The California Planet Survey II. A Saturn-Mass Planet Orbiting the M Dwarf Gl649

TL;DR

This paper reports the discovery of a Saturn-mass planet orbiting the M dwarf Gl649, providing new insights into planet formation around low-mass stars and refining the occurrence rate of giant planets in such systems.

Contribution

It presents the detection of a new giant planet around an M dwarf and updates the occurrence rate of such planets around low-mass stars.

Findings

Discovery of a Saturn-mass planet orbiting Gl649.

Occurrence rate of giant planets around M dwarfs is approximately 3.4%.

Higher occurrence rate (~10.7%) around metal-rich M dwarfs.

Abstract

We report precise Doppler measurements of the nearby (d = 10.34 pc) M dwarf Gl649 that reveal the presence of a planet with a minimum mass Msini = 0.328 Mjup in an eccentric (e = 0.30), 598.3 day orbit. Our photometric monitoring reveals Gl649 to be a new variable star with brightness changes on both rotational and decadal timescales. However, neither of these timescales are consistent with the 600-day Doppler signal and so provide strong support for planetary reflex motion as the best interpretation of the observed radial velocity variations. Gl649b is only the seventh Doppler-detected giant planet around an M dwarf. The properties of the planet and host-star therefore contribute significant information to our knowledge of planet formation around low-mass stars. We revise and refine the occurrence rate of giant planets around M dwarfs based on the California Planet Survey sample of low-mass stars (M* < 0.6 Msun). We find that f = 3.4^{+2.2}_{-0.9}% of stars with M* < 0.6 Msun harbor planets with Msini > 0.3$ Mjup and a < 2.5 AU. When we restrict our analysis to metal-rich stars with [Fe/H] > +0.2 we find the occurrence rate is 10.7^{+5.9}_{-4.2}%.