A Search for Water in a Super-Earth Atmosphere: High-resolution Optical Spectroscopy of 55Cancri e

We present the analysis of high-resolution optical spectra of four transits of 55Cnc e, a low-density, super-Earth that orbits a nearby Sun-like star in under 18 hours. The inferred bulk density of the planet implies a substantial envelope, which, according to mass-radius relationships, could be either a low-mass extended or a high-mass compact atmosphere. Our observations investigate the latter scenario, with water as the dominant species. We take advantage of the Doppler cross-correlation technique, high-spectral resolution and the large wavelength coverage of our observations to search for the signature of thousands of optical water absorption lines. Using our observations with HDS on the Subaru telescope and ESPaDOnS on the Canada-France-Hawaii Telescope, we are able to place a 3-sigma lower limit of 10 g/mol on the mean-molecular weight of 55Cnc e's water-rich (volume mixing ratio >10%), optically-thin atmosphere, which corresponds to an atmospheric scale-height of ~80 km. Our study marks the first high-spectral resolution search for water in a super-Earth atmosphere and demonstrates that it is possible to recover known water-vapour absorption signals, in a nearby super-Earth atmosphere, using high-resolution transit spectroscopy with current ground-based instruments.