Exoplanets, -moons, -comets

A Scatter of Light from a Polarized World

By Keith Cowing

Status Report

astro-ph.EP

September 25, 2025

Filed under astro-ph.EP, astronomy, atmosphere, Clouds, exoplanet, Gemini North, HD 189733b, hot jupiter, Hubble, imaging, JWST, Lick Observatory, polarimetry, Polarized light, starspot

A Scatter of Light from a Polarized World — Raw B band polarization measurements of HD 189733 obtained at Gemini North on UT 1 to 7 Aug 2018, which are dominated by observations on the last night (gold points). Top left: Stokes q (linear polarization) modulation as a function of parallactic angle. Telescope linear polarization is modulated by parallactic angle with amplitude ∆(qTP, uTP) = 0.2620±0.0034% (Wiktorowicz et al. 2023), where the best fit curve from Equation 2a is shown in black. Uncertainties in most data points are smaller than the size of the data points. Top right: Same as at top left but for Stokes u (linear polarization, Equation 2b). Bottom left: Same as at top left but for Stokes v (circular polarization, Equation 2c). Bottom right: Temporal variation of parallactic angle, which rotates rapidly as the target crosses the meridian (black dashed lines) around 20:00 LST. This rapid meridian crossing provides the bulk of the telescope polarization modulation necessary for accurate calibration. — astro-ph.EP

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 to 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 reff=0.038+0.047−0.023 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.

Sloane J. Wiktorowicz, Pushkar Kopparla, Jiazheng Li, Yuk L. Yung

Comments: 26 pages, 22 figures, accepted by AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2509.19172 [astro-ph.EP] (or arXiv:2509.19172v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2509.19172
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Submission history
From: Sloane Wiktorowicz
[v1] Tue, 23 Sep 2025 15:46:20 UTC (2,241 KB)
https://arxiv.org/abs/2509.19172
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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