Reliable detections of Earth-sized planets in the habitable zone remain
elusive in the Kepler sample, even for M dwarfs. The Kepler sample was once
thought to contain a considerable number of M dwarf stars (Teffβ<4000 K), which hosted enough Earth-sized ([0.5,1.5] Rββ) planets to
estimate their occurrence rate (Ξ·ββ) in the habitable zone. However,
updated stellar properties from Gaia have shifted many Kepler stars to earlier
spectral type classifications, with most stars (and their planets) now measured
to be larger and hotter than previously believed. Today, only one
partially-reliable Earth-sized candidate remains in the optimistic habitable
zone, and zero in the conservative zone. Here we performed a new investigation
of Kepler's Earth-sized planets orbiting M dwarf stars, using occurrence rate
models with considerations of updated parameters and candidate reliability.
Extrapolating our models to low instellations, we found an occurrence rate of
Ξ·ββ=8.58β8.22+17.94β% for the conservative habitable zone
(and 14.22β12.71+24.96β% for the optimistic), consistent with
previous works when considering the large uncertainties. Comparing these
estimates to those from similarly comprehensive studies of Sun-like stars, we
found that the current Kepler sample does not offer evidence to support an
increase in Ξ·ββ from FGK to M stars. While the Kepler sample is too
sparse to resolve an occurrence trend between early and mid-to-late M dwarfs
for Earth-sized planets, studies including larger planets and/or data from the
K2 and TESS missions are well-suited to this task.Comment: 22 pages, 11 figures, 2 tables; Accepted for publication in A