A dynamically-packed planetary system around GJ667C with three super-Earths in its habitable zone

TL;DR

This study confirms a six-planet system around GJ 667C, including three potentially habitable super-Earths, and provides evidence for a possible seventh planet, using combined Doppler data and stability analysis.

Contribution

It presents new Doppler measurements and a comprehensive analysis supporting a stable, multi-planet system with potential habitability around GJ 667C, including the possible detection of an additional planet.

Findings

Six planets are compatible with stable orbits.

Three planets are potentially habitable based on stellar flux and tidal effects.

Evidence suggests a possible seventh planet in a stable orbit.

Abstract

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. GJ 667C is already known to be orbited by two super-Earth candidates. We investigate whether the data supports the presence of additional companions. 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 activity indices and investigate quasi-periodicity. Doppler measurements of GJ 667C are described better by six 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 $\sim$ 1 M$_\oplus$ 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$\times 10^4$ solutions derived from the Bayesian samplings. 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. 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...