Nature 464, 1161-1164 (22 April 2010) | doi:10.1038/nature09013; Received 18 November 2009; Accepted 5 March 2010

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

Kevin B. Stevenson1, Joseph Harrington1, Sarah Nymeyer1, Nikku Madhusudhan2, Sara Seager2, William C. Bowman1, Ryan A. Hardy1, Drake Deming3, Emily Rauscher4 & Nate B. Lust1

  1. Planetary Sciences Group, Department of Physics, University of Central Florida, Orlando, Florida 32816, USA
  2. Department of Physics and Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
  3. Planetary Systems Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
  4. Department of Astronomy, Columbia University, New York, New York 10027, USA

Correspondence to: Kevin B. Stevenson1 Correspondence and requests for materials should be addressed to K.B.S. (Email: kevin218@knights.ucf.edu).

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 mass1, 2, and the latter constrains the planet’s temperature3, 4 and, with measurements at multiple wavelengths, its atmospheric composition. Previous work5 using transmission spectroscopy failed to detect the 1.4-μm 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 models6 for the predicted hydrogen-dominated atmosphere7, 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 mixing9 and polymerization of methane10 into substances such as ethylene may be required to explain the hot Neptune’s small CH4-to-CO ratio, which is at least 105 times smaller than predicted6.