Two exoplanets located 218 light-years from Earth, named Kepler-138c and Kepler-138d, may be water worlds consisting of deep oceans and thick, steamy atmospheres surrounding a rocky-metallic interior.
This is the first time planets have been identified as water worlds, a type of planet that astronomers have long theorized to exist. The composition of exoplanets, or planets outside of our solar system, is usually determined through analysis of their density, calculated using measurements of the transit of the star’s light by the planet and the star’s radial velocity or “wobble.” Data shows that Kepler-138c and Kepler-138d have radii 1.51 times that of Earth and masses 2.3 and 2.1 times that of Earth, respectively, resulting in densities of around 3.6 grams per cubic centimeter, similar to the density of Europa, a moon of Jupiter covered by a liquid global ocean under an icy shell. Modeling by the research team suggests that water makes up over 50% of the exoplanets’ volume, extending down to a depth of about 2,000 kilometers.
However, these exoplanets are much closer to their star, a small cool red dwarf, than Earth, resulting in temperatures that would make their water-vapor envelopes too hot to support life as we know it.