these types of exoplanets tend to be much closer to their host star than Earth is to the Sun, sub-Neptunes are too hot to have liquid water on their surface and support life. Instead, they would have atmospheres made of steam, over layers of an exotic phase of water that behaves like neither gas nor liquid. Since the existence of these “steam worlds” were first predicted 20 years ago, interest in their exact makeup and evolution has grown.

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For the first time in history, the James Webb Space Telescope (JWST) confirmed the presence of steam on a handful of sub-Neptunes. Astronomers expect JWST to observe dozens more, which is why such models are critical to connect what we see from the exoplanet’s surface to what is inside of them.

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Icy moons are small, condensed bodies with layered structures: icy crusts over liquid water oceans. Sub-Neptunes are much different. They are vastly more massive—10 to 100 times as much—and, again, they orbit much closer to their stars. So they don’t have icy crusts and liquid oceans like Europa or Enceladus. Instead, they develop thick steam atmospheres and layers of “supercritical water.”

This exotic, supercritical phase of water has been recreated and studied in laboratories on Earth, exhibiting behavior that is far more complex than simple liquid water or ice—thus, making it difficult to model accurately. Some models even suggest that, under extreme pressure and temperature conditions inside sub-Neptunes, water may even transform into “superionic ice,” a phase in which water molecules reorganize so hydrogen ions move freely through an oxygen lattice.

Very cool!