# Systematic Uncertainties in Rocky Exoplanet Reflected Light from Incomplete Laboratory BRDF Measurements

**Authors:** Leonardos Gkouvelis, Michael K. Shepard, Paul Helfenstein, Francisco J. Pozuelos, Renyu Hu  
**arXiv:** [2610.28755](https://arxiv.org/abs/2610.28755)  
**Date:** 2026-10-20  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2610.28755v1/

## Key points

- Single-geometry laboratory reflectance spectra cannot uniquely predict rocky exoplanets’ appearance across orbital phases because they do not fully constrain angular scattering.
- Multiple surface models can match the same measured spectrum yet predict substantially different phase curves, geometric albedos, and planet-to-star contrast ratios.
- Prediction uncertainties are especially large at small phase angles, when planets appear nearly fully illuminated.
- Assuming isotropic scattering introduces additional systematic biases beyond uncertainties from incomplete angular measurements.
- Multi-angular laboratory measurements sharply reduce uncertainty and are necessary for physically predictive reflected-light models; spectral coverage alone is insufficient.
- The study covers wavelengths of 0.5–1.7 micrometers, relevant to future direct-imaging missions such as the Habitable Worlds Observatory.
- Limited laboratory data can undermine interpretation of future observations regardless of observational precision.
- These conclusions come from the title and abstract only; numerical uncertainty ranges, methodological details, and consistency across materials could not be verified.

## Summary

Leonardos Gkouvelis and colleagues investigate how incomplete laboratory measurements of rocky materials limit predictions of reflected light from rocky exoplanets. Their central question is whether a surface reflectance spectrum measured at a single illumination and viewing geometry can reliably predict a planet’s appearance at different orbital phases. This question matters because interpreting reflected-light observations requires models that distinguish the effects of surface composition, scattering properties, and any atmosphere. Although atmospheric models benefit from extensive laboratory molecular-opacity databases, comparable support for rocky-surface models remains limited. This summary is based solely on the paper’s title and abstract. The full text was unavailable, so detailed numerical results and methodological specifics could not be verified. The study focuses on the bidirectional reflectance distribution function (BRDF), which describes how a surface reflects incoming light in different viewing directions. Many planetary-material reflectance libraries provide spectra measured at only one combination of illumination and viewing angles. These spectra constrain how reflectance varies with wavelength but do not fully capture its angular dependence. The authors show that multiple combinations of photometric parameters can reproduce the same laboratory spectrum while predicting different scattering behavior at other geometries. Matching a single laboratory measurement therefore does not necessarily yield a unique, physically predictive surface model. To assess the observational consequences, the authors propagate this ambiguity into disk-integrated planetary observables, including reflected-light phase curves, geometric albedos, and planet-to-star contrast ratios. Their calculations span wavelengths from 0.5 to 1.7 micrometers, a range relevant to future direct-imaging missions such as the Habitable Worlds Observatory. Phase curves describe how planetary brightness changes with the star–planet–observer angle, while geometric albedo measures a planet’s brightness at full phase relative to a reference reflector. The authors also examine biases introduced by assuming isotropic scattering and use multi-angular measurements from the Bloomsburg University Goniometer dataset to test the benefits of directly constraining angular behavior. The main finding is that single-geometry laboratory spectra allow substantial uncertainty in predicted reflected-light signals, especially at small phase angles, when a planet appears nearly fully illuminated. Surface models that agree at the measured laboratory geometry can therefore predict markedly different planetary brightnesses. The abstract also reports that isotropic-scattering assumptions introduce systematic biases beyond the uncertainty associated with incomplete angular measurements. By contrast, directly constraining scattering behavior with multi-angular laboratory data sharply reduces uncertainty in the predicted observables. The abstract does not provide numerical uncertainty ranges, bias percentages, sample sizes, or parameter values. Its principal quantitative detail is the wavelength interval of 0.5–1.7 micrometers. The authors conclude that multi-angular measurements are necessary to develop physically predictive reflected-light models of rocky exoplanets. Spectral coverage alone is insufficient; models also require experimental constraints on how materials scatter light across illumination and viewing geometries. More broadly, limitations in laboratory data can introduce systematic uncertainty into the interpretation of future observations, regardless of observational precision. However, the abstract does not establish how consistently these uncertainty reductions apply across materials and modeling choices, or how densely the relevant angles must be sampled. Assessing those limitations requires the full methodological and numerical results.