Limited laboratory angular measurements create uncertainty in rocky exoplanet reflected light

Researchers submitted a paper to arXiv on Oct. 20, 2026 showing that limited angular coverage in laboratory bidirectional reflectance distribution function measurements creates intrinsic systematic uncertainty when scientists interpret reflected light from short-period rocky exoplanets. A bidirectional reflectance distribution function describes how a surface reflects light at varying illumination and viewing angles. The Astronomical Journal has accepted the paper for publication.

The work carries the identifier arXiv:2610.28755 [astro-ph.EP] and The Astronomical Journal has accepted it for publication. Leonardos Gkouvelis submitted it to arXiv as version 1 on Oct. 20, 2026 at 18:01:36 UTC, and the file is 1,442 KB. The study frames its work around the beginning of terrestrial exoplanet characterization, which follows initial observations of short-period rocky planets. It notes that retrieval frameworks depend on forward models, and that atmospheric modeling benefits from extensive laboratory molecular opacity data.

The identifier appears under Earth and Planetary Astrophysics and Instrumentation and Methods for Astrophysics.

Rocky surface work lacks that same foundation of laboratory measurements. Reflectance libraries for planetary analog materials capture spectra at a single illumination and viewing angle. That approach limits spectral information but leaves angular scattering unmeasured. The gap matters because scientists currently interpret reflected light from short-period rocky planets using retrieval frameworks that depend on forward models.

Without multi-angular laboratory measurements, those frameworks lack the angular scattering constraints that atmospheric work derives from molecular opacity libraries. Different sets of photometric parameters can match one laboratory reflectance spectrum yet predict contrasting angular scattering and disk-integrated signals (signals summed across the planet’s visible disk). One spectrum cannot distinguish among parameter sets that would generate different disk-integrated observables.

Spectra from one geometry yield large errors in predicted signals, and the effect is strongest at small phase angles (the star–planet–observer angle). Simplified assumptions used in exoplanet studies, including isotropic scattering, can add systematic biases on top of single-geometry uncertainties. Isotropic scattering assumes that a surface reflects light equally in every direction.

The study groups this assumption with other simplifications currently used in exoplanet work as an added source of systematic error. The study extends the parameter ambiguity to reflected-light phase curves, geometric albedos and planet-star contrast ratios spanning 0.5–1.7μm. These quantities help scientists interpret reflected-light observations of exoplanets.

Researchers describe the 0.5–1.7μm interval as representative of the wavelengths targeted by future direct-imaging missions like the HWO. The authors link this interval to the HWO, a future direct-imaging mission. The Bloomsburg University Goniometer dataset supplies multi-angular laboratory measurements.

The authors show that directly constraining angular scattering behavior makes uncertainty in predicted observables collapse. Without those measurements, models cannot reliably predict reflected-light signals from rocky exoplanet surfaces. Angular information is essential for turning surface reflectance data into reliable exoplanet predictions.

The study concludes that physically predictive models of reflected light from rocky exoplanets require measurements taken at several angles. The authors base this conclusion on the collapse of uncertainty when multi-angular data directly constrain scattering behavior.