Abstract
Abstract The atmospheres on tidally locked planets likely exhibit large differences between their day- and night-sides. In this paper, we illustrate how the combined effects of aerosols and day–night temperature gradients shape transit spectra of tidally locked exoplanets and evaluate the implications for retrievals of atmospheric properties. We have developed a new code, Multi-dimensional Exoplanet TransIt Spectra (METIS), which can compute transit spectra for arbitrary longitude–latitude–altitude grids of temperature and pressure. Using METIS, we pair flexible treatments of clouds and hazes with simple parameterized day–night temperature gradients to compute transit spectra and perform retrieval experiments across a wide array of possible exoplanet atmospheric properties. Our key findings are that: (1) the presence of aerosols can increase the effects of day–night temperature gradients on transit spectra; (2) ignoring day–night temperature gradients when attempting to perform Bayesian parameter estimation will return biased results, even when aerosols are present; (3) when a day–night temperature gradient is accounted for in the retrieval, some spectra contain sufficient information to constrain temperatures and the width of the transition from day to night. The presence of clouds and hazes can actually tighten such constraints, but also weaken constraints on metallicity and reference pressure. These last findings are predicated on the assumptions made in parameterizing the day–night atmospheric structure and the assumption of thermochemical equilibrium. Our results imply that this may be a promising avenue to pursue and represent a step toward the larger goal of developing models and theory of adequate complexity to match the superior-quality data that will soon be available.
Highlights
Studying exoplanet atmospheres is compelling for a number of reasons
New methods must be developed as we enter an era of higher-resolution, higher-signal-to-noise ratio (SN R) transit spectra with the broad continuous wavelength coverage expected from upcoming missions like James Webb Space Telescope (JWST) and Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL)
We focus on hot Jupiters with day-night temperature gradients and inhomogeneous aerosol coverage that varies between the day and night side of the planet, but Multi-dimensional Exoplanet TransIt Spectra (METIS) is well-suited to computing transit spectra for atmospheres which vary between morning, evening and poles as well
Summary
Studying exoplanet atmospheres is compelling for a number of reasons. Among these are the hope to uncover trends related to planet formation and evolution (Oberg et al 2011; Piso et al 2016), make comparisons to Solar System planets (Seager & Deming 2010; Madhusudhan 2019), understand the climate and conditions on alien worlds (Heng & Showman 2015; Burrows 2014), and even search for life beyond Earth We focus on hot Jupiters with day-night temperature gradients and inhomogeneous aerosol coverage that varies between the day and night side of the planet, but METIS is well-suited to computing transit spectra for atmospheres which vary between morning, evening and poles as well. The remainder of this paper is organized as follows: section 1.1 places our study in the context of previous relevant theoretical studies and observations of tidally-locked hot Jupiters, §2 describes METIS, our newly developed code for computing transit spectra from 3D atmospheres, §3 uses models of clear transit spectra for 12 objects with phase-curve estimates of day-side and night-side temperatures to demonstrate the effects of day-night temperature gradients across different temperatures and surface gravities, §4 explores the combined effects of aerosols and day-night temperature gradients through representative parameter sensitivity studies, §5 presents the results from a series of retrieval experiments on clear, cloudy, and hazy atmospheres. We provide an appendix with auxiliary descriptions of our models and methods
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