Toward detection of terrestrial planets in the habitable zone of our closest neighbor: Proxima Centauri

The precision of radial velocity (RV) measurements to detect indirectly planetary companions of nearby stars has improved to enable the discovery of extrasolar planets in the Neptune and Super-Earth mass range. Discoveries of Earth-like planets by means of ground-based RV programs will help to determine the parameter Eta_Earth, the frequency of potentially habitable planets around other stars. In search of low-mass planetary companions we monitored Proxima Centauri (M5V) as part of our M dwarf program. In the absence of a significant detection, we use these data to demonstrate the general capability of the RV method in finding terrestrial planets. For late M dwarfs the classic liquid surface water habitable zone (HZ) is located close to the star, in which circumstances the RV method is most effective. We want to demonstrate that late M dwarfs are ideal targets for the search of terrestrial planets with the RV technique. We obtained differential RV measurements of Proxima Cen over a time span of 7 years with the UVES spectrograph at the ESO VLT. We determine upper limits to the masses of companions in circular orbits by means of numerical simulations. The RV data of Proxima Cen have a total rms scatter of 3.1 m/s and a period search does not reveal any significant signals. As a result of our companion limit calculations, we find that we successfully recover all test signals with RV amplitudes corresponding to planets with m sin i > 2 - 3 M_Earth residing inside the HZ of Proxima Cen with a statistical significance of >99%. Over the same period range, we can recover 50% of the test planets with masses of m sin i > 1.5 - 2.5 M_Earth. Based on our simulations, we exclude the presence of any planet in a circular orbit with m sin i > 1 M_Neptune at separations of a < 1 AU.