2025AJ....170..291W

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 ${r}_{\,\rm{eff}\,}=0.03{8}_{-0.023}^{+0.047}$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 A_g > 0.26 with a preferred value of A_g = 0.6. This is larger than the prior estimate of A_g = 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.