The sparkle of starlight off water could be the clincher for finding oceans on extrasolar planets. And it could be observable with the tech that will be deployed in the next generation of space telescopes.
“A glinting planet looks different from a non-glinting planet, and it’s detectable with current technology,” said Tyler Robinson, a graduate student at the University of Washington and lead author of a new paper in Astrophysical Journal Letters. “This is one step toward proving there’s liquid water at the surface of an extrasolar planet.”
 The proposed technique for finding wet worlds takes advantage of the same effect that makes sunsets on the Pacific coast so spectacular. The idea was suggested by Carl Sagan in 1993, and has been used to confirm the presence of liquid lakes on Saturn’s moon Titan.
“The oceans do a really good job of reflecting light like a mirror,” Robinson said. “Especially when you have the sun really low on the horizon, most of the sunlight comes reflected off of the water towards you. The same thing happens on the scale of a planet.”
Robinson and his colleagues showed that when a planet appears crescent-shaped to an Earthly observer, starlight reflecting off oceans can make the planet appear up to twice as bright as a planet with no oceans. They also showed that the sparkle of starlight off oceans looks different from light scattered through clouds.
Most other proposed techniques for finding water on an extrasolar planet rely on taking its spectrum, or detailed measurements of the planet’s atmosphere, and looking for the chemical fingerprint of two hydrogen atoms and one oxygen. But this strategy would show only that the planet hosts water vapor, not liquid oceans, and the technology is still a long way off.
“To get a good spectrum would require a big telescope that is still 10 or 20 years away from being designed or launched,” said exoplanet expert Darren Williams of Penn State University, who has also studied ways to search for exo-oceans but was not involved in the new work. “That’s really becoming a long-range, futuristic sort of thing.”
Robinson and his colleagues proved that the glint effect could be observable with the telescope touted as the successor to Hubble: the James Webb Space Telescope, slated to launch in 2014. If the telescope is accompanied by a shield to block starlight, as suggested in the New Worlds Observer mission concept, it will be sensitive to the light glinting off extrasolar oceans.
To test whether the glint would be visible to the new space telescope, Robinson imagined he was an alien observer looking back at Earth. He used data from weather satellites and NASA’s EPOXI mission to build a computer model of what Earth would look like to a distant observer, including weather patterns, seasonal changes and wind speeds over the oceans that would influence the height of waves.
The model “does explain what we can observe on our own planet from other spacecraft in the solar system, so you can trust the model that they’re using to do these calculations,” Williams said.
Unfortunately, even the James Webb Space Telescope won’t be able to take sharp enough images of exoplanets to tell whether the planet is in a crescent phase, much less directly see a glint. The telescope will just see a dot of light getting brighter and dimmer as it circles its star.
“We have to look for evidence of this glint when we just have this pale, tiny speck of light on our camera,” Robinson said.
So Robinson and colleagues added up all the light reflected by the model Earth to see if the glint would light up the whole planet enough to be seen from space. They found that Earth in the crescent phase would be twice as bright with a glint as without it. “That’s significant,” Robinson said. “A factor of two is a really big deal.”
The researchers also found that the glint effect is strongest in the near infrared part of the electromagnetic spectrum, just beyond what the human eye can see. These wavelengths of light are not as badly scattered as they pass through a planet’s atmosphere. Conveniently, they are also the wavelengths that the new space telescope will be most attuned to.
“The James Webb Space Telescope is really well suited to do this,” Robinson said.
Looking for the glint would not be the first line of investigation, however. Rather, Robinson imagines the technique could confirm that a good exo-Earth candidate, a plant that is about Earth’s size planet and lies the right distance from its star to support liquid water, actually does have oceans at its surface.
“We would first worry about whether the planet is even remotely Earthlike before looking for the glint,” he said.
“What’s nice about this result here is that we have a chance of doing interesting things with Earthlike planets with the James Webb Space Telescope, which is basically sitting on the hangar waiting to be launched into space,” commented Williams. “We can do that in our research lifetimes. That’s the most exciting thing about this.”
Image: 1) Astrophysical Journal Letters/Tyler Robinson. Left: NASA Astrobiology Institute’s Virtual Planetary Laboratory. Right: Earth and Moon Viewer. 2) NASA
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