Long-period magnetic activity in the K dwarf GJ 1137 and a new super-Earth on a 9-day orbit

Abstract

Aims: We investigate long-term radial velocity (RV) variability in the K-dwarf star GJ 1137 (HD 93083, HIP52521), a known Saturn-mass exoplanet host, and assess the role of stellar activity in shaping the observed signals. Methods: We analyse 13 years of archival high-precision spectroscopic observations obtained with the High Accuracy Radial velocity Planet Searcher spectrograph (HARPS). We performed an extensive spectroscopic analysis of the stellar activity indicators and applied an RV modelling approach, incorporating Keplerian fits, Gaussian process regression as a proxy for stellar activity, and other stellar activity diagnostics. Furthermore, we refined the orbital parameters and the minimum mass of the known exoplanet GJ 1137 b and searched for additional planetary candidates in the system. Results: We detect a long-period RV signal that, if interpreted as planetary, would suggest the presence of a Jovian analogue companion. However, our spectroscopic activity analysis provides strong evidence that this variability is induced by the star's long-term magnetic cycle ( Pcyc = 5870+(480)-(350) days) rather than by an orbiting planet. The signal is detected in both full width at half maximum (FWHM) of the crosscorrelation function and the chromospheric activity index log R'Hk. We measure the stellar rotation period to Prot = 32.3+(1.2)-(1.3) d and identify a significant short-period RV signal, which we attribute to a Super Earth with a period of 9.6412+(12)-(11) d and a minimum mass of 5.12+(0.70)-(0.69) Earth masses, making GJ 1137 a multiple-planet system.