Cross-section of the Earth (left) and the exoplanet Kepler-138 d (right). Like the Earth, this exoplanet has an interior composed of metals and rocks (brown portion), but Kepler-138 d also has a thick layer of high-pressure water in various forms: supercritical and potentially liquid water deep inside the planet and an extended water vapour envelope (shades of blue) above it. These water layers make up more than 50% of its volume, or a depth of about 2000 km. The Earth, in comparison, has a negligible fraction of liquid water with an average ocean depth of less than 4km.