The HARPS search for southern extra-solar planets XXXV. Super-Earths around the M-dwarf neighbors Gl433 and Gl667C

TL;DR

This paper reports the discovery and orbital analysis of super-Earth planets around the nearby M-dwarf stars Gl433 and Gl667C, including one in the habitable zone, based on extended HARPS radial velocity data.

Contribution

It provides extended radial velocity data and orbital analysis of super-Earths around two M-dwarfs, including the first detection of a habitable zone planet around Gl667C.

Findings

Gl667Cc is a super-Earth in the habitable zone of Gl667C.

Gl433b and Gl667Cb are ~5.5 M_earth planets with ~7-day periods.

Stellar metallicity does not strongly influence super-Earth occurrence.

Abstract

M dwarfs have been found to often have super-Earth planets with short orbital periods. Such stars are thus preferential targets in searches for rocky or ocean planets in the solar neighbourhood. In a recent paper (Bonfils et al. 2011), we announced the discovery of respectively 1 and 2 low mass planets around the M1.5V stars Gl433 and Gl667C. We found those planets with the HARPS spectrograph on the ESO~3.6-m telescope at La Silla Observatory, from observations obtained during the Guaranteed Time Observing program of that instrument. We have obtained additional HARPS observations of those two stars, for a total of respectively 67 and 179 Radial Velocity measurements for Gl433 and Gl667C, and present here an orbital analysis of those extended data sets and our main conclusion about both planetary systems. One of the three planets, Gl667Cc, has a mass of only M2.sin(i)~4.25 M_earth and orbits in the central habitable zone of its host star. It receives just 10% less stellar energy from Gl667C than the Earth receives from the Sun. However planet evolution in habitable zone can be very different if the host star is a M dwarf or a solar-like star, without necessarily questioning the presence of water. The two other planets, Gl433b and Gl667Cb, both have M2.sin(i) of ~5.5 M_earth and periods of ~7 days. The Radial Velocity measurements of both stars contain longer time scale signals, which we fit as longer period Keplerians. For Gl433 that signal probably originates in a Magnetic Cycle, while a longer time span will be needed to conclude for Gl667C. The metallicity of Gl433 is close to solar, while Gl667C is metal poor with [Fe/H] ~ -0.6. This reinforces the recent conclusion that the occurence of Super-Earth planets does not strongly correlate with stellar metallicity.