Exoplanets, -moons, -comets

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

By Keith Cowing

Status Report

astro-ph.EP

August 13, 2025

Filed under atmosphere, exoplanet, M-dwarf, sub-Neptune

Scaling K2 VIII: Short-Period Sub-Neptune Occurrence Rates Peak Around Early-Type M Dwarfs — Planet occurrence rates as a function of stellar temperature for short-period (P = 1 − 40 days) super-Earths (green squares) and sub-Neptunes (blue circles). A power law was fit to the super-Earths, and a double broken power law was fit to the sub-Neptunes, showing two breaks: TBr,1 = 3750+153 −97 K and TBr,2 = 5758+94 −145 K. The short-period sub-Neptunes peak around a host type of M0.5 V, then decrease in planet occurrence toward cooler M dwarfs. G and K-type stars have comparable rates of short-period super-Earths and sub-Neptunes. Another noticeable decrease in sub-Neptunes happens for host stars hotter than the Sun. Short-period super-Earths increase in occurrence toward cooler stars, with no discernible peak in the host star temperatures for which we have data from Kepler and K2. — astro-ph.EP

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.

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

Comments: 15 pages, 5 figures, 7 tables. Accepted for publication in AJ. Uniform exoplanet and stellar parameters tables for Kepler and K2 are available to download at this https URL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2508.05734 [astro-ph.EP] (or arXiv:2508.05734v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2508.05734
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Related DOI:
https://doi.org/10.3847/1538-3881/adf633
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Submission history
From: Kevin Hardegree-Ullman
[v1] Thu, 7 Aug 2025 18:00:00 UTC (2,508 KB)
https://arxiv.org/abs/2508.05734
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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