Large Interferometer For Exoplanets (LIFE)

IX. Assessing the impact of clouds on atmospheric retrievals at mid-infrared wavelengths with a Venus-twin exoplanet^★

¹ ETH Zurich, Institute for Particle Physics & Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zurich, Switzerland
e-mail: konradb@ethz.ch; sascha.quanz@phys.ethz.ch
² National Center of Competence in Research PlanetS, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
³ ETH Centre for Origin and Prevalence of Life, Wolfgang-Pauli-Str. 27, 8093 Zurich, Switzerland
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
⁶ KU Leuven, Institute of Astronomy, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁷ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
⁸ Institute of Astronomy, University of Cambridge CB3 0HA, UK
⁹ University of Bern, Center for Space and Habitability, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁰ Department of Physics and Astronomy, York University, 4700 Keele Street, North York, Ontario 3MJ 1P3, Canada
¹¹ Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK
¹² School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA

Received: 9 December 2022
Accepted: 13 March 2023

Abstract

Context. Terrestrial exoplanets in the habitable zone are likely a common occurrence. The long-term goal is to characterize the atmospheres of dozens of such objects. The Large Interferometer For Exoplanets (LIFE) initiative aims to develop a space-based mid-infrared (MIR) nulling interferometer to measure the thermal emission spectra of such exoplanets.

Aims. We investigate how well LIFE could characterize a cloudy Venus-twin exoplanet. This allows us to: (1) test our atmospheric retrieval routine on a realistic non-Earth-like MIR emission spectrum of a known planet, (2) investigate how clouds impact retrievals, and (3) further refine the LIFE requirements derived in previous Earth-centered studies.

Methods. We ran Bayesian atmospheric retrievals for simulated LIFE observations of a Venus-twin exoplanet orbiting a Sun-like star located 10 pc from the observer. The LIFESIM noise model accounted for all major astrophysical noise sources. We ran retrievals using different models (cloudy and cloud-free) and analyzed the performance as a function of the quality of the LIFE observation. This allowed us to determine how well the atmosphere and clouds are characterizable depending on the quality of the spectrum.

Results. At the current minimal resolution (R = 50) and signal-to-noise (S /N = 10 at 11.2 μ m) requirements for LIFE, all tested models suggest a CO₂-rich atmosphere (≥30% in mass fraction). Further, we successfully constrain the atmospheric pressure-temperature (P–T) structure above the cloud deck (P–T uncertainty ≤ ± 15 K). However, we struggle to infer the main cloud properties. Further, the retrieved planetary radius (R_pl), equilibrium temperature (T_eq), and Bond albedo (A_B) depend on the model. Generally, a cloud-free model performs best at the current minimal quality and accurately estimates R_pl, T_eq, and A_B. If we consider higher quality spectra (especially S/N = 20), we can infer the presence of clouds and pose first constraints on their structure.

Conclusions. Our study shows that the minimal R and S/N requirements for LIFE suffice to characterize the structure and composition of a Venus-like atmosphere above the cloud deck if an adequate model is chosen. Crucially, the cloud-free model is preferred by the retrieval for low spectral qualities. We thus find no direct evidence for clouds at the minimal R and S/N requirements and cannot infer the thickness of the atmosphere. Clouds are only constrainable in MIR retrievals of spectra with S/N ≥ 20. The model dependence of our retrieval results emphasizes the importance of developing a community-wide best-practice for atmospheric retrieval studies.