A dynamically-packed planetary system around GJ 667C with three super-Earths in its habitable zone ^{⋆,⋆⋆,⋆⋆⋆}

¹ Universität Göttingen, Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
e-mail: guillem.anglada@gmail.com
² Centre for Astrophysics, University of Hertfordshire, College Lane, Hatfield, Hertfordshire AL10 9AB, UK
³ University of Turku, Tuorla Observatory, Department of Physics and Astronomy, Väisäläntie 20, 21500 Piikkiö, Finland
e-mail: miptuom@utu.fi
⁴ Technical University of Dresden, Institute for Planetary Geodesy, Lohrmann-Observatory, 01062 Dresden, Germany
⁵ Astronomy Department, University of Washington, Box 951580, WA 98195, Seattle, USA
⁶ Leibniz Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁷ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Casilla 36- D Santiago, Chile
⁸ UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA
⁹ Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Rd. NW, 20015 Washington D.C., USA

Received: 20 February 2013
Accepted: 4 June 2013

Abstract

Context. Since low-mass stars have low luminosities, orbits at which liquid water can exist on Earth-sized planets are relatively close-in, which produces Doppler signals that are detectable using state-of-the-art Doppler spectroscopy.

Aims. GJ 667C is already known to be orbited by two super-Earth candidates. We have recently applied developed data analysis methods to investigate whether the data supports the presence of additional companions.

Methods. We obtain new Doppler measurements from HARPS extracted spectra and combined them with those obtained from the PFS and HIRES spectrographs. We used Bayesian and periodogram-based methods to re-assess the number of candidates and evaluated the confidence of each detection. Among other tests, we validated the planet candidates by analyzing correlations of each Doppler signal with measurements of several activity indices and investigated the possible quasi-periodic nature of signals.

Results. Doppler measurements of GJ 667C are described better by six (even seven) Keplerian-like signals: the two known candidates (b and c); three additional few-Earth mass candidates with periods of 92, 62, and 39 days (d, e and f); a cold super-Earth in a 260-day orbit (g) and tantalizing evidence of a ~1 M_⊕ object in a close-in orbit of 17 days (h). We explore whether long-term stable orbits are compatible with the data by integrating 8 × 10⁴ solutions derived from the Bayesian samplings. We assess their stability using secular frequency analysis.

Conclusions. The system consisting of six planets is compatible with dynamically stable configurations. As for the solar system, the most stable solutions do not contain mean-motion resonances and are described well by analytic Laplace-Lagrange solutions. Preliminary analysis also indicates that masses of the planets cannot be higher than twice the minimum masses obtained from Doppler measurements. The presence of a seventh planet (h) is supported by the fact that it appears squarely centered on the only island of stability left in the six-planet solution. Habitability assessments accounting for the stellar flux, as well as tidal dissipation effects, indicate that three (maybe four) planets are potentially habitable. Doppler and space-based transit surveys indicate that 1) dynamically packed systems of super-Earths are relatively abundant and 2) M-dwarfs have more small planets than earlier-type stars. These two trends together suggest that GJ 667C is one of the first members of an emerging population of M-stars with multiple low-mass planets in their habitable zones.