Scaling K2. VIII. Short-period Sub-Neptune Occurrence Rates Peak Around Early-type M Dwarfs

September 2025 • 2025AJ....170..183H

Authors • Hardegree-Ullman, Kevin K. • Bergsten, Galen J. • Christiansen, Jessie L. • Zink, Jon K. • Bhure, Sakhee • Boley, Kiersten M. • Fernandes, Rachel B. • Giacalone, Steven • Karpoor, Preethi R.

Abstract • We uniformly combined data from the NASA Kepler and K2 missions to compute planet occurrence rates across the entire FGK and M-dwarf stellar range. The K2 mission, driven by targets selected by guest observers, monitored nine times more M dwarfs than the Kepler mission. Combined, Kepler and K2 observed 130 short-period (P = 1–40 days) Earth to Neptune-sized candidate planets orbiting M dwarfs. K2 observed 3.5 times more of these planets than Kepler for host stars below 3700 K. Our planet occurrence rates show that short-period sub-Neptunes peak at 3750‑97+153 K and drop for cooler M dwarfs. A peak near this location was predicted by pebble accretion planet formation models and confirmed here by observations for the first time. Super-Earths continue to increase in occurrence toward cooler stars and show no clear evidence of a peak in the host star range considered here (3200–6900 K). Our observations provide critical input to further refine planet formation models. We strongly recommend further study of mid-to-late M dwarfs with TESS and soon the Nancy Grace Roman Space Telescope and PLATO to identify additional small planet trends.

Links

• PDF https://iopscience.iop.org/article/10.3847/1538-3881/adf633/pdf
• PREPRINT http://arxiv.org/abs/2508.05734
• ELECTR https://doi.org/10.3847/1538-3881/adf633