No Evidence for More Earth-sized Planets in the Habitable Zone of Kepler's M versus FGK Stars

Creators: Bergsten, Galen J.¹; Pascucci, Ilaria¹; Hardegree-Ullman, Kevin K.¹; Fernandes, Rachel B.^{1, 2}; Christiansen, Jessie L.^{3, 4}; Mulders, Gijs D.^{5, 6}

1. University of Arizona
2. Pennsylvania State University
3. NASA Exoplanet Science Institute
4. California Institute of Technology
5. Adolfo Ibáñez University
6. Millennium Institute of Astrophysics

Abstract

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 (T_eff < 4000 K), which hosted enough Earth-sized ([0.5, 1.5] R_⊕) planets to estimate their occurrence rate (η_⊕) 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 η⊕=8.58_(−8.22)^(+17.94)% for the conservative habitable zone (and 14.22_(−12.71)^(+24.96)% for the optimistic one), 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 η_⊕ 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.

Copyright and License

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

I.P. and G.B acknowledge support from the NASA Astrophysics Data Analysis Program under grant No. 80NSSC20K0446. K.H-U. and R.B.F. acknowledge support from NASA under agreement No. 80NSSC21K0593 for the program "Alien Earths." G.D.M. acknowledges support from FONDECYT project 11221206, from ANID—Millennium Science Initiative—ICN12_009, and from the ANID BASAL project FB210003. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. The results reported herein benefited from collaborations and/or information exchange within NASA's Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA's Science Mission Directorate.

Funding

I.P. and G.B acknowledge support from the NASA Astrophysics Data Analysis Program under grant No. 80NSSC20K0446. K.H-U. and R.B.F. acknowledge support from NASA under agreement No. 80NSSC21K0593 for the program "Alien Earths." G.D.M. acknowledges support from FONDECYT project 11221206, from ANID—Millennium Science Initiative—ICN12_009, and from the ANID BASAL project FB210003.

Facilities

Gaia - , Kepler - The Kepler Mission.

Software References

NumPy (van der Walt et al. 2011), SciPy (Jones et al. 2001, Virtanen et al. 2020), Matplotlib (Hunter 2007), emcee (Foreman-Mackey et al. 2013), corner (Foreman-Mackey 2016), epos (Mulders et al. 2018), KeplerPORTs (Burke & Catanzarite 2017).

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