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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 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.

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Figure 1: Secondary eclipses of GJ 436b at six Spitzer wavelengths.
Figure 2: Broadband spectrum constraints for GJ 436b.
Figure 3: Contours showing the explored mixing ratios of methane.

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We thank the Spitzer staff for rapid scheduling; M. Gillon, A. Lanotte and T. Loredo for discussions; D. Wilson for contributed code; and A. Wright for manuscript comments. We thank the following for software: the Free Software Foundation, W. Landsman and other contributors to the Interactive Data Language Astronomy Library, contributors to SciPy, Matplotlib and the Python programming language, and the open-source community. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. This material is based on work supported by the US NSF and by the US NASA through an award issued by JPL/Caltech.

Author Contributions K.B.S. wrote the paper and Supplementary Information with contributions from J.H., N.M. and R.A.H.; N.M. and S.S. produced the atmospheric models; S.N., K.B.S. and W.C.B. reduced the data; K.B.S., J.H. and D.D. analysed the results; D.D. ran an independent analysis; R.A.H. produced the orbital parameter results; and J.H., K.B.S., S.N., R.A.H., E.R. and N.B.L. wrote the analysis pipeline.

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Correspondence to Kevin B. Stevenson.

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The authors declare no competing financial interests.

Additional information

The original data are available from the Spitzer Space Telescope archive, programs 30129 and 40685.

Supplementary information

Supplementary Information

This file contains Supplementary Information comprising: Centring and Photometry; Position Sensitivity; Time-Varying Sensitivity; Determining the Best Model and a Supplementary Discussion, Supplementary Figures 1–17 with legends, Supplementary Tables 1–9, and Supplementary References. (PDF 2534 kb)

Supplementary Data

This zipped file contains Supplementary Data si1-si6, which show Spitzer lightcurves in digital form. Wavelengths used are 3.6µm (si1a-c), 4.5µm (si2), 5.8µm (si3), 8.0µm (si4), 16µm (si5) and 24µm (si6). (ZIP 26575 kb)

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Stevenson, K., Harrington, J., Nymeyer, S. et al. Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b. Nature 464, 1161–1164 (2010).

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