Exploring the Opposition Surge Effect When Studying Exoplanets

A recent study published in The Planetary Society journal explored a new theoretical framework for reflection and emission spectroscopy of rocky exoplanets, with a focus on improving the treatment of phase-dependent surface brightness (1). Although the Hapke theory of reflectance and emittance spectroscopy is widely applied in planetary science, a critical effect known as the opposition surge, which is an enhancement in brightness at small phase angles caused by shadow hiding and coherent backscattering, has been largely overlooked in exoplanet studies.

The study’s author, Leonardos Gkouvelis, who is in the Faculty of Physics at Ludwig Maximilian University (Munich, Germany), proposed an alternative formulation for modeling the opposition surge, designed to provide a physically consistent transition from smooth to sharp brightening as a function of phase angle (1). This approach explicitly incorporates wavelength-dependent scattering properties, particle sizes, and surface morphologies (1).

As part of the study, Gkouvelis compared a family of models with varying levels of simplification, from fully wavelength-dependent opposition surge treatments to models that ignore the effect entirely. The results demonstrated that neglecting opposition effects can lead to systematic errors of 20%–30% in retrieved albedos, spectral features, and phase curves, particularly within certain spectral bands (1). These inaccuracies may significantly affect interpretations of surface mineralogy, roughness, and overall composition.

The findings are especially relevant for upcoming observations with the James Webb Space Telescope (JWST), which will be able to probe rocky exoplanets orbiting M dwarfs at phase angles below ~10° (1). Accounting for opposition surge effects will therefore be essential for accurate surface characterization.

In this interview segment, Gkouvelis discusses the opposition surge effect and how his study explored a new method for modeling the opposition surge effect in studying exoplanets.

This video segment is the first part of our conversation with Gkouvelis. To stay up to date with all the latest video interviews from Spectroscopy, click here.

Reference

1. Gkouvelis, L. An Analytical Model of Wavelength-dependent Opposition Surge in Emittance and Reflectance Spectroscopy of Airless Rocky Exoplanets. Planet. Sci. J. 2025, 6 (5), 110. DOI: 10.3847/PSJ/adcba8