Effect of the Bern NGPPS “steam” water-radius update on mid-to-late M-dwarf planet populations

Determine the effect of the updated treatment of water’s contribution to sub-Neptune radii in the Bern New Generation Planetary Population Synthesis (“steam”) models on the predicted planet population around mid-to-late M dwarfs with stellar masses between 0.1 and 0.4 solar masses, including quantitative changes in the synthetic occurrence rates and radius distributions compared to the pre-update models.

Background

The paper compares its empirical occurrence-rate results to theoretical population synthesis models, including the Bern New Generation Planetary Population Synthesis (NGPPS). A recent update to these models revised how water contributes to the radii of sub-Neptunes, an adjustment reported to be necessary to reproduce the Radius Valley around Sun-like stars.

Because the authors find that the Radius Valley effectively disappears around mid-to-late M dwarfs, understanding how the Bern model’s water-radius update impacts predictions for this low-mass stellar regime is crucial. However, the updated “steam” simulations have not yet been applied to mid-to-late M dwarfs, leaving the implications of the update for these stars unknown.

References

It bears noting that the Bern model's treatment of water's contribution to the radius of sub-Neptunes has been updated in \citet{burn_steam}, which they claim is needed to replicate the Radius Valley around Sun-like stars. However, the effect of this update on their planet population around mid-to-late M dwarfs is unknown because they have not yet considered these stars in their steam simulations.

— TESS Planet Occurrence Rates Reveal the Disappearance of the Radius Valley Around Mid-to-Late M Dwarfs  (2602.23364 - Gillis et al., 26 Feb 2026) in Section 6.2 (Comparisons to Theoretical Predictions)