Can Astronomers Detect Exoplanet Oceans?

According to the following article, the answer is yes.

Furthermore, researchers have now three possible methods to detect surface liquid on an exoplanet:

1- “rotational color variability,” is based on the fact that oceans are darker and have different colors than other surface types on Earth. Over time, the variations in color of a spatially unresolved planet can betray the presence of liquid water oceans.



2- "polarization": this method relies on the alignment of the light waves being reflected (polarization). Oceans are smoother than other surface types and can align the light waves reflecting from the water's surface, thus polarizing the reflected light. Observations of polarized Earth-shine suggest that variations in polarization may help astronomers detect oceans.



3- "specular reflection " states that oceans are able to reflect light in a manner similar to a mirror, especially at crescent phases. Water can do this because it is much smoother than land, trees or snow, even though the glint spot is small, it makes a planet look abnormally bright at crescent phase. Therefore, if you could keep track of a planet's brightness as it orbits its host star, you might be able to infer the presence of a glinting ocean.


Ten-year averaged map of planetary albedo for the simulation. Green lines indicate the coast of the equatorial continents blue lines show the annually averaged extent of sea-ice. The right panel shows the zonally (longitudinally) averaged albedo profile. Image Credit: Nicolas Cowan / Northwestern University

Read more here: Astrobio

So, next step is to wait for the astronomers to apply these methods in their work at the telescopes time-span allocation around the world.
Who knows? Maybe it will be this way that the discovery of liquid water outside our solar system will be made.