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It`s wet out there

September 20, 2002

TANTALISING signs of water have been found in the atmospheres of planets orbiting distant stars. If the discovery is confirmed, it will fuel speculation that the Galaxy is teeming with life.

"This would be a historic discovery- the first detection of a prebiotic molecule in an extrasolar planet," says Cristiano Cosmovici of the Institute for Cosmic and Planetary Sciences in Rome, whose team made the discovery.

Cosmovici has looked for water near 17 stars, all of which are thought to have planetary systems or cometary clouds. His team used the 32-metre Medicina radio telescope near Bologna to look for water "maser" emissions, telltale microwaves that might come from water in a planet`s atmosphere when it`s bathed in the infrared light of its star.

Three of the planetary systems are producing these emissions, Cosmovici told the Second European Workshop on Exo/Astrobiology in Austria this week. One orbits the star Upsilon Andromedae, about 50 light years away. There are three planets in this system (see Graphic), with minimum masses of about 0.7, 2.1 and 4.6 times the mass of Jupiter. They`re all gas giants like Jupiter, although it`s possible that the system could also contain undetected rocky planets like Earth.

There are also signs of water near two much closer stars: Epsilon Eridani, a Sun-like star 10 light years away, and Lalande 21185, a red dwarf about 8 light years away. Between them they may have three planets with a similar mass to Jupiter, but the evidence is weaker than for Upsilon Andromedae`s planets.

"This result is astonishing if it`s true," says Geoff Marcy, a leading planet hunter from the University of California at Berkeley. Although having water doesn`t necessarily make a planet habitable, the result would at least show that one of the key chemicals for life is common on alien worlds. "Water`s at the top of the shopping list of ingredients for life," says Hugh Jones of Liverpool John Moores University, whose team announced a new Jupiter-mass planet this week. "This is a very exciting first step."

But these particular planets are unlikely to host life. "These gas giants presumably have no solid or liquid surface," says Tim Brown of the High Altitude Observatory in Boulder, Colorado. "Their atmospheres are probably inimical to life as we know it, and at least some of them are far too hot to give life a chance."

Nonetheless, water maser emission would allow astronomers to track the flow of gases in the planets` atmospheres. "This would tell us something about the winds that operate under the oddball circumstances these planets create," says Brown.

Everyone agrees that the results need careful checking. For one thing, says Brown, there`s a big question mark over whether Lalande 21185 and Epsilon Eridani really have planets at all. Jones worries that there`s a small chance the water signal is coming from the stars themselves, rather than any planets. He says that telescopes should be able to pinpoint the exact source of the radiation and resolve that question.

Marcy adds that he wouldn`t expect water maser emissions from the planets to be powerful enough for us to detect it anyway, so the finding would be surprising if it`s true. Then again, he adds, "It wouldn`t be the first time a surprising result came from extrasolar planets."
Hazel Muir

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