Neptune- or sub-Neptune-mass planet beyond the snow line

Determine whether a Neptune- or sub-Neptune-mass planet exists beyond the water-ice snow line in the TRAPPIST-1 planetary system by obtaining radial-velocity measurements with sufficient sensitivity to detect approximately 5 m s−1 semi-amplitude signals on long orbital periods.

Background

The NIRPS radial-velocity observations exclude planets more massive than Saturn on orbits out to 2.7 years and Neptune-mass planets on orbits shorter than 20 days. However, the available data are not sufficiently precise to rule out lower-mass planets on longer-period orbits beyond the system’s water-ice snow line.

The authors state that resolving this uncertainty would require approximately a two-fold improvement in radial-velocity sensitivity for periods shorter than 100 days and a four-fold improvement for periods extending to 2.7 years. Because the expected planetary signal would be comparable to stellar-activity signals, future observations would also need to characterize the stellar activity on short timescales.

References

Still, with the available data, we cannot exclude the possibility of a Neptune or sub-Neptune-mass planet beyond the snow line.

— Radial velocity detection of the TRAPPIST-1 planetary system with SPIRou and NIRPS  (2609.03006 - L'Heureux et al., 2 Sep 2026) in Section 3.3, “Constraints on the presence of giant planets with NIRPS”