Maximizing Precursor Knowledge of Potential Targets for the Habitable Worlds Observatory

A key objective of the upcoming Habitable Worlds Observatory (HWO) is to search for habitable and inhabited planets. The Astro2020 Decadal Survey recommended that the mission should be capable of searching for biosignatures in the atmospheres of roughly 25 habitable planets, and the NASA Exoplanet Exploration Program (ExEP) recently released a list of 164 potential target stars. While some of those targets may be excellent targets for HWO, the ExEP report cautioned that other targets may be less viable due to stellar companions, high exozodi levels, or other complicating factors. Assessing the viability of each potential target star early in the mission design process is important because the number of accessible targets depends on the mission design and the science output of HWO scales with the number of viable targets. If a high fraction of potential targets are not rejected until after major architecture choices are finalized, then HWO may not achieve the goal of surveying a sufficient number of habitable zones. In order to maximize the likelihood that HWO will detect a large number of habitable, and perhaps inhabited, planets, we have begun a comprehensive investigation of potential target stars. First, we are inspecting the astronomical literature and public data archives to assess current knowledge of the 164 targets listed in the ExEP Mission Star List including limits on the presence of planets in those systems. Second, we are supplementing the ExEP Mission Star List by adding additional stars so that HWO mission design trade studies can explore a wider region of parameter space. Third, for potential targets with known stellar or substellar companions, we are conducting dynamical simulations to determine which systems can host habitable planets over long timescales. Fourth, for each viable target, we are developing an observational plan to maximize our precursor knowledge of planetary system architecture prior to HWO observations. By assessing the suitability of potential targets early in the HWO mission design process, we will help ensure that HWO is able to observe a large sample of small, temperate planets and place meaningful constraints on the frequency of life in the universe.