The exoplanet GJ 436b has left scientists confused after defying their assumptions about the composition of its atmosphere.

Neptune-sized planets as hot as 800 Kelvin -- about 1,000 degrees Fahrenheit -- should contain high levels of methane and very little carbon monoxide. Instead, the researchers found 7,000 times less methane than expected and plenty of carbon monoxide.

Using NASA's Spitzer Space Telescope, researchers measured the dimming of light as GJ 436b passed behind its star and re-emerged. The difference in the two light levels -- measured six times at different infrared wavelengths -- represents the light emitted by the planet itself.

Size comparison of Gliese 436 b with Neptune
(photo credit: wikipedia)

The resulting data were used to determine what molecules make up the planet's atmosphere. MIT Planetary Scientists simulated millions of chemical mixes under the planet's conditions to find the ones that best matched the UCF data.

The findings surprised researchers. "It's like dipping bread into beaten eggs, frying it and getting oatmeal," said UCF Physics Professor Joseph Harrington.

The unexpected result puts GJ 436b in good company. "If you were looking at Earth from afar, you would be surprised to see oxygen gas in its atmosphere," Harrington said. "Oxygen reacts with surface materials and other gases, so you need something that continually produces it."

That something is Earth's abundant plant life. Oxygen is a "biosignature," or an indicator of life, Harrington says.

Using similar techniques to that of the UCF study, astronomers will seek oxygen and other biosignatures on habitable worlds that they soon expect to discover.

Citation: Stevenson et al., 'Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b', Nature, April 2010, 464, 1161-1164; doi:10.1038/nature09013