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Future radial velocity, astrometric, and direct-imaging surveys will findnearby Earth-sized planets within the habitable zone in the near future. Howcan we search for water and oxygen in those nontransiting planets? We show thata combination of high-dispersion spectroscopic and coronagraphic techniques isa promising technique to detect molecular lines imprinted in the scatteredlight of Earth-like planets (ELPs). In this method, the planetary signals arespectroscopically separated from telluric absorption by using the Dopplershift. Assuming a long observing campaign (a 10-day exposure) using ahigh-dispersion spectrometer (R=50,000) with speckle suppression on a 30-mtelescope, we simulate the spectra from ELPs around M dwarfs (whose stellareffective temperature is 2750-3750 K) at 5 pc. Performing a cross-correlationanalysis with the spectral template of the molecular lines, we find that rawcontrasts of $10^{-4}$ and $10^{-5}$ (using Y, J, and H bands) are required todetect water vapor at the 3 $\sigma$ and 16 $\sigma$ levels, respectively, for$T_\star$=3000 K. The raw contrast of $10^{-5}$ is required for a 6 $\sigma$detection of the oxygen 1.27 $\mu$m band. We also examine possible systematics,incomplete speckle subtraction, and the correction for telluric lines. Whenthose are not perfect, a telluric water signal appears in the cross-correlationfunction. However, we find the planetary signal is separated from thatresulting from the velocity difference. We also find that the intrinsic waterlines in the Phoenix spectra are too weak to affect the results for waterdetection. We conclude that a combination of high-dispersion spectroscopy andhigh-contrast instruments can be a powerful means to characterize ELPs in theextremely large telescope era.

Characterizing Earth-like Planets Using a Combination of High-Dispersion  Spectroscopy and High-Contrast Instruments: Doppler-shifted Water and Oxygen  Lines