The CARMENES search for exoplanets around M dwarfs - HD 147379b: A nearby Neptune in the temperate zone of an early-M dwarf

TL;DR

This paper reports the discovery of HD 147379b, a Neptune-mass exoplanet orbiting an M dwarf within the star's temperate zone, using radial velocity data from the CARMENES survey and supporting observations from Keck.

Contribution

First detection of an exoplanet around an M dwarf using CARMENES RV data, demonstrating the survey's capability to find temperate zone planets.

Findings

HD 147379b has a minimum mass of 25 Earth masses.

The planet orbits at 0.32 AU with a period of 86.54 days.

Additional stellar rotation signals observed at 21.1 days.

Abstract

We report on the first star discovered to host a planet detected by radial velocity (RV) observations obtained within the CARMENES survey for exoplanets around M dwarfs. HD 147379 ($V = 8.9$ mag, $M = 0.58 \pm 0.08$ M$_{\odot}$), a bright M0.0V star at a distance of 10.7 pc, is found to undergo periodic RV variations with a semi-amplitude of $K = 5.1\pm0.4$ m s$^{-1}$ and a period of $P = 86.54\pm0.06$ d. The RV signal is found in our CARMENES data, which were taken between 2016 and 2017, and is supported by HIRES/Keck observations that were obtained since 2000. The RV variations are interpreted as resulting from a planet of minimum mass $m_{\rm p}\sin{i} = 25 \pm 2$ M$_{\oplus}$, 1.5 times the mass of Neptune, with an orbital semi-major axis $a = 0.32$ au and low eccentricity ($e < 0.13$). HD 147379b is orbiting inside the temperate zone around the star, where water could exist in liquid form. The RV time-series and various spectroscopic indicators show additional hints of variations at an approximate period of 21.1d (and its first harmonic), which we attribute to the rotation period of the star.