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Far-Out Chemistry! Detecting an Extrasolar Planet's Atmosphere

© Linda Mamassian

Not all chemical research takes place in a laboratory, and although much chemical analysis nowadays uses sophisticated instrumentation, sometimes it's surprising just how "far out" chemical studies can be.

As a chemist with a strong interest in astronomy, I couldn't resist writing about the recent detection and chemical analysis of the atmosphere of a planet outside our solar system by instruments on the Hubble Space Telescope.

David Charbonneau, a former graduate student fellow at the National Center for Atmospheric Research, and NCAR's Timothy Brown used the Hubble telescope's spectrometer to detect the presence of sodium in the extrasolar planet's upper atmosphere. Their unique observations from NASA's space-based Hubble telescope demonstrate that it is possible to measure the chemical makeup of extrasolar planetary atmospheres - and potentially to search for chemical markers of life beyond Earth.

The planet in question is a Jupiter-sized world orbiting HD 209458, a yellow, Sun-like star that lies 150 light-years away in the constellation Pegasus. The planet was discovered in 1999, when its slight gravitational tug on its star was detected. Its mass is estimated to be about 70 percent of Jupiter's, or 220 times more massive than Earth. The planet's orbit makes it pass in front of the star, as seen from Earth. The amount of light from the star decreases very slightly during the planet's transit. Transit observations have shown that the planet is primarily gaseous rather than liquid or solid (because it is less dense than water), indicating that it is a gas giant like Jupiter or Saturn.

The planet's orbit is just four million miles (6.4 million kilometers) from the surface of the star, and a transit occurs every 3.5 days, making it a good target for repeat observations. The planet is classified as a "Hot Jupiter" with an atmospheric temperature estimated at 2,000 degrees Fahrenheit (1,100 degrees Celsius) - much too hot for life.

In the spring of 2000, Charbonneau and Brown observed four transits of the planet across HD 209458 hoping to find evidence of an atmosphere, and they succeeded! During each transit, a fraction of one percent of the star's light passed through the planet's atmosphere, and some of the light was absorbed. The starlight changed as it was filtered through the atmosphere, providing a "fingerprint" of the elements and compounds present. A spectrograph on the Hubble separated the light into its component colors based on wavelength.