First Sign of Water on Planet Outside Our System

Anne Minard
for National Geographic News
April 10, 2007
For the first time, astronomers have detected water in the atmosphere of a planet outside our solar system.

Astronomer Travis Barman announced today that he has discovered water around planet HD209458b by combining theoretical models with observations from the Hubble Space Telescope.

The results don't necessarily figure into the search for life on other planets, but they go far to reassure many astronomers that their predictions are on track, said Barman, of Lowell Observatory in Flagstaff, Arizona.

"It means that our theoretical understanding of these planets is in the ballpark," he said. "We understand enough about them to predict that water should be there, and then water is there."

Scientists have predicted water vapor in the atmospheres of most planets orbiting other stars.

The results are a "confidence booster," Barman said. They were recently accepted for publication in the Astrophysical Journal.

Planets in Transit

HD209458b and its star, HD209458, are 150 light-years from Earth in the constellation Pegasus.

Discovered in 1999, this was the first planetary system positioned in a way that allows scientists to observe the atmosphere of a planet outside our solar system. As of today, though, HD209458b is one of 14 such "transit systems."

Planets in transit systems pass between Earth and their host stars, allowing astronomers to calculate the planets' masses and measure their atmospheric compositions.

HD209458b, as seen from Earth, passes directly in front of its star every three and half days.

Using telescopes on the ground and in space, astronomers have heavily scrutinized the system.

Some of the more recently discovered planetary systems may be better candidates for such analysis, Barman said. But, he added, Hubble captured data from HD209458b with an instrument that is no longer working—so the particular kind of measurement used is no longer possible for other systems using Hubble.

Hot Jupiters

The mass of planet HD209458b is 220 times greater than Earth's. Like Jupiter, HD209458b is a gaseous planet, though it is only about seven-tenths the mass of our solar system's largest planet.

HD209458b orbits its star more closely than Mercury orbits our sun.

These traits put it into a class with other so-called hot Jupiters, which make up about 40 percent of known planets orbiting other stars. (Related: "'Hot Jupiters' Could Give Rise to Earthlike Worlds, Study Says" [September 7, 2006].)

Astronomers are interested in hot Jupiters because their size makes them the easiest exoplanets to study—and because their violent gravitational dynamics may both create and destroy Earth-size planets.

Jeremy Richardson, of NASA's ExoPlanets and Stellar Astrophysics Lab in Maryland, was lead author of an HD209458b report in the journal Nature in February.

Richardson's study found no evidence of water in the atmosphere.

But he said his team's observations were made using different techniques than Barman's—so they're not necessarily contradictory.

"The Barman paper looks at observations taken during transit," Richardson said.

"Our observations were made just before the planet disappears behind the star."

Other scientists, he said, have suggested that winds can obscure signs of water at this stage of a planet's orbit.

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