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

TL;DR

This paper surveys the frequency of Earth-like planets in the habitable zones of Sun-like stars, expressed as $η_{igoplus}$, and traces how definitions, data sets, and detection biases have driven widely varying estimates. It identifies key factors not fully incorporated in current estimates—stellar and planet multiplicity, host-star chemistry, stellar age, Galactic context, and spectral-type variation—and discusses how these could bias $η_{igoplus}$ inferences. The authors critique the limitations of transit, radial velocity, imaging, microlensing, and astrometric methods for constraining $η_{igoplus}$ today while outlining how upcoming missions (e.g., Roman, PLATO, Earth 2.0, HWO) and Gaia-enhanced stellar characterization across multiple techniques could converge toward a robust Earth-analog occurrence rate. They emphasize that robust convergence will require larger, better-characterized samples and sophisticated population models that account for complex astrophysical and observational biases, enabling more reliable mission yield estimates for biosignature-focused explorations.

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, $η_{\oplus}$, 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 $η_{\oplus}$. We first provide definitions for parameters that contribute to $η_{\oplus}$. 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 $η_{\oplus}$, and how detection techniques and surveys differ in their sensitivity and ability to accurately constrain $η_{\oplus}$. Understanding and refining the value of $η_{\oplus}$ 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.

Figures (12)

• Figure 1: Depiction of the many factors affecting $\eta_{\oplus}$ and measurements therein. The three main colors offer a loose categorization into planetary, stellar, and statistical issues (red, purple, and blue, respectively), with subcategories (in green) branching out and interconnecting
• Figure 2: A comparison of previous $\eta_{\oplus}$ estimates in log-scale, with the definition of the $\eta_{\oplus}$ regime shown on the right. Bryson2021 integrated over the optimistic habitable zone Kopparapu2013 based on individual stars' stellar effective temperature ($237 - 860$ days for a Sun-like star), while all other works integrated over a radius $R_p$ range and single period $P$ or instellation flux $S$ range for all stars. We show the Bryson2021 estimates for two bounding completeness extrapolations. Estimates come from Petigura2013ForemanMackey2014Silburt2015Burke2015, SAG13, Mulders2018Hsu2019Pascucci2019Bryson2020Kunimoto2020Bryson2021bergsten2022
• Figure 3: The DR25 Kepler exoplanet candidate population around FGK stars used in the analysis of Bryson2021, shown in period and radius, colored and sized by reliability with exoplanet radius error bars. The background color map and contours indicate detection completeness, which was only measured for orbital periods $<500$ days. The longest detectable period from Kepler data assuming three transits is indicated at 710 days. The rectangle shows the SAG13 definition of $\eta_\oplus$, Adapted from Bryson2020.
• Figure 4: Planetary radius if the primary star is the planet host, plotted against the instellation flux for the planet. The open circles denote the Kepler DR25 values thompson2018planetary, while the closed circles denote the revised values from Sullivan2022c The dark red shaded region is the region inside the optimistic HZ, while the light red shaded region is the instellation range between the conservative and optimistic HZ (calculated using koskinen2013escape). The blue shaded region falls outside both the conservative and optimistic HZs. The light and dark gray dotted lines show the 1 and 1.8 R$_{\oplus}$ boundaries, respectively. One of the planets originally outside the HZ moves into it. Almost all the planets in the HZ move above the radius gap, and 12 of the total 17 apparent super-Earths are actually sub-Neptunes, suggesting that the majority of detected planets in binaries are not true super-Earths. Figure adapted from Sullivan2022c.
• Figure 5: Studies of Earth-Jupiter correlations usually rely on a sample of small planets closer-in than the habitable zone. This figure shows the sample of systems (grey lines) with both inner super-Earths (blue circles) and outer giants (orange squares) from the California Legacy Survey Rosenthal2021Rosenthal2022; we exclude all but the first-detected planet of each type for each system. An additional example of known systems with inner small and outer giant planets can be found in Figure 4 of VanZandt2025.