Using High-Resolution Spectroscopy to Study the Composition, Temperature, and Dynamics of Exoplanet Atmospheres

Abstract

High-resolution spectroscopy, typically operating at resolving powers R greater or equal to 25,000, has matured into one of the primary techniques for characterising the atmospheres of extrasolar planets. The ability of HRS to resolve individual rotational-vibrational lines of molecular bands, combined with the large Doppler shifts experienced by close-in planets during their orbits, allows planetary signals to be separated from quasi-stationary telluric and stellar contamination. Since the pioneering detection of carbon monoxide in the transmission spectrum of HD 209458b, HRS has enabled the identification of more than a dozen chemical species, including H2O, CH4, HCN, TiO, VO, Na, K, Li, H-alpha, He I, Mg, Ca, V, Cr, Mn, Fe, Co, Ni, and Ti, in a wide variety of transiting, non-transiting and directly imaged exoplanets . In addition to chemical abundances, HRS constrains the vertical temperature structure through the pressure dependence of line depths, and reveals atmospheric dynamics through Doppler shifts and asymmetries imprinted on the planetary cross-correlation function. This review synthesises observational and methodological progress from the past fifteen years with a focus on how current and forthcoming high-resolution facilities HARPS, ESPRESSO, NIRPS, CARMENES, CRIRES+, SPIRou, GIANO, and ultimately the ANDES, METIS and HARMONI instruments on the Extremely Large Telescope are reshaping our empirical view of exoplanet atmospheres.