HADES RV Programme with HARPS-N at TNG VI. GJ 3942 b behind dominant activity signals

TL;DR

This study analyzes five years of spectroscopic and photometric data of GJ 3942 to distinguish stellar activity from planetary signals, leading to the discovery of a super-Earth and potential additional planet candidate.

Contribution

It presents a detailed methodology combining Gaussian-process regression and MCMC to disentangle stellar activity from radial velocity signals, resulting in the detection of a new super-Earth around GJ 3942.

Findings

Discovered GJ 3942 b, a super-Earth with 6.9-day orbit and 7.1 Me mass.

Identified stellar rotation period at 16.3 days.

Detected a possible second planet candidate with 10.4-day period.

Abstract

Short- to mid-term magnetic phenomena on the stellar surface of M-type stars cannot only resemble the effects of planets in radial velocity data, but also may hide them. We analyze 145 spectroscopic HARPS-N observations of GJ 3942 taken over the past five years and additional photometry to disentangle stellar activity effects from genuine Doppler signals as a result of the orbital motion of the star around the common barycenter with its planet. To achieve this, we use the common methods of pre-whitening, and treat the correlated red noise by a first-order moving average term and by Gaussian-process regression following an MCMC analysis. We identify the rotational period of the star at 16.3 days and discover a new super-Earth, GJ 3942 b, with an orbital period of 6.9 days and a minimum mass of 7.1 Me. An additional signal in the periodogram of the residuals is present but we cannot claim it to be related to a second planet with sufficient significance at this point. If confirmed, such planet candidate would have a minimum mass of 6.3 Me and a period of 10.4 days, which might indicate a 3:2 mean-motion resonance with the inner planet.