Early Exploration of the Scientific Discovery Space for the Habitable Worlds Observatory

The Habitable Worlds Observatory (HWO) is a future NASA flagship mission concept identified by the Astro2020 Decadal Survey as the highest priority for large space missions. It aims to conduct transformative astrophysics and search for biosignatures in the atmospheres of roughly 25 potentially Earth-like planets. To advance early development, NASA formed the Science, Technology, Architecture Review Team (START), which invited the scientific community to contribute through working groups.

This paper emerged from that collaborative process, presenting 70 science cases across four pillars: growth of galaxies (15 cases), evolution of the elements (13 cases), solar systems in context (32 cases), and living worlds (10 cases). Together they address 27 of the 30 science questions and discovery areas identified by Astro2020.

The 70 investigations require 140 observing programs, with 87% of cases needing spectroscopic observations and 30% photometric ones, spanning ultraviolet (UV) to near-infrared (NIR) wavelengths. High-contrast imaging and polarimetric capabilities are each needed for 34% and 27% of cases, respectively. UV access is critical: 83% of cases require data at wavelengths up to 2000 nm. Additional capabilities include precise astrometry for planet mass measurement, rapid response, a large instantaneous field of regard, non-sidereal tracking, saturation mitigation, and high dynamic range.

This portfolio defines the broad capability set HWO must possess to meet its dual goals of transformative astrophysics and biosignature discovery, providing a foundation for early mission design and technology prioritization.