Atmospheric Retrieval of Terrestrial Solar System Planets for LIFE

Context

A long-term goal of exoplanet research is to characterize the atmospheres of a sizable sample of temperate terrestrial exoplanets. Such studies will augment our knowledge on the diversity of terrestrial worlds and might enable the discovery of habitable or even inhabited worlds.

Missions capable of measuring spectra of temperate terrestrial exoplanets have been proposed (LUVOIR/HabEx — optical & near-infrared; Large Interferometer For Exoplanets (LIFE) — mid-infrared (MIR) [see Quanz et al., this conference]). The MIR thermal emission measured by LIFE provides exclusive probes to important molecules (e.g. the potential bioindicators CH₄ and O₃). Further, MIR observations can provide direct constraints on a planet’s pressure-temperature (PT) profile, radius, and surface conditions.

Methods/Results

We present our recent atmospheric retrieval studies of detailed LIFE mock-observations. We investigate a cloud-free Earth-twin [1] and, to our knowledge for the first time, a cloudy Venus-twin [Konrad et al., in prep.] exoplanet around a sun-like star at 10 pc.

We simulate MIR planet emission spectra with petitRADTRANS (a 1D radiative transfer model) [2] and use LIFESim [Ottiger et al., subm.] to estimate the wavelength-dependent astrophysical noise expected for exoplanet observations with LIFE.

Our retrieval suite uses petitRADTRANS as atmospheric model (including a parametric cloud model) and the MultiNest [3] parameter estimation algorithm. We retrieve for the planet’s radius, PT profile, surface pressure, molecular abundances, and cloud structure, and constrain the planet mass using the mass-radius relation forecaster [4]. By retrieving spectra of different wavelength ranges, resolutions, and noise levels, we determine the requirements to:

1. distinguish Earth- from Venus-like MIR spectra

2. characterize the structure/composition of atmospheres

3. detect potential biomarkers in an Earth-twin

4. infer cloud presence

5. constrain cloud structure/composition in a Venus-twin

We also discuss the challenges for retrievals of MIR exoplanet spectra from LIFE and how differences in the quality of the spectra affect them.

Conclusion

With these studies and an additional retrieval study for Earth at different times [Alei et al., this conference], we find first constraints for the LIFE instrument requirements and identify important limitations and challenges of MIR exoplanet retrievals.

References

[1] Konrad, B.S., et al., 2021, arXiv:2112.02054

[2] Mollière, P., et al., 2019, A&A, 627:A67

[3] Feroz, F., et al., 2009, MNRAS, 398(4):1601–1614

[4] Chen, J., Kipping, D.M., 2016, arXiv:1603.08614