Exploring and Validating Exoplanet Atmospheric Retrievals with Solar System Analog Observations

Abstract

Solar system observations that serve as analogs for exoplanet remote sensing data can provide important opportunities to validate ideas and models related to exoplanet environments. Critically, and unlike true exoplanet observations, solar system analog data benefit from available high-quality ground- or orbiter-derived “truth” constraints that enable strong validations of exoplanet data interpretation tools. In this work, we first present a versatile atmospheric retrieval suite, capable of application to reflected light, thermal emission, and transmission observations spanning a broad range of wavelengths and thermochemical conditions. The tool—dubbed rfast—is designed, in part, to enable exoplanet mission concept feasibility studies. Following model validation, the retrieval tool is applied to a range of solar system analog observations for exoplanet environments. Retrieval studies using Earth reflected light observations from NASA’s EPOXI mission provide a key proof of concept for exo-Earth direct imaging concept missions under development. Inverse modeling applied to an infrared spectrum of Earth from the Mars Global Surveyor Thermal Emission Spectrometer achieves good constraints on atmospheric gases, including many biosignature gases. Finally, retrieval analysis applied to a transit spectrum of Titan derived from the Cassini Visual and Infrared Mapping Spectrometer provides a proof of concept for interpreting more feature-rich transiting exoplanet observations from NASA’s James Webb Space Telescope. In the future, solar system analog observations for exoplanets could be used to verify exoplanet models and parameterizations, and future exoplanet analog observations of any solar system worlds from planetary science missions should be encouraged.

In Search of the Edge: A Bayesian Exploration of the Detectability of Red Edges in Exoplanet Reflection Spectra

“…In particular, our demonstration that sharp features like the modern Earth's vegetation red edge can be reliably retrieved (to 150 nm for cloud-free models at S/ N = 5) is very promising for a proposed future large IR/ optical/UV space-based telescope (e.g., Decadal Survey on Astronomy & Astrophysics 2020). While we find that clouds can result in overestimated surface albedos (in agreement with results from Feng et al 2018, Robinson & Salvador 2023, and Damiano & Hu 2022, the red edge's wavelength location can nonetheless be correctly retrieved for modern Earth-like planets even for Earth-like cloud coverage.…”

“…We prescribe three further free parameters for the planetary radius and gravity (evaluated at 1 mbar) and the surface pressure. These choices are similar to those made by other reflected-light retrieval studies (e.g., Feng et al 2018;Damiano & Hu 2022;Robinson & Salvador 2023;Wang et al 2022).…”

“…Techniques for retrieving reflected light from exoplanets were initially developed for directly imaged cool giant exoplanets, largely in preparation for the Roman Space Telescope (Lupu et al 2016;Nayak et al 2017;Lacy et al 2019;Carrión-González et al 2020Mukherjee et al 2021). Subsequent studies have extended reflected light retrievals to directly imaged sub-Neptunes and terrestrial planets (Feng et al 2018;Damiano & Hu 2021Robinson & Salvador 2023;Wang et al 2022), which would be observable with a future large IR/optical/UV space-based telescope.…”

“…Reflection spectral retrievals from terrestrial planets must additionally consider surface reflection. Most previous studies have assumed a uniform-in-wavelength surface albedo (Feng et al 2018;Damiano & Hu 2022;Robinson & Salvador 2023). Consequently, the spectral imprint of a wavelength-dependent surface, including the red edge, has seen little investigation.…”

Large Interferometer For Exoplanets (LIFE)

“…Earth-centered retrieval studies and retrieval studies on theoretical spectra of habitable worlds are often used to investigate the characterization performance for different quality spectra (e.g., von Paris et al 2013;Brandt & Spiegel 2014;Feng et al 2018;Quanz et al 2021;Léger et al 2019;Carrión-González et al 2020;Robinson & Salvador 2022). Venus -to our knowledge -has not yet been considered in a comparable retrieval study.…”