NIRPS tightens the mass estimate of GJ 3090 b and detects a planet near the stellar rotation period

We present an updated characterization of the planetary system orbiting the nearby M2 dwarf GJ 3090 (TOI-177; $d = 22$ pc), based on new high-precision radial velocity (RV) observations from NIRPS and HARPS. With an orbital period of 2.85 d, the transiting sub-Neptune GJ 3090 b has a mass we refine to $4.52 \pm 0.47 M_{\oplus}$, which, combined with our derived radius of $2.18 \pm 0.06 R_{\oplus}$, yields a density of $2.40^{+0.33}{-0.30}$ g cm$^{-3}$. The combined interior structure and atmospheric constraints indicate that GJ 3090 b is a compelling water-world candidate, with a volatile-rich envelope in which water likely represents a significant fraction. We also confirm the presence of a second planet, GJ 3090 c, a sub-Neptune with a 15.9 d orbit and a minimum mass of $10.0 \pm 1.3 M{\oplus}$, which does not transit. Despite its proximity to the star’s 18 d rotation period, our joint analysis using a multidimensional Gaussian process (GP) model that incorporates TESS photometry and differential stellar temperature measurements distinguishes this planetary signal from activity-induced variability. In addition, we place new constraints on a non-transiting planet candidate with a period of 12.7 d, suggested in earlier RV analyses. This candidate remains a compelling target for future monitoring. These results highlight the crucial role of multidimensional GP modelling in disentangling planetary signals from stellar activity, enabling the detection of a planet near the stellar rotation period that could have remained undetected with traditional approaches.

💡 Research Summary

The paper presents a comprehensive re‑characterisation of the planetary system orbiting the nearby M2 dwarf GJ 3090 (TOI‑177, distance ≈ 22 pc) by combining new high‑precision radial‑velocity (RV) measurements from the Near‑InfraRed Planet Searcher (NIRPS) with an expanded HARPS dataset. The authors obtained 84 NIRPS RVs (median precision ≈ 1.8 m s⁻¹) and 109 HARPS RVs (median precision ≈ 1.9 m s⁻¹), together with simultaneous differential stellar temperature (ΔT) measurements from HARPS‑HAM and long‑baseline photometry from TESS (four sectors) and ASAS‑SN (≈ 11 yr).

A central methodological advance is the implementation of a multidimensional Gaussian Process (GP) framework that simultaneously models three observables: the TESS light curve (capturing stellar rotation and possible long‑term magnetic cycles), the ΔT activity indicator (a physically motivated proxy for spot‑plage temperature contrast), and the RV time series. Each dataset shares a common quasi‑periodic covariance kernel (rotation period ≈ 18 days) but retains independent offsets and jitter terms for each instrument. This joint modelling allows the authors to disentangle activity‑induced RV variations from genuine planetary signals, especially crucial because one of the planets’ orbital periods lies close to the stellar rotation period.

The refined analysis yields a precise mass for the transiting sub‑Neptune GJ 3090 b: M = 4.52 ± 0.47 M⊕, combined with an updated radius of 2.18 ± 0.06 R⊕ derived from the full TESS dataset, giving a bulk density of 2.40 g cm⁻³. Interior structure modelling, incorporating both core‑mantle composition (informed by NIRPS‑derived Fe/Mg ratios) and possible volatile layers, indicates that water (or water‑rich ices) likely constitutes a substantial fraction (30‑60 %) of the planet’s total volume. This places GJ 3090 b among the most compelling water‑world candidates in the sub‑Neptune regime, bridging the radius valley between super‑Earths and larger Neptunes.

In addition to confirming the previously reported non‑transiting planet GJ 3090 c, the authors detect a clear Keplerian signal at a period of 15.9 days with a minimum mass of 10.0 ± 1.3 M⊕. Although this period is close to the stellar rotation period, the multidimensional GP successfully isolates it as a planetary signal, demonstrating the power of the approach for systems where orbital and rotation periods overlap.

A third signal, with a period of 12.7 days, emerges in the RV residuals at modest significance. While not yet confirmed, the authors treat it as a promising candidate for future follow‑up, emphasizing that continued high‑precision monitoring (e.g., with ESPRESSO or future NIR spectrographs) will be required to validate its planetary nature.

The activity diagnostics reveal that the ΔT indicator is the most sensitive tracer of the 18‑day rotation signal for this M dwarf, outperforming traditional chromospheric indices such as Hα or bisector span. This underscores the advantage of near‑infrared spectroscopy for probing stellar heterogeneities in cool stars.

Overall, the study showcases three key contributions: (1) a substantial improvement in the mass precision of GJ 3090 b (≈ 10 % uncertainty), (2) a robust demonstration that multidimensional GP modelling can recover planetary signals even when they sit within the stellar rotation window, and (3) an enriched picture of the GJ 3090 system architecture, comprising at least two confirmed sub‑Neptune planets and a tentative third. These results highlight the synergy between next‑generation near‑infrared RV instruments and sophisticated statistical tools, paving the way for more reliable detection and characterization of low‑mass planets around active M dwarfs.