Universe Today
We’ve found over 5,500 exoplanets so far. Dozens of them have been in the habitable zone 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 in pre-print 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 the 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, and rock, and soil, doesn’t glint. They are what physicists call Lambertian surfaces - they bounce light in all directions equally.
Water is different. 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.
Fraser discusses the possibility of "Hycean" worlds that are completely covered in oceans.In the case of an exoplanet, this shallow angle occurs during its crescent phase, where we’re only looking at mostly the nightside of a planet with only a sliver of it 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. And decades before that, a team of researchers including Carl Sagan used a Galileo fly-by 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.
That might be about to change. The researchers turned to an atmospheric tool to modify it for the purpose of detecting glint. Modifying the tool, known as rfast, they incorporated a model known as the Cox-Munk ocean model to reflect how wind speeds and wave ripples would affect the light reflected back to a telescope. Critically, they found that, even at relatively high noise thresholds, the model proved it could differentiate between planets that either had or didn’t have an ocean, provided they were viewed at a phase angle of more than 120°.
Fraser talks about the technology behind corongraphs - the technique used to suppress starlight from host stars.Why 120°? At that angle, another phenomenon kicks in called glint reddening. At these high angles, starlight that skims off the surface of an ocean has to travel much longer through the planet’s atmosphere. While doing so, it is subjected to Rayleigh scattering, the atmospheric effect that scatters blue light, and famously makes our sky blue. By the time the light has reached a telescope, that means most of the blue wavelengths have been stripped away, leaving primarily a red coloration.
This is all good news for telescope designers, especially those working on the Habitable Worlds Observatory (HWO). Originally theoretical papers thought ocean glint might only be observable at 130°, and higher degree of phase angle means the planet must be closer to its parent star when it's observed. Given that blocking out the light from the parent star is one of the biggest challenges facing HWO’s designers, adding an extra 10° degree to operate allows for some breathing room.
But the paper also points out some limitations in their model. Clouds are the most obvious one. In their simulations, they assumed 50% cloud cover for an Earth-like world. But there are plenty of examples, even in our own solar system, of planets that are much more covered in clouds. Results from those planets can go one of two ways - either they completely blot out the ocean entirely, or, in some forms, like high altitude cirrus clouds, they can scatter starlight towards the telescope, mimicking a glint flash where there isn’t actually one.
There are ways around the second problem at least, including differentiating between gas and liquid absorption bans, but it shows the challenges of trying to find an ocean on a planet light years away. The researchers point out that we don’t even have a lot of data collected about Earth when it’s in its crescent phase, but providing some as training data will be crucial if this tool is to help HWO scientists make sense of what they’re looking at. But, at least for now, it’s good to know that we could potentially find a world covered in oceans, if only we look at it from the correct angle.
Learn More:
Eleanor Cornish & Tyler D. Robinson - Retrieving Ocean Glint Reflectance Signatures from Directly Imaged Earth-like Exoplanets
UT - Finding An Ocean On An Exoplanet Would Be Huge and the Habitable Worlds Observatory Could Do It
