Are We There Yet? Challenges in Quantifying the Frequency of Earth Analogs in the Habitable Zone

December 2025 • 2025PASP..137l1001F

Authors • Fernandes, Rachel B. • Johnson, Samson • Bergsten, Galen J. • Bhure, Sakhee • Boley, Kiersten M. • Boss, Alan P. • Bryson, Steve • DeRocco, William • Dietrich, Jamie • Duck, Alison • Giacalone, Steven • Gupta, Arvind F. • He, Matthias Y. • Kunimoto, Michelle • Ment, Kristo • Sagear, Sheila • Silverstein, Michele L. • Sullivan, Kendall • Vrijmoet, Eliot Halley • Wagner, Kevin • Wilson, Robert F. • Brefka, Lucas • Belikov, Ruslan • Chakrabarty, Aritra • Christiansen, Jessie L. • Ciardi, David R. • Dattilo, Anne • Fitzmaurice, Evan • Ford, Eric B. • Hotnisky, Andrew • Jones, Sinclaire • Kar, Aman • Kopparapu, Ravi • Lowson, Nataliea • Mamajek, Eric E. • Mennesson, Bertrand • Meyer, Michael R. • Millholland, Sarah • Mulders, Gijs D. • Mullally, Susan E. • Murlidhar, Arjun • Pascucci, Ilaria • Ragozzine, Darin • Robertson, Paul • Stapelfeldt, Karl • Wright, Jason

Abstract • Searching for life elsewhere in the universe is one of the most highly prioritized pursuits in astronomy today. However, the ability to observe evidence of Earth-like life through biosignatures is limited by the number of planets in the solar neighborhood with conditions similar to Earth. The occurrence rate of Earth-like planets in the habitable zones of Sun-like stars, η_⊕, is therefore crucial for addressing the apparent lack of consensus on its value in the literature. Here we present a review of the current understanding of η_⊕. We first provide definitions for parameters that contribute to η_⊕. Then, we discuss the previous and current estimated parameter values and the context of the limitations on the analyses that produced these estimates. We compile an extensive list of the factors that go into any calculation of η_⊕, and how detection techniques and surveys differ in their sensitivity and ability to accurately constrain η_⊕. Understanding and refining the value of η_⊕ is crucial for upcoming missions and telescopes, such as the planned Habitable Worlds Observatory and the Large Interferometer for Exoplanets, which aim to search for biosignatures on exoplanets in the solar neighborhood.

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• PREPRINT http://arxiv.org/abs/2511.05660