A new study challenges long-held assumptions about how water forms on planets, finding that many close-in sub-Neptunes — among the most common planets in the galaxy — may generate large amounts of water internally rather than acquiring it from icy material formed far from their stars.
The research, led by Prof. Dan Shim of Arizona State University and Prof. Alona Vazan of the Open University of Israel, shows that under the extreme pressures and temperatures inside sub-Neptune planets, dense hydrogen can react with molten rock to create substantial quantities of water. The findings were published in Nature.
Sub-Neptunes, which range from two to four times Earth’s radius, typically have rocky and metallic cores wrapped in thick hydrogen-rich atmospheres. Previous models suggested that planets rich in water must have formed beyond the so-called snow line, where temperatures are low enough for water ice to condense, before migrating inward to their present orbits. But the new experiments indicate that hydrogen-magma reactions alone can generate water reaching tens of weight percent — far more than earlier predictions.
Using a diamond anvil cell and pulsed laser heating, the team recreated the high-pressure, high-temperature conditions at the boundary between a sub-Neptune’s rocky interior and its hydrogen envelope. At pressures exceeding several gigapascals and temperatures of about 3,000 Kelvin, hydrogen reduced silicon and iron in the molten silicate, releasing oxygen that bonded with hydrogen to form water. The researchers detected both water and silicon-hydrogen compounds in the reacted material.
Earlier models based on low-pressure assumptions had suggested only trace water could form through such reactions. The new experiments, however, measured water production 2,000 to 3,000 times higher than those estimates. Computer simulations in the study found that once water forms, convection can transport it throughout the planet’s deep interior, allowing production to continue for billions of years as long as molten silicates remain present.
The results imply that hydrogen-rich sub-Neptunes may evolve naturally into water-rich planets, bridging two categories once considered distinct. This process reduces the need to invoke large-scale migration to explain the presence of water-rich sub-Neptunes orbiting extremely close to their stars, where temperatures are too high for ice to survive during formation.
According to the study, even sub-Neptunes formed from dry materials can ultimately contain tens of weight percent water. If these planets later lose a large portion of their hydrogen atmospheres through stellar radiation, they could be stripped down to super-Earths that still retain significant interior water. That water may eventually reach the surface or atmosphere as the interior cools.
Prof. Shim said the findings reshape expectations about which planets might become water rich. Prof. Vazan added that the work challenges the long-assumed link between a planet’s orbital position and its water content. The study concludes that detecting large amounts of water in an exoplanet’s atmosphere should not automatically be taken as evidence that the planet formed far from its star.