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Methane mystery
on Gliese 436b

Posted: 08 September 2010

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Observations by NASA’s Spitzer Space Telescope of light from the exoplanet Gliese 436b have identified an atmosphere filled with carbon monoxide but exhibiting a puzzling lack of methane, says Professor Joseph Harrington of the University of Central Florida, who presented the results at the Exoclimes conference at the University of Exeter this week.

Artwork depicting the methane depleted Gliese 436b and its parent star. Image: NASA/JPL–Caltech/R Hurt (SSC/Caltech).

Gliese 436b is a Neptune-sized planet orbiting at a distance of just 4.3 million kilometres from its cool red dwarf star. A transiting planet, Gliese 436b was discovered in 2004, thirty light years from Earth, and can be seen transiting the red dwarf. It has subsequently become one of the first planets to be investigated as part of a dedicated ‘secondary eclipse’ observing programme by Spitzer, which observes at infrared wavelengths. A secondary eclipse occurs when a planet moves behind its star; just before it disappears the dayside of the planet is facing us, and the dayside light is mixed in with the star’s light. By subtracting the light of the star alone (when the planet is behind it) from the light of the star and planet together, we are left with just the light of the planet, which contains distinct emission lines from atoms and molecules within its atmosphere. When Spitzer conducted secondary eclipse observations of Gliese 436b, what it saw was puzzling, says Harrington.

Theoretical models suggest that a planet like Gliese 436b, which has an average atmospheric temperature of a little over 400 degrees Celsius, should have significant amounts of methane in their hydrogen-dominated atmospheres, and no carbon monoxide. “We see exactly the opposite,” says Harrington, who published the results with his PhD student and lead author Kevin Stevenson in the 22 April edition of Nature. Methane characteristically appears at an infrared wavelength of 3.6 microns (a micron is a millionth of a metre), and yet it was at a level 7,000 times less than what it should have been. “It’s absence was conspicuous,” he adds.

Additionally, carbon monoxide molecules were surprisingly abundant at 4.5 microns, but the trouble is that carbon monoxide should not become dominant until temperatures begin to exceed 900 degrees Celsius. “Carbon, when it is cold, likes to hold onto hydrogen, but if it is hotter it likes to throw off the the hydrogen and steal oxygen from, say, water molecules, to make carbon monoxide,” explains Harrington.

So, what is happening on Gliese 436b? Harrington suggests that there could be methane on the dark side of the planet that we can’t see, and as winds blow it around to the dayside, it is destroyed by ultraviolet radiation from the star. It may be possible to prove this by detecting the star’s light filtered through the planet’s atmosphere as it transits. From our point of view, the atmosphere would be detectable as a thin ring around the silhouette of the planet. This cross-section of the atmosphere is on the day/night border, and it is possible that methane could exist here, having not yet had chance to have been destroyed by ultraviolet radiation. If methane does exist, it would absorb some of the starlight, creating giveaway absorption lines in the star’s spectrum. Meanwhile, the carbon monoxide may be forming deeper within the planet, where temperatures are higher, before upwelling to the surface thanks to complex atmospheric flows.

Results from several other planets are set to be published in the next few months, says Harrington – what surprises will they have in store?