Senior star may have comets.

Astronomers report what could be the first evidence of water-bearing objects that orbit a star other than our sun. Since water is an essential ingredient for life, this finding could increase the odds that life exists outside the solar system.

The researchers detected a surprisingly high abundance of water vapor in the vicinity of a bloated, elderly star called CW Leonis. The composition of the star strongly suggests that the vapor does not come from CW Leonis itself, says Gary J. Melnick of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. The star contains much more carbon than oxygen, and the carbon would grab any available oxygen atoms to form carbon monoxide. That would leave little oxygen to combine with hydrogen to form water, he notes.

Where did the water come from? In the July 12 NATURE, Melnick's team proposes that the vapor it detected with the Submillimeter Wave Astronomy Satellite originated in a swarm of icy bodies--comets--that might be orbiting CW Leonis.

As part of its aging process, CW Leonis recently increased its girth at least several hundredfold--equivalent to our sun growing so voluminous that it would bump against Jupiter. The heat of the swollen star could easily turn an outlying population of comets into steam, Melnick's team says. The researchers calculate that this vaporization could last a million years.

To account for the team's observation, CW Leonis must have vaporized several hundred billion comets lying at distances from the star ranging from 75 to 300 times Earth's distance from the sun. The water vapor's total mass, about 10 times that of Earth, would approximate that of the Kuiper belt--a reservoir of comets in our solar system that lies beyond the orbit of Neptune. If CW Leonis does indeed have a belt of comets, which are one of the building blocks of planets, the star might have an entire planetary system, the team proposes.

All that conjecture may seem a bit much to eke out of observations of water vapor around a carbon-rich star, comments theorist Alan R Boss of the Carnegie Institution of Washington (D.C.). But the finding "really is best explained by an unseen population of comets," he says.

Water vapor might serve as a sign for recognizing dying stars that have orbiting planets, Boss suggests. Scientists already know how to detect younger stars with planets (SN: 6/16/01, p. 375).

Melnick says the best bet for follow-up studies is the European Space Agency's Herschel Space Observatory, a far-infrared and submillimeter telescope scheduled for launch in 2007. That observatory is expected to determine the location of the water vapor around CW Leonis. If the vapor really originates in a reservoir of comets, the highest water concentration should lie at a distance from the center of CW Leonis comparable to that of the Kuiper belt from the sun.