Long-period magnetic activity in the K dwarf GJ1137 and a new super-Earth on a 9-day orbit

A cautionary tale for Jovian-analogue detection from Doppler surveys^★

¹ Department of Astronomy, Sofia University “St Kliment Ohridski”, 5 James Bourchier Blvd, BG-1164 Sofia, Bulgaria
² Instituto de Astrofísica de Canarias, 38205 La Laguna, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Spain
⁴ Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko shosse Blvd., 1784 Sofia, Bulgaria
⁵ Ss. Cyril and Methodius University in Skopje, Faculty of Natural Sciences and Mathematics-Skopje, Institute of Physics, Arhimedova 3 1000 Skopje, North Macedonia
⁶ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁷ Centre for Exoplanets and Habitability, University of Warwick, Coventry CV4 7AL, UK
⁸ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone (RM), Italy
⁹ Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹⁰ Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy
¹¹ Consejo Superior de Investigaciones Científicas (CSIC), 28006 Madrid, Spain
¹² Light Bridges S. L., 35004 Las Palmas de Gran Canaria, Spain
¹³ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁴ Landessternwarte, Zentrum für Astronomie der Universtät Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 23 October 2025
Accepted: 22 January 2026

Abstract

Context. Detection and characterisation of Jovian analogues in precision radial velocity (RV) measurements is gaining momentum due to the constantly increasing observational baseline of Doppler surveys. The occurrence rate of Jovian-mass exoplanets is crucial to understanding the architecture of planetary systems. However, long-period RV signals in Doppler surveys could also be induced by stellar magnetic cycles, leading to misinterpretations of planetary candidates.

Aims. We investigate long-term RV variability in the K-dwarf star GJ 1137 (HD 93083, HIP 52521), 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 (P_cyc = 5870₋₃₅₀⁺⁴⁸⁰ days) rather than by an orbiting planet. The signal is detected in both full width at half maximum (FWHM) of the cross-correlation function and the chromospheric activity index log R′_HK. We measure the stellar rotation period to P_rot = 32.3_−1.3^+1.2 d and identify a significant short-period RV signal, which we attribute to a Super Earth with a period of 9.6412₋₍₁₁₎⁺⁽¹²⁾ d and a minimum mass of 5.12_−0.69^+0.70 M_⊕, making GJ 1137 a multiple-planet system.