Dry Planets Can Brew Their Own Oceans After All

For years, close orbiting exoplanets that seemed drenched in water were treated as cosmic migrants, born far from their stars and hauled inward later in life. A new set of laser driven experiments suggests something more startling. Dry, hydrogen rich worlds can manufacture oceans from the inside out, rewriting how scientists connect a planet’s water supply to its birthplace.

In work led by Lawrence Livermore National Laboratory postdoctoral researcher Harrison Horn and published in Nature, researchers recreated the brutal conditions at the boundary between a rocky, molten core and a thick hydrogen atmosphere on sub Neptune sized planets. There, at pressures of several gigapascals and searing temperatures, molten silicate rock meets dense hydrogen fluid. The team found that this interface is not a quiet dividing line. It is a chemical engine that can turn rock and hydrogen into water in surprising abundance.

A New Route From Rock To Ocean

Sub Neptunes, with radii roughly 2 to 4 times that of Earth, are among the most common planets in the galaxy, yet our solar system has no direct example. Interior models usually split them into two classes. Some appear to be wrapped in hydrogen dominated atmospheres with little water, often called dry planets. Others seem loaded with water on top of a rocky and metallic core, the so called wet planets. Until now, the dominant idea was that water rich versions had to form beyond the snow line, where ice can condense, then migrate inward or import water by comet and asteroid delivery.

Horn and colleagues tested a very different story. Using a laser heated diamond anvil cell, they squeezed and melted mixtures of silicate minerals and iron in a hydrogen rich environment, pushing them to pressures and temperatures comparable to those at the core envelope boundary inside sub Neptunes. Under these conditions, hydrogen does not just sit above the magma like an atmosphere. It invades the melt, breaks bonds, and rearranges elements.

“Our experiments are the first to look at interactions between hydrogen and silicates at the pressure temperature conditions expected at their interface in sub Neptune exoplanets,” said Horn. “We show that water does not need to come from further out in the solar system. It can be produced within a planet itself.”

In the experiments, silicon in the silicate melt is partially stripped of oxygen and incorporated into iron rich alloys or silicon hydrides. The freed oxygen then combines with hydrogen to form water. Measurements of the resulting phases, including iron silicon alloys, iron hydrides, silicon hydride and clear spectroscopic signals from water, reveal that the process is far more efficient than many previous models suggested.

The team estimates that under the right conditions, these reactions can generate water contents of up to a few tens of weight percent, orders of magnitude higher than some low pressure extrapolations had predicted. In planetary language, that is the difference between a modest trace of water and a world that could justifiably be called ocean rich.

Worlds That Change From Within

Because sub Neptunes are expected to maintain molten silicate interiors for billions of years beneath their hydrogen blankets, the reactions seen in the laboratory are not a brief spark. They could run for geologic timescales. Dense hydrogen can penetrate deep into the magma ocean, while convection stirs the interior, continually exposing fresh silicate to hydrogen and carrying newly formed water upward.

The researchers also show that planetary composition matters. The ratio of magnesium to silicon, for example, shapes how much silicon is available for these reactions and how easily melts can convect. For some plausible exoplanet chemistries, endogenic water production reaches tens of weight percent, enough to transform a hydrogen rich sub Neptune into an object that looks, in mass and radius, like a canonical water world.

Crucially, this interior chemistry plays out while other processes, such as atmospheric loss, are stripping away light gases. If a planet loses much of its hydrogen envelope while retaining heavier water, the result can be a super Earth with surface or interior water that was never directly accreted as ice. It was built by hydrogen magma reactions deep below the surface and later released as the planet cooled and solidified.

That possibility has unsettling implications for how astronomers read the spectra of exoplanet atmospheres. Finding abundant water around a close in planet has often been taken as circumstantial evidence that it formed beyond the snow line and migrated inward. Horn’s study warns that this link between apparent water content and formation distance may not hold.

Rethinking What Water Means

From a habitability perspective, the work does not prove that any specific sub Neptune is friendly to life. It does something more basic that may be just as important. It broadens the menu of planetary histories that can end in water rich worlds.

Instead of demanding that water always arrive from cold, distant reservoirs, the experiments show that rock, hydrogen and time can do the job under the right pressure and temperature. Planets born from dry starting materials, orbiting relatively close to their stars, can still become wet planets.

As Horn puts it, the results speak to the foundations of planetary science itself.

“These results help further our understanding of how planets form, a rapidly growing field in the era of space and ground based telescope exoplanetary search efforts,” he said.

The next time observers report a close in world wrapped in water vapor, the story behind that moisture may turn out to be less like a migration tale and more like an origin story written in magma, hydrogen and the quiet chemistry of a hidden boundary layer.

Study Details

Journal: Nature

Article: Building wet planets through high pressure magma hydrogen reactions

Authors: H. W. Horn, A. Vazan, S. Chariton, V. B. Prakapenka, S.-H. Shim

DOI: 10.1038/s41586-025-09630-7