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.
Subscribe to Journal
Get full journal access for 1 year
only $3.90 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
Gillon, M. et al. Detection of transits of the nearby hot Neptune GJ 436 b. Astron. Astrophys. 472, L13–L16 (2007)
Torres, G. The transiting exoplanet host star GJ 436: a test of stellar evolution models in the lower main sequence, and revised planetary parameters. Astrophys. J. 671, L65–L68 (2007)
Deming, D. et al. Spitzer transit and secondary eclipse photometry of GJ 436b. Astrophys. J. 667, L199–L202 (2007)
Demory, B.-O. et al. Characterization of the hot Neptune GJ 436 b with Spitzer and groundbased observations. Astron. Astrophys. 475, 1125–1129 (2007)
Pont, F., Gilliland, R. L., Knutson, H., Holman, M. & Charbonneau, D. Transit infrared spectroscopy of the hot Neptune around GJ 436 with the Hubble Space Telescope. Mon. Not. R. Astron. Soc. 393, L6–L10 (2009)
Burrows, A. & Sharp, C. M. Chemical equilibrium abundances in brown dwarf and extrasolar giant planet atmospheres. Astrophys. J. 512, 843–863 (1999)
Figueira, P. et al. Bulk composition of the transiting hot Neptune around GJ 436. Astron. Astrophys. 493, 671–676 (2009)
Rogers, L. A. & Seager, S. A framework for quantifying the degeneracies of exoplanet interior compositions. Astrophys. J. 712, 974–991 (2010)
Saumon, D. et al. Ammonia as a tracer of chemical equilibrium in the T7.5 dwarf Gliese 570D. Astrophys. J. 647, 552–557 (2006)
Zahnle, K., Marley, M. S. & Fortney, J. J. Thermometric soots on hot Jupiters? Preprint at 〈http://arxiv.org/abs/0911.0728〉 (2009)
Werner, M. W. et al. The Spitzer Space Telescope mission. Astrophys. J. Suppl. Ser. 154, 1–9 (2004)
Ford, E. B. Quantifying the uncertainty in the orbits of extrasolar planets. Astrophys. J. 129, 1706–1717 (2005)
Knutson, H. A., Charbonneau, D., Allen, L. E., Burrows, A. & Megeath, S. T. The 3.6–8.0 μm broadband emission spectrum of HD 209458b: evidence for an atmospheric temperature inversion. Astrophys. J. 673, 526–531 (2008)
Harrington, J., Luszcz, S., Seager, S., Deming, D. & Richardson, L. J. The hottest planet. Nature 447, 691–693 (2007)
Deming, D., Seager, S., Richardson, L. J. & Harrington, J. Infrared radiation from an extrasolar planet. Nature 434, 740–743 (2005)
Caceres, C. et al. High cadence near infrared timing observations of extrasolar planets: I. GJ 436b and XO-1b. Astron. Astrophys. 507, 481–486 (2009)
Maness, H. L. et al. The M dwarf GJ 436 and its Neptune-mass planet. Publ. Astron. Soc. Pacif. 119, 90–101 (2007)
Madhusudhan, N. & Seager, S. A temperature and abundance retrieval method for exoplanet atmospheres. Astrophys. J. 707, 24–39 (2009)
Lodders, K. & Fegley, B. Atmospheric chemistry in giant planets, brown dwarfs, and low-mass dwarf stars. I. Carbon, nitrogen, and oxygen. Icarus 155, 393–424 (2002)
Zahnle, K., Marley, M. S., Freedman, R. S., Lodders, K. & Fortney, J. J. Atmospheric sulfur photochemistry on hot Jupiters. Astrophys. J. 701, L20–L24 (2009)
Swain, M. R. et al. A ground-based near-infrared emission spectrum of the exoplanet HD189733b. Nature 463, 637–639 (2010)
Cho, J., Menou, K., Hansen, B. M. S. & Seager, S. Atmospheric circulation of close-in extrasolar giant planets. I. Global, barotropic, adiabatic simulations. Astrophys. J. 675, 817–845 (2008)
Alonso, R. et al. Limits to the planet candidate GJ 436c. Astron. Astrophys. 487, L5–L8 (2008)
Castelli, F. & Kurucz, R. L. New grids of ATLAS9 model atmospheres. Preprint at 〈http://arxiv.org/abs/astro-ph/0405087〉 (2004)
Bean, J. L., Benedict, G. F. & Endl, M. Metallicities of M dwarf planet hosts from spectral synthesis. Astrophys. J. 653, L65–L68 (2006)
Swain, M. R. et al. Molecular signatures in the near-infrared dayside spectrum of HD 189733b. Astrophys. J. 690, L114–L117 (2009)
Atreya, S. K., Mahaffy, P. R., Niemann, H. B., Wong, M. H. & Owen, T. C. Composition and origin of the atmosphere of Jupiter — an update, and implications for the extrasolar giant planets. Planet. Space Sci. 51, 105–112 (2003)
Lodders, K. & Fegley, B. Jr. The origin of carbon monoxide in Neptune’s atmosphere. Icarus 112, 368–375 (1994)
Mandel, K. & Agol, E. Analytic light curves for planetary transit searches. Astrophys. J. 580, L171–L175 (2002)
Reiners, A. Activity-induced radial velocity jitter in a flaring M dwarf. Astron. Astrophys. 498, 853–861 (2009)
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.
The authors declare no competing financial interests.
The original data are available from the Spitzer Space Telescope archive, programs 30129 and 40685.
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)
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)
About this article
Cite this article
Stevenson, K., Harrington, J., Nymeyer, S. et al. Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b. Nature 464, 1161–1164 (2010). https://doi.org/10.1038/nature09013
Beyond Equilibrium Temperature: How the Atmosphere/Interior Connection Affects the Onset of Methane, Ammonia, and Clouds in Warm Transiting Giant Planets
The Astronomical Journal (2020)
Research in Astronomy and Astrophysics (2020)
Implications of three-dimensional chemical transport in hot Jupiter atmospheres: Results from a consistently coupled chemistry-radiation-hydrodynamics model
Astronomy & Astrophysics (2020)
Monthly Notices of the Royal Astronomical Society (2020)
Molecular cross-sections for high-resolution spectroscopy of super-Earths, warm Neptunes, and hot Jupiters
Monthly Notices of the Royal Astronomical Society (2020)