The atmospheric chemistry of the warm Neptune GJ 3470b: Influence of metallicity and temperature on the CH₄/CO ratio

Abstract

Context. Current observation techniques are able to probe the atmosphere of some giant exoplanets and get some clues about their atmospheric composition. However, the chemical compositions derived from observations are not fully understood. For instance, the CH 4 /CO abundance ratio is often inferred to be different from the value that has been predicted by chemical models. Recently, the warm Neptune GJ 3470b has been discovered, and because of its close distance from us and high transit depth, it is a very promising candidate for follow-up characterisation of its atmosphere. Aims. We study the atmospheric composition of GJ 3470b to compare to the current observations of this planet and to prepare for future ones but also to understand the chemical composition of warm (sub-)Neptunes as a typical case study. The metallicity of such atmospheres is totally uncertain and are likely to vary to values up to 100× solar. We explore the space of unknown parameters to predict the range of possible atmospheric compositions. Methods. We use a one-dimensional chemical code to compute a grid of models with various thermal profiles, metallicities, eddy diffusion coefficient profiles, and stellar UV incident fluxes. Thanks to a radiative transfer code, we then compute the corresponding emission and transmission spectra of the planet and compare them with the observational data already published. Results. Within the parameter space explored we find that methane is the major carbon-bearing species in most cases. We, however, find that for high metallicities with a sufficiently high temperature, the CH 4 /CO abundance ratio can become lower than unity, as suggested by some multiwavelength photometric observations of other warm (sub-)Neptunes, such as GJ 1214b and GJ 436b. As for the emission spectrum of GJ 3470b, brightness temperatures at infrared wavelengths may vary between 400 and 800 K depending on the thermal profile and metallicity. Conclusions. Combined with a hot temperature profile, a substantial enrichment in heavy elements by a factor of ≥100 with respect to the solar composition can shift the carbon balance in favour of carbon monoxide at the expense of methane. Nevertheless, current observations of this planet do not allow us yet to determine which model is more accurate.

Tea: A Code Calculating Thermochemical Equilibrium Abundances

“…In general, we see, as expected (e.g., Lodders & Fegley 2002;Line et al 2011;Venot et al 2014), that the shapes of the vertical distributions are mostly preserved for all metallicities. However, the thermochemical mixing ratio of CO 2 , CO, H 2 O, N 2 , HS, and H 2 S vary by several orders of magnitude over the range of metallicities, while CH 4 and hydrocarbons change very little.…”

Pseudo 2D chemical model of hot-Jupiter atmospheres: application to HD 209458b and HD 189733b

“…Furthermore, in cooler planets such as hot Neptunes orbiting M dwarfs (e.g., GJ 436b) the temperature contrast between day and nightsides decreases because the cooling rate scales with the cube of temperature (e.g., A73, page 18 of 21 Lewis et al 2010), and therefore the composition is expected to be even more homogeneous with longitude than in warmer planets such as HD 209458b and HD 189733b. However, unlike hot Jupiters, hot Neptunes may have an atmospheric metallicity much higher than solar (Line et al 2011;Moses et al 2013;Agúndez et al 2014;Venot et al 2014), which makes it interesting to investigate the extent of the spatial variation of molecular abundances in their atmospheres.…”

“…The circulation pattern in these planets is characterized by an equatorial superrotating eastward jet. On the other hand, the chemical composition of hot Jupiters has been investigated by one-dimensional models, which currently account for thermochemical kinetics, vertical mixing, and photochemistry (Zahnle et al 2009;Line et al 2010Line et al , 2011Moses et al 2011Moses et al , 2013Kopparapu et al 2012;Venot et al 2012Venot et al , 2014Agúndez et al 2014). These models have revealed the existence of three different chemical regimes in the vertical direction.…”

The effect of metallicity on the atmospheres of exoplanets with fully coupled 3D hydrodynamics, equilibrium chemistry, and radiative transfer

“…Alternatively, several theoretical studies (Moses et al 2013a,b;Agúndez et al 2014b;Venot et al 2014;Menou 2012;Kataria et al 2014;Charnay et al 2015a) have explored the parameter space by performing a range of simulations that vary the elemental composition and investigated the effect on the atmosphere properties, such as the temperature, wind velocities, chemical composition and simulated transmission and emission spectra.…”