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

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 by54 the number of planets in the solar neighborhood with conditions similar to Earth. The occurrence rate55 of Earth-like planets in the habitable zones of Sun-like stars, η⊕, is therefore crucial for addressing56 the apparent lack of consensus on its value in the literature. Here we present a review of the current57 understanding of η⊕. We first provide definitions for parameters that contribute to η⊕. Then, we58 discuss the previous and current estimated parameter values and the context of the limitations on the59 analyses that produced these estimates. We compile an extensive list of the factors that go into any60 calculation of η⊕, and how detection techniques and surveys differ in their sensitivity and ability to61 accurately constrain η⊕. Understanding and refining the value of η⊕is crucial for upcoming missions62 and telescopes, such as the planned Habitable Worlds Observatory and the Large Interferometer for63 Exoplanets, which aim to search for biosignatures on exoplanets in the solar neighborhood.64