A new experimental study suggests that many exoplanets don’t need to import ice from the outer reaches of their systems to become “water-rich” – they can generate water internally, thanks to reactions in their fiery cores. Published in Nature, the new research reveals that hydrogen-rich planets (especially sub-Neptunes) can produce substantial water by reacting hydrogen with their own molten rock.

Using a diamond-anvil cell heated by a pulsed laser, the team recreated the intense pressures (several GPa) and temperatures (thousands of K) thought to exist at the boundary between a rocky core and a hydrogen-rich envelope. Under these conditions, silicate magma breaks down: silicon is stripped from rock to form metal-silicon alloys and hydrides, while oxygen is liberated and combines with hydrogen to form water – up to tens of weight percent, far more than previous low-pressure models suggested.

This finding has profound implications for how we interpret exoplanet compositions. Many sub-Neptunes are modeled as either “dry” (hydrogen-dominated) or “wet” (water-dominated). The traditional view holds that water-rich planets must have formed far from their star, and then migrated inward. But this new work suggests a very different possibility: hydrogen-rich and water-rich sub-Neptunes may not be fundamentally different types of planets, but points on a continuum, linked by interior chemistry.

In practice, as hydrogen is consumed in the reaction, a planet’s outer envelope may gradually become more water-rich. Moreover, convection within the planet could mix this newly made water, potentially leading to deep, water-rich layers.

Perhaps most striking, the authors argue that detecting large amounts of water in an exoplanet’s atmosphere is not necessarily a sign that it migrated in from the cold outer disk – it could simply have “made” that water from its own core. That challenges a long-standing assumption linking a planet’s location of formation to its present-day composition.

Read the original paper for more details.