculiar eclipses, KH 15D, is 2,400 light-years from Earth, in the constellation Monoceros and part of a region well-known as a birthplace of stars. In cosmic time, the 3-million-year-old KH 15D is just emerging from infancy. The sun is about 4.6 billion years old.

"We've monitored thousands of these stars over the years, and this is the only one that behaves this way," Herbst said. "Essentially, the star winks at us."

The observations were described at a conference on extrasolar planets at the Carnegie Institution in Washington, sponsored in part by the National Aeronautics and Space Administration. Since 1995, at least 100 planets have been detected orbiting stars other than the sun, and this has stimulated increasing research into the variety and seeming ubiquity of other planetary systems. But nearly all of the planets are large gaseous bodies the size of Jupiter or bigger.

HOPING FOR CLUES

Alan Boss, a Carnegie astrophysicist who specializes in studies of extrasolar planets but is not involved in the new observations, said the star KH 15D "might turn out to be a Rosetta stone for deciphering some of the mysteries of planet formation."

"It is not an exact analog of our own system," said Boss, but KH 15D gives astronomers a unique opportunity to gain understanding about the life and times of planets such as those in the sun's family.

Astronomers have previously photographed and studied protoplanetary disks around other stars closer to Earth. Although they cannot actually see the KH 15D disk, only its eclipses of the star, astronomers think the phenomenon should reveal rapid changes in circumstellar conditions that influence the creation of planets and how they migrate through the disk.

MORE WORK AHEAD

This may take years of further observations, data analysis and theoretical work, Herbst said. The mass and extent of the circumstellar disk cannot yet be determined. The eclipsing matter appears to be closer to the star than Mercury is to the sun.

"Is there 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?" Herbst asked at a news conference. "We think that's very possible."

Some astronomers think they see the effects of the dust and rocks clumping into one or possibly two blobs, which orbit close to the star and are responsible for obscuring most of its light.

What especially intrigues scientists is that they have detected changes in the blobs in recent months. No previous observations of protoplanetary disks were prolonged enough or sufficiently detailed to detect such structural changes over such a brief period.

Boss speculated that clumping around the star had already created a planet. "It would be fairly newly formed," he said, but the discovery of a planet around such a young star would force theorists to revise their models of planetary formation.

TAKING NOTICE IN '90s

Attention was first drawn to KH 15D in observations in the late 1990s, mostly by Wesleyan undergraduate students. They recognized that the star's light fades out every 48.3 days and stays faint, at about 4 percent of its maximum brightness, for 18 days. No single object such as another star could do such a thing, the astronomers reasoned, because it would have to be too huge to exist that close to the star.

As fascination with the eclipses spread, so did the observations. In the last year, other astronomers in Germany, Israel, Uzbekistan and the United States took to their telescopes.

These observations showed that alternate eclipses have slightly different shapes, the astronomers said, suggesting that there may be two clumps, similar but not identical, orbiting the star.

If such clumps or something like a hidden planet are determined to be in the disk, the Wesleyan astronomers said, their gravity could generate tides or waves running through the spreading dust and making the distant star wink at curious but puzzled scientists.

• Online: KH 15D: http://www.astro.wesleyan.edu/kh15d