# Chasing rainbows and ocean glints: Inner working angle constraints for the Habitable Worlds Observatory

**Authors:** Sophia R. Vaughan, Timothy D. Gebhard, Kimberly Bott, Sarah L. Casewell, Nicolas B. Cowan, David S. Doelman, Matthew Kenworthy, Johan Mazoyer, Maxwell A. Millar-Blanchaer, Victor J. H. Trees, Daphne M. Stam, Olivier Absil, Lisa Altinier, Pierre Baudoz, Ruslan Belikov, Alexis Bidot, Jayne L. Birkby, Markus J. Bonse, Bernhard Brandl, Alexis Carlotti  
**arXiv:** [2307.15137](https://arxiv.org/abs/2307.15137)  
**Date:** 2023-07-27  
**Summary page:** https://www.summarizepaper.com/en/paper/2307.15137v1/

## Key points

- NASA is planning for a space mission called HabWorlds to detect rocky planets in habitable zones and determine their habitability
- The presence of surface liquid water is crucial for defining planetary habitability
- The observatory will use photometric and polarimetric phase curves of starlight reflected by exoplanets to reveal optical phenomena such as ocean glint, rainbows, and scattering effects caused by clouds or atmospheric gas
- The coronagraph used in HabWorlds may obscure the phase angles where these optical features are most prominent
- Accessibility of different phase angles depends on factors like the planet's orbital inclination and the inner working angle (IWA) of the coronagraph
- A catalog of 164 stars relevant to HabWorlds was used to estimate the number of exo-Earths that could be searched for ocean glint, rainbows, and polarization effects resulting from Rayleigh scattering
- Most exo-Earth planetary systems can observe the polarimetric Rayleigh scattering peak
- Rainbows caused by water clouds at phase angles ranging from approximately 20-60 degrees would be accessible in around 46 systems with Earth equivalent instellation
- Ocean glint signature at phase angles between approximately 130-170 degrees would be observable in about 16 systems assuming an IWA of 62 milliarcseconds (mas)
- Improving the IWA to 41 mas would increase accessibility to rainbows and glints by factors of approximately 2 and 3 respectively
- Observing these scattering features can help detect surface oceans and water cycles, which are key indicators of habitability

## Summary

NASA is currently planning for a Habitable Worlds Observatory (HabWorlds), a space mission aimed at detecting rocky planets in habitable zones and determining their habitability. The presence of surface liquid water is crucial in defining planetary habitability. To achieve this goal, the observatory will utilize photometric and polarimetric phase curves of starlight reflected by exoplanets to reveal various optical phenomena such as ocean glint, rainbows, and scattering effects caused by clouds or atmospheric gas. While direct imaging missions are typically optimized for planets near quadrature, it is important to note that the coronagraph used in HabWorlds may obscure the phase angles where these optical features are most prominent. Therefore, the accessibility of different phase angles for a given exoplanet will depend on factors such as the planet's orbital inclination and the inner working angle (IWA) of the coronagraph. In this study, the authors utilized a recently-created catalog consisting of 164 stars relevant to HabWorlds to estimate the number of exo-Earths that could be searched for ocean glint, rainbows, and polarization effects resulting from Rayleigh scattering. They found that the polarimetric Rayleigh scattering peak can be observed in most exo-Earth planetary systems. Specifically, they determined that rainbows caused by water clouds at phase angles ranging from approximately 20-60 degrees would be accessible with HabWorlds in around 46 systems if the planet has an Earth equivalent instellation. Additionally, they found that the ocean glint signature at phase angles between approximately 130-170 degrees would be observable in about 16 systems assuming an IWA of 62 milliarcseconds (mas), which corresponds to three times lambda divided by telescope diameter (3λ/D). Furthermore, they demonstrated that improving the IWA to 41 mas (corresponding to two times lambda divided by telescope diameter; 2λ/D) would increase accessibility to rainbows and glints by factors of approximately 2 and 3 respectively. By observing these scattering features, HabWorlds has potential to detect surface oceans and water cycles which are key indicators of habitability. This research provides valuable insights into inner working angle constraints for Habitable Worlds Observatory and highlights importance of considering phase angles in search for habitable exoplanets.