Out of the thousands of exoplanets detected to date, the few that have been directly imaged are excellent targets for studying orbital configurations and atmospheric chemical compositions. However, direct imaging and characterization faces several technical challenges owing to the small angular separation and high contrast between exoplanets and their host stars. High-contrast imaging (HCI) systems mitigate these effects by suppressing diffracted star light, that may otherwise overwhelm the planet signal, with an extreme adaptive optics system and a coronagraph. Current state-of-the-art high contrast imaging instruments, such as the Gemini Planet Imager at the Gemini South telescope and SPHERE at the Very Large Telescope, are able to achieve better than 1/100000 star light suppression level at a few tenths of an arcsec, which allows for the detection of gas giant planets and brown dwarfs orbiting nearby young stars.
Star light suppression can be further improved by coupling a high-resolution spectrograph (HRS) with a coronagraphic system. In this High Dispersion Coronagraphy (HDC) scheme, the coronagraphic component serves as a spatial filter to separate the light from the star and the planet. The HRS serves as spectral filter taking advantage of differences in spectral features between the stellar spectrum and the planetary spectrum, e.g., different absorption lines and radial velocities (RV).
The HDC technique may be the only viable way of searching for biosignatures in the atmospheres of Earth-like planets using next-generation extremely large ground-based telescopes. The HDC technique technique also provides a way of relaxing very stringent requirement of starlight suppression for space missions aiming at detecting and characterizing terrestrial planets.