Water was thought to arrive on planets aboard comets and asteroids. A 2025 Nature study shows sub-Neptunes, the galaxy's most common planets, can make their own by reacting hydrogen atmospheres with molten rock.
For a long time the story of how a planet gets its water had one main plot. Water forms easily in the cold outer reaches of a young star system, freezes into the ice that coats comets and distant ast… [+4812 chars]