Exploring the Atmospheres of Hot Neptune and Gas Giant Exoplanets

EXPLORING ATMOSPHERES OF HOT MINI-NEPTUNES AND EXTRASOLAR GIANT PLANETS ORBITING DIFFERENT STARS WITH APPLICATION TO HD 97658b, WASP-12b, CoRoT-2b, XO-1b, AND HD 189733b

Authors:

Y. Miguel and L. Kaltenegger

Abstract:

We calculated an atmospheric grid for hot mini-Neptune and giant exoplanets that links astrophysical observable parameters—orbital distance and stellar type—with the chemical atmospheric species expected. The grid can be applied to current and future observations to characterize exoplanet atmospheres and serves as a reference to interpret atmospheric retrieval analysis results. To build the grid, we developed a one-dimensional code for calculating the atmospheric thermal structure and linked it to a photochemical model that includes disequilibrium chemistry (molecular diffusion, vertical mixing, and photochemistry). We compare the thermal profiles and atmospheric composition of planets at different semimajor axes (0.01 AU ≤ a ≤ 0.1 AU) orbiting F, G, K, and M stars. Temperature and UV flux affect chemical species in the atmosphere. We explore which effects are due to temperature and which are due to stellar characteristics, showing the species most affected in each case. CH4 and H2O are the most sensitive to UV flux, H displaces H2 as the most abundant gas in the upper atmosphere for planets receiving a high UV flux. CH4 is more abundant for cooler planets. We explore vertical mixing, to inform degeneracies on our models and in the resulting spectral observables. For lower pressures, observable species like H2O or CO2 can indicate the efficiency of vertical mixing, with larger mixing ratios for a stronger mixing. By establishing the grid, testing the sensitivity of the results, and comparing our model to published results, our paper provides a tool to estimate what observations could yield. We apply our model to WASP-12b, CoRoT-2b, XO-1b, HD189733b, and HD97658b.