Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b

The nearby extrasolar planet GJ 436b-which has been labelled as a 'hot Neptune'-reveals itself by the dimming of light as it crosses in front of and behind its parent star as seen from Earth. Respectively known as the primary transit and secondary eclipse, the former constrains the planet's radius and mass(1,2), and the latter constrains the planet's temperature(3,4) and, with measurements at multiple wavelengths, its atmospheric composition. Previous work(5) using transmission spectroscopy failed to detect the 1.4-mu m-water vapour band, leaving the planet's atmospheric composition poorly constrained. Here we report the detection of planetary thermal emission from the dayside of GJ 436b at multiple infrared wavelengths during the secondary eclipse. The best-fit compositional models contain a high CO abundance and a substantial methane (CH4) deficiency relative to thermochemical equilibrium models(6) for the predicted hydrogen-dominated atmosphere(7,8). Moreover, we report the presence of some H2O and traces of CO2. Because CH4 is expected to be the dominant carbon-bearing species, disequilibrium processes such as vertical mixing(9) and polymerization of methane(10) into substances such as ethylene may be required to explain the hot Neptune's small CH4-to-CO ratio, which is at least 10(5) times smaller than predicted(6).