This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: We've found more than 5,500 exoplanets so far. Dozens of them are in the habitable zones of their parent stars, where, at least in theory, they could host an ocean.
But we've never definitively found a liquid ocean on another planet. A new paper from researchers Eleanor Cornish and Tyler Robinson, of the University of Arizona, available as a preprint on arXiv and submitted to The Astrophysical Journal, looks at a unique way we might be able to find one—by looking for its "glint." Glint is the common term for what physicists call specular reflection. On Earth, it's a common sight at beaches when sunlight hits the water at a low angle and the horizon lights up with a brilliant flash of gold.
But that same feature can happen on other surfaces—famously, gold has a "glint" when seen from the right angle. But most of what makes up exoplanets, like sand, rock and soil, doesn't glint. They are what physicists call Lambertian surfaces—they bounce light in all directions equally.
It acts sort of like a mirror. When seen from straight above, light mostly penetrates straight down or reflects weakly. But when it bounces off water at a shallow angle, it reflects, and physicists understand how it does.
In the case of an exoplanet, this shallow angle occurs during its crescent phase, when we're looking at mostly the nightside of a planet with only a sliver actually illuminated. When a planet is at this angle, incoming starlight would bounce straight off an ocean and directly into a telescope, causing the planet to brighten dramatically from the telescope's perspective. This phenomenon isn't theoretical.
In 2009, Cassini detected a specular glint off the hydrocarbon lakes on the surface of Titan. Decades before that, a team of researchers including Carl Sagan used a Galileo flyby of Earth to spot glint reflecting off our own oceans. In other words, we know we can do it, but so far we haven't done so for exoplanets.
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