For decades, scientists have debated the origin of Earth’s water. One hypothesis suggests that comets and asteroids from the outer regions of the Solar System delivered it after the planet had already formed. Another theory proposes that water could have formed within Earth itself through chemical reactions during its early development. Until recently, this second hypothesis could not be tested under realistic conditions. However, an international team of researchers has now conducted laboratory experiments that confirm the possibility of water forming internally within developing planets. The results were published on October 30, 2025, in Nature.

Experiment Simulating Planet Formation

To test this idea, scientists led by Anat Shahar of the Carnegie Institution for Science in Washington used sub-Neptunes—the most common type of exoplanet in our galaxy—as their model. These worlds are larger than Earth but smaller than Neptune and are believed to have rocky cores surrounded by dense hydrogen atmospheres.

In the lab, the researchers recreated conditions typical of a young planetary body. Using a diamond anvil cell, a device capable of generating extreme pressure, they compressed samples of molten, iron-rich rock to 600,000 atmospheres while heating them to over 4,000°C. These parameters match the internal conditions of a planet enveloped by a thick hydrogen cloud during its early formation stages.

Chemistry Under Pressure: How Water Forms

Under such extreme conditions, hydrogen readily dissolves in magma and reacts with iron oxides, producing liquid water according to the reaction:
3FeO + 2H → Fe₃O₄ + H₂O

The process occurs spontaneously and in significant quantities. Anat Shahar and her colleagues reported that water becomes a natural byproduct of chemical reactions inside a forming planet. The hydrogen atmosphere acts as a thermal blanket, trapping heat for millions or even billions of years and allowing these reactions to continue long enough to accumulate substantial amounts of water.

Implications for Earth and Other Worlds

The discovery compels scientists to reconsider the origins of Earth’s oceans. A substantial portion of them may have formed during the planet’s molten stage, rather than being delivered from space. This would explain why signs of liquid water appear in Earth’s geological record only a few hundred million years after its formation.

For exoplanets, the implications are even more significant. If water forms as a natural outcome of planetary formation, then it must be far more common in the universe than previously believed. This is especially relevant for sub-Neptunes—of which more than a thousand have been detected in the Milky Way—many of which may harbor internal oceans beneath thick layers of ice or atmosphere.

New Horizons in Astrobiology

Anat Shahar emphasized that this finding fundamentally changes the way scientists approach the search for habitable worlds. Water, she explained, should no longer be seen as a rare cosmic coincidence but as a natural consequence of planet formation. Combined with the presence of heat and organic compounds—both frequently found in young planetary systems—this greatly expands the potential habitable zone across the galaxy.

Future telescopes such as PLATO and ARIEL will be able to examine exoplanetary atmospheres for signs of water vapor. Missions to icy moons like Europa and Enceladus will also gain new context: subsurface water there may not only be a remnant of ancient oceans but also a product of ongoing internal processes.

In Brief

Laboratory experiments have demonstrated for the first time that water can form inside planets during their birth through reactions between hydrogen and iron oxides in magma under extreme pressure and temperature. This makes water a widespread phenomenon in the universe—particularly on sub-Neptunes—and explains the early emergence of oceans on Earth. The discovery significantly broadens the prospects for finding life beyond the Solar System.