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, eta circle plus, 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 eta circle plus. We first provide definitions for parameters that contribute to eta circle plus. 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 eta circle plus, and how detection techniques and surveys differ in their sensitivity and ability to accurately constrain eta circle plus. Understanding and refining the value of eta circle plus 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.