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 which, combined with our derived radius of ̊adius,R_⊕, yields a density of -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 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.