Feasibility of spectro-polarimetric characterization of exoplanetary atmospheres with direct observing instruments

Context. Spectro-polarimetry of reflected light from exoplanets is anticipated to be a powerful method for probing atmospheric composition and structure. Aims. We aim to evaluate the feasibility of the search for a spectro-polarimetric feature of water vapor using a high-contrast polarimetric instrument on a 30–40 m-class ground-based telescope. Methods. Three types of errors are considered: (a) errors from the difference between efficiencies for two orthogonally polarized states; (b) errors caused by speckle noises; and (c) errors caused by photon noise from scattered starlight. Using the analytically derived error formulas, we estimate the number of planets for which feasible spectro-polarimetric detection of water vapor is possible. Results. Our calculations show that effective spectro-polarimetric searches for water vapor are possible for 5 to 14 known planets. Spectro-polarimetric characterization of exoplanetary atmospheres is feasible with an extremely large telescope and a direct observing spectro-polarimeter