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

Context: 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. Detections of extremely low mass planets, even as small as 1 Earth mass or below, in short-period orbits now appears conceivable in ongoing RV planet searches. Discoveries of these Earth-like planets by means of ground-based RV programs will help to determine the parameter eta⊕, the frequency of potentially habitable planets around other stars. Aims: 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. Methods: Using the iodine cell 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. Results: The RV data of Proxima Cen have a total rms scatter of 3.1~m s-1 and a period search does not reveal any significant signals. In contrast to our earlier results for Barnard's star, the RV results for the active M dwarf Proxima Cen are only weakly correlated with Halpha line index measurements. 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_⊕ 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_⊕. 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. Based on observations collected at the European Southern Observatory, Paranal, Chile, programmes 65.L-0428, 66.C-0446, 267.C-5700, 68.C-0415, 69.C-0722, 70.C-0044, 71.C-0498, 072.C-0495, 173.C-0606 and 078.C-0829. Table 1 is only available in electronic form at http://www.aanda.or