All wet?
Hubble data suggests moisture in the atmosphere of extrasolar planet HD209458b, but not everyone buys it.

Alien Water Find Iffy

An astronomer using data collected by the Hubble Space Telescope claims he has found the first evidence of water on a planet outside our solar system. But the scientist who pioneered the investigative technique says the water may just be a mirage.

As far as anyone knows, water is the key to life. Space missions sent to Mars, Jupiter, and Saturn in recent years have found signs of water--present or past--on those planets or their moons (ScienceNOW, 13 December 2006). At the same time, astronomers have been scanning more than 200 known extrasolar planets for signs of H2O, but until now its existence had not been confirmed outside our solar system.

The new discovery was made by Travis Barman of Lowell Observatory in Flagstaff, Arizona. Barman examined spectral data collected by the Hubble Space Telescope for HD209458b, which is a transiting planet, meaning it passes between Earth and the parent star. During each passage, which occurs about every 3.5 days, HD209458b blocks part of the light of the star. At the same time, the planet's atmosphere absorbs specific parts of the spectrum of that starlight, and that property allows astronomers to identify the gases in the atmosphere. Previously, researchers had discovered sodium and hydrogen in HD209458b's atmosphere using the technique, but in a paper accepted for publication in the Astrophysical Journal, Barman used a computer model he has developed to interpret previously published data. That interpretation, he says, shows the telltale signature of water molecules absorbing the starlight.

That's a possibility but not yet certain, says astrophysicist David Charbonneau of the Harvard Center for Astrophysics in Cambridge, Massachusetts, whose data and technique formed the basis for Barman's conclusions. The problem, Charbonneau says, is that interpretation of spectral information from an extrasolar planet requires more precision and stability than may be possible with current instrumentation. Inherent variations in the Hubble's spectrograph mean that "we can't determine whether those variations are due to the spectrograph itself or originate in the planet's atmosphere," Charbonneau says. He says additional measurements are required. But they will have to wait, because the Hubble spectrograph has stopped functioning, so either it needs to be repaired or researchers must arrange to use an alternative space- or ground-based telescope.

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