Continuous Habitable Zone Metric for Prioritizing Habitable Worlds Observatory Targets

TL;DR

This paper introduces a Bayesian-based metric to prioritize exoplanet targets for future direct imaging missions by estimating the likelihood of planets being in a long-term habitable zone, aiding efficient resource allocation.

Contribution

It develops a novel CHZ$_2$ likelihood metric to assess and rank Sun-like stars for habitable exoplanet detection based on continuous habitability duration.

Findings

CHZ$_2$ likelihood peaks at 3-4 Gyr for late-F and early-G stars.

Stars earlier than F3 or hotter than 6600 K are less likely to host long-term habitable zones.

The metric helps prioritize targets based on habitable zone accessibility and extent.

Abstract

Future direct imaging space telescopes, such as NASA's Habitable Worlds Observatory (HWO), will be the first capable of both detecting and characterizing terrestrial exoplanets in the habitable zones (HZ) of nearby Sun-like stars. Since this will require a significant amount of time and resources for even a single system or exoplanet, the likelihood that a system will host detectable life should be considered when prioritizing observations. One method of prioritization is to estimate the likelihood that an exoplanet has remained continuously within the HZ long enough for life to emerge and make a detectable impact on the atmosphere. We utilize a Bayesian method to calculate the likelihood that a given orbital radius around a star is currently in the 2 Gyr continuous habitable zone (CHZ$_2$), the approximate time it took life on Earth to significantly oxygenate the atmosphere. We apply this method to the 164 stars in the NASA Exoplanet Exploration Program Mission Star List (EMSL) for HWO, representing a preliminary sample of Sun-like stars with HZs most accessible to a future direct imaging mission. By considering the CHZ$_2$ likelihood at all orbital radii outside a hypothetical inner working angle for HWO, we define a metric for prioritizing targets according to the accessibility and total extent of the CHZ$_2$. We find that the CHZ$_2$ metric peaks between $3-4$ Gyr for late-F and early-G dwarfs, but tentatively determine that stars earlier than $\sim {F3}$ or hotter than $\sim 6600$ K are unlikely to have a CHZ$_2$ at the time of observation.