Astronomers find 'winking' star

The star was seen to dim (right) regularly

By Dr David Whitehouse BBC News Online science editor

Astronomers have observed a star being eclipsed in a way never seen before - not by another star or planet, but by dust grains, rocks and maybe even asteroids orbiting in a clumpy circumstellar disc.

The discovery enables for the first time the detailed structure of such a disc to be studied. Researchers believe it will inform them about how the Earth and our Solar System formed.

The star, named KH 15D, is in the constellation of Monoceros, and is about 2,400 light-years away. It is part of a well known cluster of young stars called NGC 2264 that reside in a nebulous region close to the famous Cone Nebula.

Such regions are stellar nurseries and KH 15D has all the markings of youth. It is estimated to be just three million years old.

Regular timetable

Observations made at the Van Vleck Observatory during the late 1990s led Kristin Kearns and William Herbst of Wesleyan University to realise that KH 15D was a potentially unique and important object.

Scientist hope to better understand how our Solar System was formed

"Basically, the star winked at us," said Herbst. "On most nights, it was at its standard brightness, but sometimes it would be nearly gone - shining by only a tiny fraction of its normal luminosity."

After several years of study, the researchers recognised a pattern to the star's behaviour - it fades out every 48.3 days and stays faint for about 18 days.

This indicated that something was orbiting the star and obscuring its light on a regular timetable. This is not an uncommon event in astronomy - there are many known examples of eclipsing binary stars.

However, what was unique in the case of KH 15D was the length of the eclipse as well as its depth.

Close to star

The star was totally obscured for more than one third of the period of the orbiting matter. No single object such as a star or planet could do such a thing, since it would require an object much too large to fit in the space available.

The researchers realised that only a collection of smaller objects - dust grains, rocks or perhaps asteroids, orbiting together in a strung out, clumpy arc - could possibly explain such a lengthy eclipse.

The eclipsing material is closer to the star than Mercury is to our Sun. Astronomers have never directly measured material this close to a star.

The newly studied system could eventually shed light on the earliest stages of planet formation, said Alan Boss of the Carnegie Institution.

"This gives us a chance to see what's happening right down in the maelstrom where planets like Earth form," he added.