Scaling K2 VIII: Short-Period Sub-Neptune Occurrence Rates Peak Around Early-Type M Dwarfs

TL;DR

This study combines Kepler and K2 data to analyze short-period planet occurrence rates around FGK and M dwarfs, revealing a peak in sub-Neptune planets around early-type M dwarfs, aligning with planet formation models.

Contribution

First comprehensive combined analysis of Kepler and K2 data across the full stellar range, identifying a peak in sub-Neptune occurrence around early M dwarfs.

Findings

Sub-Neptune occurrence peaks near 3750 K host stars.

K2 observed 3.5 times more planets around M dwarfs than Kepler.

Super-Earth occurrence increases toward cooler stars.

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 $3750^{+153}_{-97}$ 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 K$-$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.