Unlike anything in our solar system, Kepler-138c and Kepler-138d are much bigger and lighter than Earth, despite having roughly twice our mass. This difference in density suggests a significant portion of their volume, up to half, could be water. It's important to note that scientists haven't directly detected liquid water on these planets.
The findings, published in the journal Nature Astronomy, are based on comparing the planets' sizes and masses with planetary models. The conclusion is that these exoplanets likely contain a large amount of material lighter than rock but heavier than hydrogen or helium. Water is the most likely candidate for this mysterious material.
These water worlds are theorized to be quite different from Earth. One possibility is that they have scorching atmospheres of superheated steam instead of liquid water on the surface, due to their proximity to their star. However, there could be vast oceans of liquid water beneath these atmospheres.
This discovery is a significant step forward in our search for water worlds, a type of planet long theorized by astronomers. As telescopes and observation techniques improve, scientists hope to find many more of these fascinating worlds in the future.