Characterization of potentially habitable planets: Retrieval of atmospheric and planetary properties from emission spectra

TL;DR

This study assesses the potential of infrared emission spectroscopy from future space missions to characterize habitable exoplanets, finding limited capabilities for atmospheric composition and temperature determination with current designs.

Contribution

It quantifies the accuracy of atmospheric parameter retrieval from emission spectra and evaluates the limitations of upcoming mission designs for planetary habitability assessment.

Findings

Emission spectroscopy provides weak limits on surface conditions.

Current mission designs are unlikely to characterize atmospheric composition.

Increasing SNR improves CO2 detection but biosignature detection remains marginal.

Abstract

An increasing number of potentially habitable terrestrial planets and planet candidates are found by ongoing planet search programs. The search for atmospheric signatures to establish planetary habitability and the presence of life might be possible in the future. We want to quantify the accuracy of retrieved atmospheric parameters which might be obtained from infrared emission spectroscopy. We use synthetic observations of hypothetical habitable planets, constructed with a parametrized atmosphere model, a high-resolution radiative transfer model and a simplified noise model. Classic statistical tools such as chi2 statistics and least-square fits were used to analyze the simulated observations. When adopting the design of currently planned or proposed exoplanet characterization missions, we find that emission spectroscopy could provide weak limits on surface conditions of terrestrial planets, hence their potential habitability. However, these mission designs are unlikely to allow to characterize the composition of the atmosphere of a habitable planet, even though CO2 is detected. Upon increasing the signal-to-noise ratios by about a factor of 2-5 (depending on spectral resolution) compared to current mission designs, the CO2 content could be characterized to within two orders of magnitude. The detection of the O3 biosignature remains marginal. The atmospheric temperature structure could not be constrained. Therefore, a full atmospheric characterization seems to be beyond the capabilities of such missions when using only emission spectroscopy during secondary eclipse or target visits. Other methods such as transmission spectroscopy or orbital photometry are probably needed in order to give additional constraints and break degeneracies. (abridged)