High Dispersion Coronagraphy

Several thousand exoplanets have been discovered over the past 20 years. The solar system is now seen as presenting just one example among a mind-boggling variety of system architectures: from circumbinary exoplanets, systems with tightly packed inner planets, water-worlds, potential Earth twins, super-Earths, sub- and super-Neptunes, and hazy hot Jupiters all the way to extremely long-period lonely massive objects looking more like failed stars than giant planets. So far, the vast majority of these planetary systems have been discovered indirectly by techniques looking at tiny variations in their host star's motion and/or brightness. These techniques have limited remote-sensing capabilities, yet have ushered in an entirely new branch of astrophysics named comparative exoplanetology, putting the solar system and its planets into a universal perspective. Radial velocity and transit techniques provide mass and radius measurements, density and thus rough interior composition when combined. However, these approaches suffer from degeneracy (the mass-radius relationships overlap for various compositions, see, e.g.,  Seager et al. 2007, Valencia et al. 2007), highlighting the need to directly diagnose the chemical compositions of exoplanets via  spectroscopy. Taking spectra of the light filtering through the atmosphere during primary transits, or reflected off the surface when directly imaged, emitted thermal light during secondary eclipses or from young hot giant planets, are complementary means of probing the atmosphere of other worlds (Morley et al. 2016). Although directly-imaged exoplanets are excellent targets for chemical composition studies (Konopacky et al. 2013; Oppenheimer et al. 2013; Bonnefoy et al. 2015; Rajan et al. 2015), overcoming the small angular separation and high contrast between exoplanets and their host stars remains a technical challenge. 

Coupling a high-resolution spectrograph with a high-contrast imaging instrument is the next big step in the direct characterization of exoplanet atmospheres (http://www.caltech.edu/news/inventing-tools-detecting-life-elsewhere-54515). In this scheme, the high-contrast imaging system serves as a spatial filter to separate the light from the star and the planet, and the high-resolution spectrograph serves as spectral filter, which differentiates between features in the stellar and planetary spectra, e.g., between different absorption lines and radial velocities. High-resolution spectroscopy has three game-changing benefits: detailed species-by-species molecular characterization, Doppler measurements (planet spin, orbital velocity, plus mapping of atmospheric and/or surface features), and last but not least, improved detection capability by side-stepping speckle noise calibration issues affecting low-resolution spectroscopic data from current integral field spectrographs such as SPHERE (Beuzit et al. 2008) and GPI (Macintosh et al. 2015). Most direct imaging results so far have indeed focused on photometry and very low-resolution spectroscopy, due to the lack of instruments designed to optimally merge high contrast imaging (wavefront control and coronagraphy) and high-resolution spectroscopy at small angles from the host star. 

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Co-Investigators: J. Wang (Caltech), G. Ruane (Caltech), N. Jovanovic (Caltech), J.-R. Delorme (Caltech), N. Klimovich (Caltech), W. J. Wenhao (Pomona College), D. Echeverri (Caltech), M. Randloph (CalPoly SLO), J.K. Wallace (JPL), J. Fucik (Caltech), G. Vasisht (JPL), B. Mennesson (JPL), E. Choquet (JPL/Caltech), R. Dekany (Caltech), E. Serabyn (JPL), R. Hu (JPL/Caltech), B. Benneke (Caltech/Univ. Montreal) 

© Dimitri Mawet 2017