The discovery of these water-dominated exoplanets challenges the earlier assumption that super-Earth-sized planets were merely scaled-up versions of our planet. As Bjorn Benneke, a co-author of the study, explained, Kepler-138c and d are fundamentally different, likely consisting of thick layers of water surrounding rocky cores. This makes them more similar to icy moons like Europa and Enceladus in our Solar System, but on a much larger scale.

However, these planets may not feature Earth-like surface oceans. Instead, Kepler-138d could have a dense atmosphere of water vapor due to high temperatures that exceed water’s boiling point. Beneath this atmosphere, liquid water may exist under extreme pressure, or water could even be in a supercritical state, where it behaves as both a liquid and gas.

The findings offer the best evidence yet for the existence of water worlds, a type of planet long theorized but rarely observed. The planets’ low densities, combined with their proximity to a relatively dim star, provide astronomers with a unique opportunity to study the potential for such worlds in other star systems.

NASA highlighted that, as observational techniques improve, researchers may discover many more planets like these, further expanding our understanding of planetary diversity in the galaxy. The discovery also reinforces the importance of red dwarf stars as hosts for intriguing and potentially habitable planets.

The potential for liquid water, even in extreme conditions, makes these planets key targets for future study. By deepening our understanding of their composition and atmospheric conditions, astronomers hope to unravel more secrets about the types of planets that exist in our universe.