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

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.

💡 Research Summary

The authors present a comprehensive re‑analysis of the K‑dwarf GJ 1137 (HD 93083, HIP 52521) using 13 years of high‑precision HARPS spectra (140 observations spanning 2004–2017). The data are split into pre‑ and post‑fiber‑upgrade subsets, and both SERVAL template‑matching radial velocities (RVs) and CCF‑derived RVs from the RACCOON pipeline are examined. In addition to RVs, a suite of activity diagnostics—BIS, contrast, FWHM, dLW, Hα, Na I D, CRX, and the chromospheric index log R′HK—are extracted from the HARPS‑RVBank.

Stellar parameters are refined via spectral‑energy‑distribution fitting with astroARIADNE, employing multiple atmospheric models (BT‑Settl, PHOENIX, etc.) and MIST isochrones. The star is found to be a K2 IV‑V dwarf with a mass of 0.836 ± 0.023 M⊙, radius 0.837 ± 0.026 R⊙, and an age of ~10.7 Gyr—significantly higher mass than previously assumed, which in turn raises the minimum mass of the known planet GJ 1137 b to ≈0.44 MJ.

A periodogram analysis of the RV time series reveals two dominant signals: a short‑period signal at 144.7 days, corresponding to the known Saturn‑mass planet GJ 1137 b, and a long‑period signal at ≈5 640 days (≈15.5 yr). The long‑period signal is also present in several activity indicators, most notably log R′HK, FWHM, and dLW, with Pearson correlation coefficients around 0.4–0.5 between the RV residuals (after subtracting the b‑planet) and these indices. This suggests that the long‑term RV variation may be driven by stellar magnetic activity rather than an additional massive companion.

To disentangle planetary and activity contributions, the authors employ a quasi‑periodic Gaussian Process (GP) model as a proxy for stellar activity. The GP kernel includes hyper‑parameters for the stellar rotation period (P_rot ≈ 32.3 days), the decay timescale of active regions (λ ≈ 5 870 days), and the amplitude (h ≈ 0.9 m s⁻¹). Joint modeling of the RVs with two Keplerian components (for planets b and a putative long‑period companion) plus the GP yields a Bayesian Information Criterion (BIC) improvement of >12 when the long‑period signal is modeled as activity, strongly favoring the activity hypothesis.

After accounting for the GP activity model, a residual periodicity at 9.6412 ± 0.12 days remains with an RV semi‑amplitude of ~1.8 m s⁻¹. This signal shows no correlation with any activity indicator, and its phase is stable over the 13‑year baseline. The inferred minimum mass is 5.12 ± 0.70 M⊕, placing the object in the super‑Earth regime. The authors designate this new planet GJ 1137 c, making the system a multi‑planet system comprising a Saturn‑mass planet at 144 days and a super‑Earth at 9.6 days.

The paper discusses the broader implications: long‑term magnetic cycles in K and M dwarfs can mimic the RV signature of Jovian analogues, potentially leading to false detections if activity is not properly modeled. The successful use of a GP with a quasi‑periodic kernel demonstrates a robust method for separating activity from planetary signals, especially in data sets that span multiple stellar cycles. The newly discovered super‑Earth adds to the growing inventory of low‑mass planets around nearby K dwarfs, making GJ 1137 a promising target for future high‑resolution spectroscopy (e.g., ELT‑ANDES) and possibly direct imaging campaigns.

In summary, the study (1) confirms that the ≈16‑year RV variation in GJ 1137 is caused by a magnetic activity cycle, (2) refines the orbital and physical parameters of the known planet GJ 1137 b, and (3) reports the detection of a new super‑Earth, GJ 1137 c, on a 9.64‑day orbit. The work underscores the necessity of careful activity modeling in the quest for long‑period exoplanets, especially around low‑mass stars.