A Methane-Free GJ 436b?

Details A Methane-Free GJ 436b? A Methane-Free GJ 436b?

Oct 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.6206h&link_type=abstract

American Astronomical Society, DPS meeting #42, #62.06; Bulletin of the American Astronomical Society, Vol. 42, p.1090

Astronomy and Astrophysics

Astronomy

Scientific paper

We observed (Stevenson et al. 2010, Nature 646, 1161-1164) six secondary eclipses of GJ 436b, a hot Neptune-sized planet orbiting an M-dwarf star, with the Spitzer Space Telescope in six photometric channels (3.6, 4.5, 5.8, 8, 16, 24 µm). Eclipse depths were 0.041 ± 0.003, <0.010, 0.033 ± 0.014, 0.054 ± 0.008, 0.140 ± 0.027, and 0.175 ± 0.041 and brightness temperatures were 1,120 ± 20, <700, 720 ± 110, 740 ± 40, 980 ± 130, and 960 ± 170 K, respectively, with the 4.5 µm non-detection given as a 3-sigma upper limit. The unexpectedly strong signal at 3.6 µm could indicate a thermal inversion and CH4 in emission, but an inversion would also cause much higher signals than observed in the 4.5, 5.8, and 8 µm bands, due to CO (and/or CO2), H2O, and CH4, respectively. We are driven to the uncomfortable conclusion, based on extensive modeling of millions of candidate spectra, that there is no inversion, H2O is in absorption, CO is abundant, and CH4, which equilibrium chemistry predicts to be abundant at these temperatures in an H2-dominated atmosphere, is somehow depleted. Photochemical destruction of CH4 in the UV-rich irradiation from the M-dwarf star is a possible depletion mechanism. Vertical mixing could explain the presence of CO (Madhusudhan & Seager 2010, arXiv:1004.5121v1). We discuss the recent transit photometry of Beaulieu et al. (2010, arXiv:1007.0324v1), which detects CH4 at the limb. Spitzer is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA, which provided support for this work.

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