A Scatter of Light from a Polarized World
-
1.
The Aerospace Corporation
- 2. Solafune, Inc., Solix Shibuya 401, 3-chōme-6-15, Shibuya, Shibuya City, Tokyo 150-0002, Japan
-
3.
Macau University of Science and Technology
-
4.
California Institute of Technology
Abstract
Many known exoplanets harbor clouds, which lead to degeneracies in spectroscopic models between particle composition and size. Polarimetry, however, provides independent assessment. Here we report the 7.2σ discovery of linearly polarized, scattered light from the hot Jupiter HD 189733b in B band (390–475 nm) peaking near quarter phase with Δp = 40.9 ± 7.1 ppm. Polarization measurements, obtained with the POLISH2 polarimeter at both Gemini North and the Lick Observatory 3 m, are best explained by silicate (SiO2 or MgSiO3) particles with effective radius r_(eff) = 0.038_(−0.023)^(+0.047) μm (90% confidence). This is broadly consistent with results from both Hubble transmission spectroscopy and JWST secondary eclipse spectroscopy suggesting small, SiO2 scattering particles. It is difficult to reconcile large polarization and moderate Hubble secondary eclipse depth via pure Rayleigh, silicate, or MnS scatterers. The measured polarization of HD 189733b is detected with such high confidence that we place a 2σ lower limit on its B-band geometric albedo of Ag > 0.26 with a preferred value of Ag = 0.6. This is larger than the prior estimate of Ag = 0.226 ± 0.091 from Hubble secondary eclipse photometry, and it presents HD 189733b as one of the most reflective known exoplanets in B band. It also validates Rayleigh scattering from the exoplanet, as opposed to starspot contamination, as the cause of HD 189733’s blue optical slope in transmission spectroscopy. Assuming other known exoplanets harbor atmospheres like HD 189733b, we model dozens to be detectable with at least 5σ confidence after one week of Gemini time each.
Copyright and License
© 2025. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Acknowledgement
The authors wish to acknowledge scattering calculations performed by Max Michaelis and Sebastian Wolf with the POLARIS code extended to model scattered light from exoplanets.
This work was supported by NASA XRP grant 80NSSC19K1578 and based on POLISH2 observations obtained at both the University of California Observatories/Lick Observatory and the international Gemini Observatory (program ID GN-2018A-C-1), a program of NSF NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the U.S. National Science Foundation on behalf of the Gemini Observatory partnership: the National Science Foundation (United States), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea). This work was enabled by observations made from the Gemini North telescope, located within the Maunakea Science Reserve and adjacent to the summit of Maunakea.
Facilities
Gemini:Gillett - Gillett Gemini North Telescope (POLISH2), Shane - Lick Observatory's 3m Shane Telescope (POLISH2).
Files
Wiktorowicz_2025_AJ_170_291.pdf
Additional details
Related works
- Is new version of
- Discussion Paper: arXiv:2509.19172 (arXiv)
Funding
- National Aeronautics and Space Administration
- 80NSSC19K1578
Dates
- Accepted
-
2025-09-19
- Available
-
2025-10-29Published
Caltech Custom Metadata
- Caltech groups
- Division of Geological and Planetary Sciences (GPS)
- Publication Status
- Published