What if water and hydrogen don’t stay separated inside planets like Earth and Neptune? New research from UCLA and Princeton shows that under extreme heat and pressure, these elements mix and even “rain out” within young planets.
- When planets are newly formed, they can be extremely hot, with atmospheres made of a uniform mixture of hydrogen and water.
- As these planets cool over time, hydrogen and water begin to separate inside the atmosphere.
- This separation leads to a “rainout” of water deep within the planet, releasing heat and altering the planet’s atmospheric composition.
- These internal changes can significantly impact the long-term evolution of the planet over billions of years.
- The findings also apply to exoplanets with hydrogen-rich atmospheres and water oceans, especially if their internal temperatures are high enough to prevent hydrogen and water from separating.
Water and Gas May Mix Deep Inside Planets
All planets are made from a mix of gas, ice, rock, and metal. For a long time, scientists assumed these materials didn’t chemically react with one another during planet formation. But what if they do? Researchers from UCLA and Princeton explored that question and found something unexpected: under the intense heat and pressure inside young planets, water, and hydrogen gas can react, creating surprising chemical mixtures in their atmospheres – and even triggering a kind of deep, internal “rainfall.”
Most planets in our galaxy fall between the sizes of Earth and Neptune. Studies show these worlds often form with thick hydrogen atmospheres. Over time, hydrogen interacts with the molten interiors of these planets, sometimes for millions or even billions of years. These interactions between the atmosphere and interior turn out to be a key part of how such planets form, evolve, and develop their internal structure.
Simulating Extreme Planetary Conditions
However, the temperatures and pressures involved are so extreme that laboratory experiments to study them are nearly impossible. The researchers took advantage of UCLA and Princeton supercomputers to conduct quantum mechanical molecular dynamics simulations to investigate how hydrogen and water — two of the most important planetary constituents — interact over a wide range of pressure and temperature in planets Neptune-sized and smaller. The results were published on March 24 in The Astrophysical Journal Letters.
“We usually think of basic physics and chemistry as being known already,” said study co-author Lars Stixrude, a UCLA earth, planetary, and space sciences professor. “We know when things are going to melt and 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.”
Atoms in Action: Hydrogen Meets Water
The researchers set up simulations of a system split into hydrogen and water, with several hundred atoms of each, and calculated how they interact with each other at the quantum level. The atoms responded in a natural way, as they would in a laboratory experiment under the same conditions.
Planets can be extremely hot when they are born or if they are very close to their parent stars, and these computational experiments showed that such planets would have an atmosphere composed of a homogenous mixture of hydrogen and water. But as the planets age, their temperature decreases, and the hydrogen and water begin to separate. The subsequent rainout of water could not only generate an unexpected amount of heat deep inside these worlds but reshape the composition of atmospheres and evolution of these planets for billions of years.
Planetary Evolution Through Atmospheric Separation
“Over time, as the planet cools down, in the outer regions of the atmosphere, clouds begin to form as water condenses out,” said first author Akash Gupta, who conducted the research as a UCLA doctoral student and is now a 51 Pegasi b and Harry H. Hess Postdoctoral Fellow at Princeton University. “Shortly thereafter, water and hydrogen would begin to separate deep within the atmosphere — 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 as heavier water sinks while the lighter hydrogen rises, resulting in an outer, hydrogen-rich envelope and an inner, water-rich one.”
Could Rainout Explain Uranus’ Mystery?
The finding could also help solve the mystery of why Uranus emits much less heat than Neptune even though these planets are very similar in size.
“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 said. “This could explain why Uranus exhibits significantly lower heat flow compared to Neptune.”
Implications for Habitable Exoplanets
The work has implications for planets outside our solar system, such as K2-18 b and TOI-270 d, which have been argued to be potentially habitable worlds with a hydrogen atmosphere overlying a water ocean. However, the internal temperatures of such exoplanets, if high enough, could lie entirely in the regime where hydrogen and water can’t separate, so that they would consist of a single homogeneous hydrogen-water fluid.
Toward a New Framework for Planetary Interiors
“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, study co-author and UCLA earth, planetary, and space sciences professor.
“On the other hand, planets that are colder could have a separate layer enriched in water, possibly in liquid form.”
The research offers a physics-based framework to better identify which planetary systems in our galaxy might host water-rich exoplanets with oceans, and which ones may instead have atmospheres where hydrogen and water are fully mixed. It also sheds light on the conditions that determine this split.
Reference: “The Miscibility of Hydrogen and Water in Planetary Atmospheres and Interiors” by Akash Gupta, Lars Stixrude and Hilke E. Schlichting, 24 March 2025, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/adb631
The research was funded by NASA, the National Science Foundation, the Heising-Simons Foundation and Princeton University.
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