Why Estimating η⊕ Is Difficult: A Kepler-Centric Perspective

η_⊕, the occurrence rate of rocky habitable zone exoplanets orbiting Sun-like stars, is of great interest to both the astronomical community and the general public. The Kepler space telescope has made it possible to estimate η_⊕, but estimates by different groups vary by more than an order of magnitude.

We identify several causes for this range of estimates. We first review why, despite being designed to estimate η_⊕, Kepler’s observations are not sufficient for a high-confidence estimate, due to Kepler’s detection limit coinciding with the η_⊕ regime.

This results in a need to infer η_⊕, for example extrapolating from a regime of non-habitable zone, non-rocky exoplanets. We examine two broad classes of causes that can account for the large discrepancy in η_⊕ found in the literature: a) differences in definitions and input data between studies, and b) fundamental limits in Kepler data that lead to large uncertainties and poor accuracy.

We highlight the risk of large biases when using extrapolation to describe small exoplanet populations in the habitable zone. We discuss how η_⊕ estimates based on Kepler data can be improved, such as reprocessing Kepler data for more complete, higher-reliability detections and better exoplanet catalog characterization. We briefly survey upcoming space telescopes capable of measuring η_⊕, and how they can be used to supplement Kepler data.

Steve Bryson, Michelle Kunimoto, Ruslan Belikov, Galen J. Bergsten, Sakhee Bhure, William J. Borucki, Douglas A. Caldwell, Aritra Chakrabarty, Rachel B. Fernandes, Matthias Y. He, Jon M. Jenkins, Kristo Ment, Michael R. Meyer, Gijs D. Mulders, Ilaria Pascucci, Peter Plavchan

Comments: 24 pages, 9 figures, submitted to PASP
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2511.05658 [astro-ph.EP] (or arXiv:2511.05658v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2511.05658
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Submission history
From: Michelle Kunimoto
[v1] Fri, 7 Nov 2025 19:00:05 UTC (1,908 KB)
https://arxiv.org/abs/2511.05658

Astrobiology,