The Impending Opacity Challenge in Exoplanet Atmospheric Characterization

Abstract

With a new generation of observatories coming online this decade, the process of characterizing exoplanet atmospheres will need to be reinvented. Currently mostly on the instrumental side, characterization bottlenecks will soon stand by the models used to translate spectra into atmospheric properties. Limitations stemming from our stellar and atmospheric models have already been highlighted. Here, we show that the current limitations of the opacity models used to decode exoplanet spectra propagate into an accuracy wall at ~0.5-1.0 dex (i.e., 3 to 10x) on the atmospheric properties, which is an order of magnitude above the precision targeted by JWST Cycle 1 programs and needed for, e.g., meaningful C/O-ratio constraints and biosignatures identification. We perform a sensitivity analysis using nine different opacity models and find that most of the retrievals produce harmonious fits owing to compensations in the form of >5$\sigma$ biases on the derived atmospheric parameters translating in the aforementioned accuracy wall. We suggest a two-tier approach to alleviate this problem involving a new retrieval procedure and guided improvements in opacity data, their standardization and optimal dissemination.

16 Pages, 12 Figures

References in corpus (17)

• petitRADTRANS: a Python radiative transfer package for exoplanet characterization and retrieval
• Fingering convection and cloudless models for cool brown dwarf atmospheres
• The 2020 release of the ExoMol database: molecular line lists for exoplanet and other hot atmospheres
• ExoMol line lists -- XXXIX. Ro-vibrational molecular line list for CO${}_2$
• Five carbon- and nitrogen-bearing species in a hot giant planet's atmosphere
• A hybrid line list for CH${}_4$ and hot methane continuum
• Phosphine as a Biosignature Gas in Exoplanet Atmospheres
• Effects of a fully 3D atmospheric structure on exoplanet transmission spectra: retrieval biases due to day-night temperature gradients
• The full infrared spectrum of molecular hydrogen
• Highlights in the Study of Exoplanet Atmospheres
• On degeneracies in retrievals of exoplanetary transmission spectra
• Toward the analysis of JWST exoplanet spectra: Identifying troublesome model parameters
• Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New H${}_2$O Cross-Sections
• Information Content Analysis for Selection of Optimal JWST Observing Modes for Transiting Exoplanet Atmospheres
• An analytical formalism accounting for clouds and other "surfaces" for exoplanet transmission spectroscopy
• Molecular Simulations for the Spectroscopic Detection of Atmospheric Gases
• The Need for Laboratory Measurements and Ab Initio Studies to Aid Understanding of Exoplanetary Atmospheres

Cited by in corpus (4)

• A Catalogue of Exoplanet Atmospheric Retrieval Codes
• A 1.55 R${}_{\oplus }$ habitable-zone planet hosted by TOI-715, an M4 star near the ecliptic South Pole
• The impact of spectral line wing cut-off: Recommended standard method with application to MAESTRO opacity database
• Radiative Transfer and Inversion codes for characterizing planetary atmospheres: an overview