ARES

VI. Viability of one-dimensional retrieval models for transmission spectroscopy characterization of exo-atmospheres in the era of JWST and Ariel

¹ LATMOS/IPSL, UVSQ Université Paris-Saclay, Sorbonne Université, CNRS, Guyancourt, France
e-mail: yassin.jaziri@latmos.ipsl.fr
² Observatoire Astronomique de l’Université de Genève, département d’astronomie Chemin Pegasi 51, 1290 Versoix, Switzerland
³ Dipartimento di Fisica, La Sapienza Università di Roma, Piazzale Aldo Moro 2, 00185 Roma, Italy
⁴ Institut d’Astrophysique de Paris – CNRS, Sorbonne Université, Paris, France
⁵ LESIA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CNRS, 5 place Jules Janssen, 92195 Meudon, France
⁶ Laboratoire de Météorologie Dynamique, IPSL, CNRS, Sorbonne Université, Ecole Normale Supérieure, Université PSL, Ecole Polytechnique, Institut Polytechnique de Paris, 75005 Paris, France
⁷ School of Natural Sciences, University of Tasmania, Private Bag 37 Hobart, Tasmania 70001, Australia
⁸ Université de Paris Cité, Institut de Physique du Globe de Paris, CNRS 75005 Paris, 1 rue Jussieu, 75005 Paris, France
⁹ Laboratoire AIM, CEA, CNRS, Univ. Paris-Sud, UVSQ, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
¹⁰ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
¹¹ School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
¹² Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹³ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁴ Université Paris Cité and Univ. Paris Est Créteil, CNRS, LISA, 75013 Paris, France

Received: 6 July 2023
Accepted: 11 December 2023

Abstract

Observed exoplanet transit spectra are usually retrieved using 1D models to determine atmospheric composition. However, planetary atmospheres are 3D. With the new state-of-the-art James Webb Space Telescope (JWST) and future space telescopes such as Ariel (Atmospheric Remote-sensing Infrared Exoplanet Large-survey), we will be able to obtain increasingly accurate transit spectra. The 3D effects on the spectra will be visible, and we can expect biases in the 1D extractions. In order to elucidate these biases, we have built theoretical observations of transit spectra, from 3D atmospheric modeling through transit modeling to instrument modeling. For this purpose, we used a global climate model (GCM) to simulate the atmosphere, a 3D radiative transfer model to calculate theoretical transmission spectra, and adapted instrument software from JWST and Ariel to reproduce telescope noise. Next, we used a 1D radiative transfer inversion model to retrieve the known input atmosphere and disentangle any biases that might be observed. The study was done from warm planets to ultra-hot planets to assess biases as a function of average planet temperature. Three-dimensional effects are observed to be strongly nonlinear from the coldest to the hottest planets. These effects also depend on the planet’s metallicity and gravity. Considering equilibrium chemistry, 3D effects are observed through very strong variations in certain features of the molecule or very small variations over the whole spectrum. We conclude that we cannot rely on the uncertainty of retrievals at all pressures, and that we must be cautious about the results of retrievals at the top of the atmosphere. However the results are still fairly close to the truth at mid-altitudes (those probed). We also need to be careful with the chemical models used for planetary atmosphere. If the chemistry of one molecule is not correctly described, this will bias all the others, and the retrieved temperature as well. Finally, although fitting a wider wavelength range and higher resolution has been shown to increase retrieval accuracy, we show that this could depend on the wavelength range chosen, due to the accuracy on modeling the different features. In any case, 1D retrievals are still correct for the detection of molecules, even in the event of an erroneous abundance retrieval.