Dear Colleagues,

We’re thrilled to share draft science cases that will help shape the top-level science goals, instrument needs, and mission architecture for the Habitable Worlds Observatory (HWO) — NASA’s next flagship mission in formulation for astrophysics, as recommended by the Astro2020 Decadal Survey.

These 60 Science Case Development Documents (SCDDs), written by members of the four HWO Science Working Groups over the past 18 months, span a wide range of topics. They reflect the mission’s exciting scientific potential to usher in a new era of astrophysics discovery and address one of humanity's oldest questions "Are we alone?"

We invite the research community to join the cases as endorsers through the HWO Science Case Portal.

Endorsers will be acknowledged in the documents, similar to their acknowledgement in Astro2020 white papers. Endorsement signals support for the scientific goals of the case and HWO’s proposed role for achieving them — not authorship or responsibility for its content.

The portal hosts all current SCDDs, with documents being added through 30 May 2025. A subset of the authors are inviting feedback, while all welcome endorsement from members of the broader community. This open, transparent process ensures shared ownership of HWO’s scientific direction and supports future planning by the HWO Technology Maturation Project Office and the forthcoming Community Science and Instrument Team (CSIT).

The HWO SCDDs will be published through two venues: on arXiv and in the proceedings of the HWO25 Meeting. To support this, there are two deadlines:

Endorsers who sign on by 22 June 2025 will be acknowledged in the arXiv post (planned for the week of 30 June), as well as the HWO25 proceedings.

A second deadline, 15 August 2025, allows additional time for community engagement and incorporation of feedback from the HWO25 Meeting; endorsers who sign on by then will be acknowledged in the HWO25 proceedings.

Summary of Key Dates

19 – 30 May 2025	SCDDs released to research community
22 June 2025	Endorser deadline for inclusion in arXiv posting
30 June 2025	Target week for arXiv posting
28 – 31 July 2025	HWO25 Science & Technology Meeting
15 August 2025	Endorser deadline for inclusion in HWO25 proceedings

Note that the inclusion, form, and timing of acknowledgments, as well as participation in publication activities, are at the discretion of the authors (not all SCDDs may participate in every step uniformly).

We’re grateful to our many colleagues who helped create these science cases and are excited to invite the broader community to engage with them. Your feedback and support will be vital as we advance the development of the HWO mission together.

On behalf of the HWO Science Working Groups,

Janice Lee & Paul Scowen	Evolution of the Elements Co-Chairs
Giada Arney & Niki Parenteau	Living Worlds Co-Chairs
Marc Postman & Swara Ravindranath	Galaxy Growth Co-Chairs
Evgenya Shkolnik & Tyler Robinson	Solar Systems in Context Co-Chairs
Stephanie LaMassa & Jessica Noviello	HWO Science Case Curators