Water Found in Extrasolar Planet's Atmosphere

This is an artist's impression of the gas-giant planet orbiting the yellow, Sun-like star HD 209458, 150 light-years from Earth. Astronomers used NASA's Hubble Space Telescope to look at this world and make the first direct detection of an atmosphere around an extrasolar planet.

Astronomers have detected water in the atmosphere of a planet outside our solar system for the first time.

The finding, to be detailed in an upcoming issue of Astrophysical Journal, confirms previous theories that say water vapor should be present in the atmospheres of nearly all the known extrasolar planets. Even hot Jupiters, gaseous planets that orbit closer to their stars than Mercury to our Sun, are thought to have water.

The discovery, announced today, means one of the most crucial elements for life as we know it can exist around planets orbiting other stars.

“We know that water vapor exists in the atmospheres of one extrasolar planet and there is good reason to believe that other extrasolar planets contain water vapor,” said Travis Barman, an astronomer at the Lowell Observatory in Arizona who made the discovery.

HD209458b is a world well-known among planet hunters. In 1999, it became the first planet discovered around a normal star outside our solar system and, a few years later, was the first exoplanet confirmed to have oxygen and carbon in its atmosphere.

HD209458b is separated from its star by only about 4 million miles (7 million kilometers)—about 100 times closer than Jupiter is to our Sun—and is so hot scientists think about it is losing about 10,000 tons of material every second as vented gas.

Thus, despite the discovery of water in its atmosphere, it is unlikely that any life exists on or in the gaseous planet.

Using a combination of previously published Hubble Space Telescope measurements and new theoretical models, Barman found strong evidence for water absorption in the atmosphere of the extrasolar planet HD209458b.

Barman took advantage of the fact that HD209458b is a so-called “transiting planet,” meaning it passes directly in front of its star as seen from Earth. It transits every three-and-a-half days.

When this happens, water vapor in the planet’s atmosphere magnifies some of the star’s infrared light relative to its visible light.

Barman found the water signature after applying new theoretical models he developed to visible and infrared Hubble data collected by Harvard student Heather Knutson last year.

“It is encouraging that theoretical predictions of water in extrasolar planets seem to agree reasonably well with observations,” Barman said.

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