Will the second time be the charm? After the apparent discovery of water on a planet outside our solar system was met with skepticism 3 months ago, an international team of researchers now claims to have found water on a different world using a more powerful method. If the find holds up, it could provide the means of locating the liquid on other alien worlds.
Most scientists think that life cannot exist without water. That's why, ever since astronomers confirmed the first planet outside of our solar system in 1995, they have been looking for signs of water on the 200-plus exoplanets now known. Earlier this year, planetary scientist Travis Barman of Lowell Observatory in Flagstaff, Arizona, claimed in the Astrophysical Journal to have found evidence of water in the atmosphere of an exoplanet (ScienceNOW, 11 April). But researchers cautioned that the find was inconclusive because some of the data could have resulted from signal noise picked up by the detector.
The researchers, led by Giovanna Tinetti based at the Institut d'Astrophysique in Paris, France, say their new analysis is more reliable because the observations were taken in infrared light, which is far better at detecting light-absorption patterns unique to water. Using NASA's Spitzer Space Telescope, they examined the light spectrum from a gas-giant planet called HD 189733b, whose orbit passes in front of a star located about 64 light-years away in the constellation Vulpecula. The team then plugged their observations into a computer program containing more than 500 million specific light-absorption "signatures." The computer confirmed that the spectral data matched that of water vapor.
Even this analytical technique has limitations. Reporting in the 12 July Nature, the scientists note that they can't tell the density of water vapor in HD 189733b's atmosphere--in other words, whether it is present in only trace amounts or at much higher levels. Astrophysicist and team member Robert Barber of University College London in the U.K. wrote in an e-mail that he hopes "future observations will enable us to compute the actual amount of water vapor present" in the planet's atmosphere, as well as the atmospheres of other exoplanets, which they are planning to study.
But other experts say there's a big difference between water vapor, as discovered in HD 189733b's atmosphere, and liquid water, which is what astronomers are really hoping to nail down. The new findings are "entirely reasonable," but they fall short of the main goal of the search, says astrobiologist Margaret Turnbull of the Space Telescope Science Institute in Baltimore, Maryland. Water vapor "is actually fairly common in the universe," she says, "what is rare is liquid water, and that's the key to life as we know it."