Detecting Atmospheric Molecules of Temperate Terrestrial Exoplanets using High-Resolution Spectroscopy in the Mid Infrared Domain

TL;DR

This study explores the potential of high-resolution mid-infrared spectroscopy to detect key atmospheric molecules like CO2 and H2O in temperate terrestrial exoplanets, aiding in habitability assessment.

Contribution

It introduces a differential spectroscopy method at quadrature phases to detect atmospheric molecules despite host star spectrum uncertainties.

Findings

Detectability of CO2 and H2O in nearby systems with a 6.5-meter telescope.

Sensitivity to CO2 and N2O at 1-10^3 ppm mixing ratios.

Spectral resolution above 10,000 offers limited improvements in molecule detection.

Abstract

Motivated by the development of high-dispersion spectrographs in the mid-infrared (MIR) range, we study their application to the atmospheric characterization of nearby non-transiting temperate terrestrial planets around M-type stars. We examine the detectability of CO$_2$, H$_2$O, N$_2$O, and O$_3$ in high-resolution planetary thermal emission spectra at 12-18 $\mu $m assuming an Earth-like profile and a simplified thermal structure. The molecular line width of such planets can be comparable to or broader than the Doppler shift due to the planetary orbital motion. Given the likely difficulty in knowing the high-resolution MIR spectrum of the host star with sufficient accuracy, we propose to observe the target system at two quadrature phases and extract the differential spectra as the planetary signal. In this case, the signals can be substantially suppressed compared with the case where the host star spectrum is perfectly known, as some parts of the spectral features do not remain in the differential spectra. Despite this self-subtraction, the CO$_2$ and H$_2$O features of nearby ($\lesssim $ 5~pc) systems with mid-/late-M host stars would be practical with a 6.5-meter-class cryogenic telescope, and orbital inclination could also be constrained for some of them. For CO$_2$ and N$_2$O in a 1~bar Earth-like atmosphere, this method would be sensitive when the mixing ratio is 1-10$^3$ ppm. The detectability of molecules except O$_3$ is not significantly improved when the spectral resolution is higher than $\mathcal{R}\gtrsim 10,000$, although the constraint on the orbital inclination is improved. This study provides some benchmark cases useful for assessing the value of MIR high-resolution spectroscopy in terms of characterization of potentially habitable planets.