Atmospheres of new planets might have unexpected mixtures of hydrogen and water
by Holly Ober for UCLA News
Los Angeles CA (SPX) Mar 30, 2025
All planets are made of gas, ice, rock and metal, and models of how planets form usually assume that these materials don’t react chemically with each other. But what if some of them do? UCLA and Princeton planetary scientists asked that question and got a surprising answer: Under the intense heat and pressure of newborn planets, water and gas react with each other, creating unexpected mixtures in the atmospheres of young Earth-to-Neptune-sized planets and a “rainfall” deep inside the atmospheres.
Recent studies show that the most common type of planets in our galaxy, those between the sizes of Earth and Neptune, typically form with a hydrogen atmosphere, resulting in conditions where hydrogen and the planet’s molten interior interact for millions to billions of years. Interactions between the atmosphere and the interior are thus crucial to understanding the formation and evolution of these bodies and what might lie beneath these atmospheres.
But 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 are published 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.”
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.
“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.”
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.”
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.
“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 thus further provides a physics-inspired framework to narrow the search for planetary systems in our galaxy in which water-rich exoplanets could have water oceans or if they might have atmospheres where hydrogen and water are completely mixed and uncovers what possibly governs this bifurcation.
Research Report:The Miscibility of Hydrogen and Water in Planetary Atmospheres and Interiors
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