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

TL;DR

This study refines the mass and composition of GJ 3090 b, confirms a second planet GJ 3090 c using advanced Gaussian process modeling, and demonstrates the importance of multidimensional analysis in disentangling planetary signals from stellar activity.

Contribution

The paper introduces a multidimensional Gaussian process approach to distinguish planetary signals from stellar activity, enabling detection of planets near stellar rotation periods.

Findings

Refined mass of GJ 3090 b to 4.52 ± 0.47 M⊕

Confirmed GJ 3090 c with 15.9-day orbit and 10.0 ± 1.3 M⊕

Detected a potential non-transiting planet candidate at 12.7 days

Abstract

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.