Original URL: http://www.theregister.co.uk/2007/02/21/spitzer_extrasolar_planet_spectra/

NASA spies on dry, dusty worlds

Tooling up in the hunt for aliens

By Christopher Williams

Posted in Science, 21st February 2007 22:38 GMT

For the first time, astronomers have measured the composition of a planet outside the solar system, in what NASA says is a key step in its quest for extraterrestrial life.

NASA pointed its Spitzer Space Telescope at two gas giant exoplanets in the Vulpecula and Pegasus constellations, 370 and 904 trillion miles away, for long enough that detailed measurements of the infrared spectrum of light reflected by the planet could be made.

In a paper published in this week's Nature, the team based at the California Institute of Technology in Pasadena report two surprises from the data. First, the snappily named HD 209458b and HD 189733b seem much drier than calculations predicted - the spike in the spectrum where the water should show itself was not as big as the models said it should be.

Lead author of the study Jeremy Richardson said: "It is virtually impossible for water, in the form of vapor, to be absent from the planet, so it must be hidden."

Happily, the second surprise provided an answer to the conundrum. The spectrum from HD 209458b revealed that the water could be being masked by the planet's upper atmosphere, which is thick with dusts clouds formed from tiny silicate particles similar to sand. Nothing similar to the clouds is present in our solar system.

To make the new observations, the team monitored the change in the the light from each system when the planet as it was "eclipsed" by its star. Jet Propulsion Laboratory scientist Mark Swain, who also worked on the data, said: "With these new observations, we are refining the tools that we will one day need to find life elsewhere if it exists. It's sort of like a dress rehearsal." The plan now is to apply the technique to rocky extrasolar planets.

More than 200 extrasolar planets have been detected since the first definitive discovery was made in 1995 through measuring gravitational "wobbles" experienced by the star 51 Pegasi. The new Spitzer data is the first time astronomers have been able to test their predictions about the "hot Jupiter" worlds, which have been the most common type detected because of their huge mass and proximity to their star. Earlier Hubble images collected only basic information on the presence of elements like carbon and oxygen on hot Jupiters. ®