Detecting the Glint of Starlight on the Oceans of Distant Planets

TL;DR

This paper demonstrates that analyzing phase light curves can help astronomers identify water surfaces on distant exoplanets by detecting glint and polarization effects, aiding in the search for Earth-like worlds.

Contribution

It introduces a modeling approach showing how surface water and atmospheric conditions affect observable light curves and polarization, enabling water detection on exoplanets.

Findings

Water-covered planets show brighter glint near crescent phases.

Planets with mixed surfaces polarize reflected light significantly.

Water surfaces influence brightness and polarization patterns in observable light curves.

Abstract

We propose that astronomers will be eventually be able to discriminate between extrasolar Earth-like planets with surface oceans and those without using the shape of phase light curves in the visible and near-IR spectrum. We model the visible light curves of planets having Earth-like surfaces, seasons, and optically-thin atmospheres with idealized diffuse-scattering clouds. We show that planets partially covered by water will appear measurably brighter near crescent phase (relative to Lambertian planets) because of the efficient specular reflection (i.e., glint) of starlight incident on their surfaces at a highly oblique angle. Planets on orbits within 30 degrees of edge-on orientation (half of all planets) will show pronounced glint over a sizeable range of orbital longitudes, from quadrature to crescent, all outside the glare of their parent stars. Also, water-covered planets will appear darker than a Lambertian disk near full illumination. Finally, we show that planets with a mixed land/water surface will polarize the reflected signal by as much as 30-70 percent. These results suggest several new ways of directly identifying water on distant planets.