CARMENES: Blue Planets Orbiting Red Stars

M dwarfs are very interesting targets for searches for habitable planets. They are the most numerous type of star; therefore our closest cosmic neighbors may well live on a planet orbiting an M dwarf. It is also easier to look for Earth-like planets around low-mass stars, as they give rise to relatively large radial-velocity variations and deep transits. Third, planets of M dwarfs may be different from those of the Solar System as they experience stronger UV and X-ray irradiation. This provides opportunities for comparative studies of planetary atmospheres under varying conditions.

We are conducting a survey of 300 nearby M dwarfs (average distance only 13pc), with the goal of finding terrestrial planets in their habitable zones. We have built a pair of spectrographs that are optimized specifically for measuring precise radial velocities of cool stars. This CARMENES instrument is now operational at the 3.5m telescope of Calar Alto Observatory in Spain.

We have detected the signatures of several previously known planets as well as discovered new ones, with masses down to a few Earth masses. The CARMENES survey is also generating a unique data set for studies of M star atmospheres, rotation, and activity. The spectra cover important diagnostic lines for activity, as well as molecular lines sensitive to magnetic fields. Correlating the time series of these features with each other, and with wavelength-dependent radial velocities, provides excellent discrimination between planetary companions and stellar radial velocity “noise”, and generates new insight into the physical properties of M dwarf atmospheres.

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