Alien oceans could be detected by looking for surface glint

By Ryan Whitwam

We’ve gotten fairly good at spotting exoplanets out there in the vastness of space, but we still know very little about them. Astronomers can usually only surmise about the surface conditions based on a planet’s size and proximity to its star, but NASA researchers are toying with the idea of looking for the glint of alien oceans as a way of detecting water.

We don’t yet have the technology to study exoplanets directly, but that day is fast approaching with the instruments like the James Webb Space Telescope just a few years away. To prepare for that day, astronomers are using the only habitable planet we know of as a model — Earth. What would Earth look like if we were studying it from a few light years away?

We’re used to seeing Earth from space as a brilliant blue marble in a sea of stars, but that’s only what it looks like up close. From a distance, Earth goes through phases like the moon does. During the crescent phases, the reflection of light from the oceans gets very bright — so bright that the right instrument might be able to detect it from very far away.

An example of this was seen in 2009 when NASA’s Lunar Crater Observation and Sensing Satellite(LCROSS) spacecraft caught a glimpse of Earth and the moon from the dark side of the moon (seen above). Scientists at NASA’s Ames Research Center analyzed the data acquired from LCROSS and found that the ultraviolet and visible light signature provided a good approximation of what Earth’s surface looks like with respect to land and water coverage. It was good enough to pick out details like the Pacific and Atlantic ocean, at least.

The LCROSS data showed that even when less of the Earth’s surface is visible during the crescent phases, its brightness could increase from 40-80%. This kind of marked increase detected on an exoplanet would offer extremely strong evidence of liquid water on its surface. A planet with oceans should behave in more or less the same manner as Earth when observed from a distance, but we also have to figure in things like its mass and location in the solar system. A planet might have a highly reflective surface, but be outside of the habitable zone. That could indicate ice or some other liquid on the surface. Even excessive cloud cover could cause an increase in reflectivity.

Any detection of the glint from a probable body of water would be a huge deal, even if it takes additional studies of the exoplanet’s atmosphere to confirm it. We’ve had such limited information on the planets themselves that any hints could be revolutionary.