2011/04/30 by Eliza M.-R. Kempton, Eliza Miller-Ricci Kempton, Kevin Zahnle +1 · 177 citations
Chemistry · Physics and Astronomy · #Astrobiology #Astrophysics #Astrophysics and Star Formation Studies #Atmosphere (unit) #Atmospheric chemistry #Atmospheric physics #Atmospheric sciences #Chemical physics #Chemistry #Haze #Hydrogen #Meteorology #Methane #Organic chemistry #Ozone #Physics #Planet #Spectroscopy and Laser Applications #Stellar, planetary, and galactic studies #astro-ph.EP
paper · pdf · doi:10.1088/0004-637x/745/1/3
published in The Astrophysical Journal 745(1), 3 (IOP Publishing) · Accepted to ApJ; 32 pages, 8 figures, 1 table
arxiv created 2011/12/22 · openalex publication_date 2011/12/27 · arxiv updated 2015/05/28 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
Recent observations of the transiting super-Earth GJ 1214b reveal that its atmosphere may be hydrogen-rich or water-rich in nature, with clouds or hazes potentially affecting its transmission spectrum in the optical and very-near-IR. Here, we further examine the possibility that GJ 1214b does indeed possess a hydrogen-dominated atmosphere, which is the hypothesis that is favored by models of the bulk composition of the planet. We study the effects of non-equilibrium chemistry (photochemistry, thermal chemistry, and mixing) on the planet's transmission spectrum. We furthermore examine the possibility that clouds could play a significant role in attenuating GJ 1214b's transmission spectrum at short wavelengths. We find that non-equilibrium chemistry can have a large effect on the overall chemical composition of GJ 1214b's atmosphere, however these changes mostly take place above the height in the atmosphere that is probed by transmission spectroscopy. The effects of non-equilibrium chemistry on GJ 1214b's transmission spectrum are therefore minimal, with the largest effects taking place if the planet's atmosphere has super-solar metallicity and a low rate of vertical mixing. Interestingly, we find that the best fit to the observations of GJ 1214b's atmosphere in transmission occurs if the planet's atmosphere is deficient in CH 4 , and possesses a cloud layer at a pressure of ∼200 mbar. This is consistent with a picture of efficient methane photolysis, accompanied by formation of organic haze that obscures the lower atmosphere of GJ 1214b at optical wavelengths. However, for methane to be absent from GJ 1214b's transmission spectrum, UV photolysis of this molecule must be efficient at pressures of greater than ∼1 mbar, whereas we find that methane only photolyzes to pressures less than 0.1 mbar, even under the most optimistic assumptions. An alternative explanation of the observations of GJ 1214b is that the atmosphere is water-rich, although this interpretation conflicts with the findings of Croll et al., who measure a low mean molecular weight for the planet's atmosphere. Additional observations at wavelengths corresponding to mid-IR water and methane features in GJ 1214b's transmission spectrum should break the degeneracy between the two possible cases.