For decades, scientists have assumed that planets are formed from a mix of gas, ice, rock, and metal, with these materials largely maintaining their distinct chemical identities. Traditional models of planetary formation treat these elements as inert building blocks, assembling into planets but not reacting significantly with one another. However, groundbreaking new research from UCLA and Princeton University suggests that this assumption might be wrong.
By investigating what happens under the extreme conditions inside planets, researchers have discovered that hydrogen and water—two of the most abundant substances in the universe—can chemically interact in surprising ways. Their findings challenge long-standing planetary models and provide new insights into the formation, evolution, and even habitability of planets far beyond our solar system.
A New Perspective on Planetary Formation
Recent studies indicate that the most common type of planets in our galaxy—those between the sizes of Earth and Neptune—typically form with a hydrogen-rich atmosphere. This hydrogen doesn’t simply sit above the planet’s surface like a separate layer of gas; instead, it interacts with the planet’s molten interior over millions or even billions of years.
This interaction is crucial to understanding what lies beneath the thick atmospheres of these planets. However, studying these processes directly is nearly impossible. The temperatures and pressures inside a planet are so extreme that no laboratory experiment on Earth can replicate them. Instead, the scientists at UCLA and Princeton turned to the next best option: powerful supercomputers.
By running advanced quantum mechanical molecular dynamics simulations, the researchers recreated how hydrogen and water behave under the intense conditions of young, Neptune-sized planets. Their results, published in The Astrophysical Journal Letters, reveal a surprising picture of planetary interiors and atmospheres.
The Surprising Chemistry of Hydrogen and Water
“We usually think of basic physics and chemistry as being known already,” said Lars Stixrude, a professor of Earth, planetary, and space sciences at UCLA and a co-author of the study. “We know when things are going to melt, when they’re going to dissolve, and when they’re going to freeze. But when it comes to the deep insides of planets, we just don’t know. There’s no textbook where we can look these things up, and we have to predict them.”
To explore these unknowns, the research team simulated a system containing several hundred atoms of hydrogen and water. By observing how these atoms interacted at the quantum level, they discovered that at extremely high temperatures, hydrogen and water mix uniformly, forming a single, homogeneous atmosphere.
However, as the planet cools, the mixture begins to separate, with water forming clouds and condensing into droplets. Over time, this leads to a dramatic “rainfall” deep inside the planet’s atmosphere. Unlike the rain we experience on Earth, which falls from the clouds to the ground, this deep-atmosphere rain consists of water sinking toward the planet’s interior while lighter hydrogen rises toward the outer layers.
This process, known as hydrogen-water rainout, could significantly alter the structure and evolution of a planet.
How Rainout Changes a Planet from the Inside Out
At first glance, the separation of hydrogen and water might not seem particularly important, but this rainout process has profound implications for planetary physics.
As water droplets descend into the planet’s depths, they release energy in the form of heat. This additional heat source could play a major role in shaping a planet’s atmosphere and internal structure.
According to Akash Gupta, the study’s first author and a postdoctoral fellow at Princeton University, “Shortly after the first clouds form, water and hydrogen would begin to separate deep within the atmosphere. This is a pivotal event, given that the majority of the planet’s hydrogen and water reserves lie in these depths. This would then lead to a ‘rainfall’ deep inside the planet’s atmosphere, resulting in an outer hydrogen-rich envelope and an inner water-rich one.”
This mechanism could explain some long-standing mysteries in our own solar system—most notably, why Uranus and Neptune appear so different despite their similarities.
Solving the Uranus-Neptune Mystery
Uranus and Neptune are often considered planetary twins. They are similar in size, composition, and distance from the Sun. Yet, Neptune emits significantly more heat than Uranus, and scientists have long puzzled over why Uranus appears to be so much colder.
The new study suggests that hydrogen-water rainout could be the missing piece of this puzzle.
“Rainout of water may have so far occurred to a greater extent in Neptune than in Uranus, thus generating more internal heat within Neptune,” Gupta explained. “This could explain why Uranus exhibits significantly lower heat flow compared to Neptune.”
If Neptune experienced more water sinking deep into its interior, this would have released more heat over time. Uranus, on the other hand, may have undergone less of this process, leading to its surprisingly low temperature.
Implications for Habitable Exoplanets
While this research sheds light on planets within our solar system, its implications extend far beyond Uranus and Neptune. It may also reshape the way scientists think about potentially habitable exoplanets.
Some exoplanets, like K2-18 b and TOI-270 d, have been proposed as potential candidates for hosting life because they appear to have thick hydrogen atmospheres overlaying vast water oceans. However, if the internal temperatures of these planets are too high, the hydrogen and water might remain completely mixed, forming a single, uniform fluid rather than distinct atmospheric and oceanic layers.
This could drastically change the habitability of such worlds. If hydrogen and water do not separate, these planets would lack a clear boundary between atmosphere and ocean, making them very different from Earth.
“If water and hydrogen are indeed substantially mixed throughout a planet’s interior, the structure and thermal evolution of Earth- and Neptune-like exoplanets can be substantially different from the standard models typically used in the field,” said Hilke Schlichting, a study co-author and UCLA professor.
On the other hand, planets that are cooler might experience enough hydrogen-water separation to form a distinct water-rich layer, possibly in liquid form. Such planets could have stable, long-lasting water oceans, increasing their chances of being habitable.
A New Way to Identify Water-Rich Worlds
The findings from this study provide scientists with a physics-based framework to refine their search for habitable planets. Instead of looking for planets with just the right temperature and atmospheric composition, astronomers may now need to consider whether hydrogen and water are likely to separate inside a given planet.
This could be an important factor in determining whether an exoplanet hosts a true liquid water ocean or whether its interior consists of a homogenous hydrogen-water mix.
Understanding the conditions under which hydrogen and water separate could help astronomers identify planets where life might be possible. It could also offer clues about the formation and evolution of planetary systems across the Milky Way.
Conclusion: The Hidden Chemistry That Shapes Worlds
This groundbreaking research challenges our long-held assumptions about planetary formation and evolution. The discovery that hydrogen and water can mix and separate under extreme conditions reshapes how we think about planets in our solar system and beyond.
From explaining why Neptune is warmer than Uranus to refining the search for habitable exoplanets, the study reveals a hidden chemistry that governs the fate of worlds. As scientists continue to explore these ideas using advanced simulations and future space missions, we may soon gain even deeper insights into the unseen forces that shape the cosmos.
In the words of Lars Stixrude, “When it comes to the deep insides of planets, we just don’t know. There’s no textbook where we can look these things up, and we have to predict them.”
With new discoveries like this, we are one step closer to writing that missing textbook—one that unlocks the secrets of planets near and far.
Reference: Akash Gupta et al, The Miscibility of Hydrogen and Water in Planetary Atmospheres and Interiors, The Astrophysical Journal Letters (2025). DOI: 10.3847/2041-8213/adb631