Water from fire: How baby planets may make oceans out of magma

The most common planets in our galaxy may be far wetter than we thought. New experiments show that young, rocky worlds wrapped in thick hydrogen atmospheres can produce large amounts of water when their molten surfaces react with the gas.

The work, led by Carnegie Science’s Francesca Miozzi and Anat Shahar, helps explain one of planetary science’s oldest puzzles: where all the water on rocky planets comes from.

Testing the water-making idea

Sub-Neptunes – planets smaller than Neptune but bigger than Earth – dominate the exoplanet census. They’re thought to have rocky interiors overlain by hefty, hydrogen-rich atmospheres.

That makes them perfect testbeds for an idea that has hovered in theory for years but hadn’t been proven in the lab.

The concept is simple: magma oceans and hydrogen blankets can team up to make water right on the planet, without needing comets or water-rich asteroids.

“Our rapidly increasing knowledge about the vast diversity of exoplanets has enabled us to envision new details about the earliest stages of rocky planet formation and evolution,” Miozzi explained.

“This opened the door to considering a new source for planetary water supplies – a long-debated mystery among Earth and planetary scientists – but experiments designed with this purpose in mind were absent.”

A planet-making problem

For Earth, the story of water has never been straightforward. Some of it likely arrived on icy bodies. Some of it may have been trapped in the mantle from the start.

But models from the past decade point to a third path. A young planet with a deep magma ocean and a thick hydrogen blanket could make water when that gas reacts with iron in the melt. Until now, though, this was mostly math.

Miozzi and Shahar decided to see if nature actually does what the equations said it should. The work was conducted at AEThER (Atmospheric Empirical, Theoretical, and Experimental Research), an interdisciplinary project founded by Shahar.

AEThER aims to connect what we see in exoplanet atmospheres to what’s happening deep inside those worlds.

Bringing together astronomers, cosmochemists, petrologists, mineral physicists, and dynamicists, the project is trying to answer a deceptively simple question: what planetary setups are most likely to make – and keep – liquid water?

Recreating a baby planet with water

Together with colleagues from the Institut de Physique du Globe de Paris and UCLA, the team built a miniature version of an early rocky planet.

The experts began with an iron-rich silicate melt, a stand-in for a global magma ocean. Next, they exposed it to molecular hydrogen, representing the bloated early atmosphere that forms when a planet develops in a gas-rich disk. Then they increased the pressure and temperature.

The researchers squeezed the samples to nearly 60 gigapascals – about 600,000 times Earth’s surface pressure – and heated them above 4,000°C (7,200°F).

Those are the kinds of conditions that exist deep inside young, molten planets and can persist if a hydrogen envelope acts as a thermal blanket, keeping the planet hot for millions or even billions of years.

“Our work provided the first experimental evidence of two critical processes from early planetary evolution,” said Miozzi.

“We showed that a copious amount of hydrogen is dissolved into the melt and significant quantities of water are created by iron-oxide reduction by molecular hydrogen.”

Why that matters for habitability

Those two results are the heart of the paper. First, the magma soaks up a lot of hydrogen. Second, the hydrogen reacts with iron oxides in the melt and makes water.

If that’s happening across an entire magma ocean, the planet now has an internal water source running right where its crust and atmosphere are forming.

That water can eventually escape upward, become locked in minerals, or outgas later as the planet cools. However it moves, the key is that planet formation naturally generates water rather than relying on a rare, fortunate delivery.

Building blocks of habitability

Together, these findings show that the magma ocean can store large amounts of hydrogen while forming water. That has knock-on effects.

Hydrogen dissolved in magma can change the melt’s density and cooling rate. It can also affect how the core separates from the mantle and shape the planet’s later atmosphere.

A planet with this chemistry could form surface oceans far more easily than a dry, airless world.

“The presence of liquid water is considered critical for planetary habitability,” Shahar concluded.

“This work demonstrates that large quantities of water are created as a natural consequence of planet formation. It represents a major step forward in how we think about the search for distant worlds capable of hosting life.”

Rethinking sub-Neptunes – and Earth

There’s a bigger implication hiding in the exoplanet angle. Sub-Neptunes – the workhorses of the Milky Way – commonly have rocky interiors and hydrogen blankets early on.

If magma–atmosphere chemistry works as efficiently as the experiments show, countless worlds across the galaxy may have started with more water than we assumed.

Some of those planets may later lose their hydrogen atmospheres to their stars, leaving behind smaller, water-bearing super-Earths. That, in turn, reframes how we interpret exoplanet atmospheres. What we observe today may be the final chapter of a story that began in a magma ocean.

By tying atmospheric observations to deep-interior processes, the AEThER team is giving astronomers a way to read that story backward – from gas, to chemistry, to the likelihood of oceans.

In other words: if you give a young planet a hot molten surface and wrap it in hydrogen, it may just make its own seas.

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