Space telescope analyses exoplanetary atmospheres

Stephen Withers
Thursday, 22 February 2007 07:27

Science - Space

NASA scientists have for the first time identified constituents of the atmosphere of an exoplanet (a planet outside our solar system) by using the Spitzer space telescope.

Spitzer is equipped with a spectrograph which splits light into different wavelengths. Different chemicals are associated with particular patterns of wavelengths, so analysis of this data indicates the makeup of the object being observed.

The problem is capturing sufficient light reflected by an exoplanet to allow such analysis. "This is an amazing surprise," said Spitzer project scientist Michael Werner of NASA's Jet Propulsion Laboratory. "We had no idea when we designed Spitzer that it would make such a dramatic step in characterizing exoplanets."

The 'trick' is that Spitzer is an infrared telescope, and planets are relatively bright in these wavelengths so they are not swamped by the light from their stars as they are in the visible wavelengths. When one of these planet passes behind its sun, the dip in the infrared light is big enough to be detected, and the 'missing' light must be that coming from the planet. Subtracting one set of measurements from the other provides the spectrum of the planet.

Two planets have been studied in this way, both gas giants similar to Jupiter but closer to the stars they orbit, and therefore hotter. One is HD 189733b (370 trillion miles away in the constellation Vulpecula), the other HD 209458b (904 trillion miles away in the constellation Pegasus).

The big surprise was that water was not detected in either planet's atmosphere even though theorists expected it to be plentiful. "The theorists' heads were spinning when they saw the data," said Jeremy Richardson of NASA's Goddard Space Flight Center.

But silicates - tiny sand grains - were detected in HD 209458b, leading scientists to suggest that the water molecules are obscured by dust clouds.  "It is virtually impossible for water, in the form of vapor, to be absent from the planet, so it must be hidden, probably by the dusty cloud layer we detected in our spectrum," said Richardson.

"When we first set out to make these observations, they were considered high risk because not many people thought they would work," said Carl Grillmair of NASA's Spitzer Science Center at the California Institute of Technology, who led the team that captured the spectrum of HD 189733b. "But Spitzer has turned out to be superbly designed and more than up to the task."

The project's achievements for 2006 included the first day and night temperature measurements of an exoplanet, and the discovery of nearly 2300 planet-forming discs within the Orion nebula.

Launched in August 2003, Spitzer was named in honour of astrophysicist Lyman Spitzer Jr, who (among other things) originally proposed the idea of large space telescopes.