Photochemistry and Spectral Characterization of Temperate and Gas-Rich Exoplanets

Jupiter as a test case. The planet modeled is a Jupiter-mass and Jupiter-radius planet at a 5.2-AU orbit of a Sun-like star, having an atmospheric metallicity of 3×solar. (a) The solid line is the pressure-temperature profile adopted from Galileo probe measurements and Cassini CIRS measurements in Jupiter (the solid line; Seiff et al. 1998; Simon-Miller et al. 2006) and the dashed line is the pressure-temperature profile calculated by the atmospheric structure model. (b) The eddy diffusion coefficient profile adopted in this work. (c) The calculated mixing ratio profiles of CH4, NH3, and major photochemical products. The solid lines are the results using the measured temperature profile, the dashed lines are the results using the modeled temperature profile (i.e., without the temperature inversion), and the dotted lines are the results using the modeled temperature profile and the photodissociation quantum yield of C2H2 set to unity (see discussion in Section 2.3). In comparison are the abundance data of major hydrocarbons and HCN in Jupiter’s atmosphere, as compiled in Morrissey et al. (1995); Gladstone et al. (1996); Davis et al. (1997); Yelle et al. (2001); Moses et al. (2005).

Exoplanets that receive stellar irradiance of approximately Earth's or less have been discovered and many are suitable for spectral characterization.

Here we focus on the temperate planets that have massive H2-dominated atmospheres, and trace the chemical reactions and transport following the photodissociation of H2O, CH4, NH3, and H2S, with K2-18 b, PH2 b, and Kepler-167 e representing temperate/cold planets around M and G/K stars.

We find that NH3 is likely depleted by photodissociation to the cloud deck on planets around G/K stars but remains intact in the middle atmosphere of planets around M stars. A common phenomenon on temperate planets is that the photodissociation of NH3 in presence of CH4 results in HCN as the main photochemical product.

The photodissociation of CH4 together with H2O leads to CO and CO2, and the synthesis of hydrocarbon is suppressed. Temperate planets with super-solar atmospheric metallicity and appreciable internal heat may have additional CO and CO2 from the interior and less NH3 and thus less HCN.

Our models of K2-18 b can explain the transmission spectrum measured by Hubble, and indicate that future observations in 0.5-5.0 um would provide the sensitivity to detect the equilibrium gases CH4, H2O, and NH3, the photochemical gas HCN, as well as CO2 in some cases.

Temperate and H2-rich exoplanets are thus laboratories of atmospheric chemistry that operate in regimes not found in the Solar System, and spectral characterization of these planets in transit or reflected starlight promises to greatly expand the types of molecules detected in exoplanet atmospheres.

Renyu Hu

Comments: Accepted by ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2108.04419 [astro-ph.EP] (or arXiv:2108.04419v1 [astro-ph.EP] for this version)
Submission history
From: Renyu Hu
[v1] Tue, 10 Aug 2021 02:59:45 UTC (1,074 KB)

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