In the 1995 post-apocalyptic action film "Waterworld" Earth's polar ice caps have completely melted, and the sea level has risen to over 5 miles, covering nearly all of the land. Astronomers have uncovered a pair of planets that are true "water worlds," unlike any planet found in our solar system.

Slightly larger than Earth, they don't have the density of rock. And yet, they are denser than the gas-giant outer planets orbiting our Sun. So, what are they made of? The best answer is that these exoplanets have global oceans at least 500 times deeper than the average depth of Earth's oceans, which simply are a wet veneer on a rocky ball.

The soggy worlds orbit the red dwarf star Kepler-138, located 218 light-years away in the constellation Lyra. The planets were found in 2014 with NASA's Kepler Space Observatory. Follow-up observations with the Spitzer and Hubble space telescopes found that the planets must be composed largely of water. The spectral signature of water wasn't directly observed. But this conclusion is based on their density, which is calculated from comparing their size and mass.

Don't expect to find fish in the global oceans. They are probably too warm and under very high pressure, and so there's no such thing as a discrete boundary between the ocean surface and planet atmosphere.