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

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.