What if the insides of planets are more complicated than we thought?
A new study from scientists at UCLA and Princeton suggests that hydrogen and water, two common ingredients in planets, can mix in surprising ways—especially in young planets—and even “rain” deep inside their atmospheres.
Most models of how planets form assume that materials like gas, ice, rock, and metal don’t react much with each other.
But this new research shows that under the extreme heat and pressure found in planets between the size of Earth and Neptune, hydrogen and water can mix together and then separate as the planet cools down.
Using powerful supercomputers at UCLA and Princeton, researchers ran complex simulations to see how hydrogen and water behave under different conditions.
Instead of trying to recreate the intense environments in a lab (which is almost impossible), they used computer models based on quantum mechanics—the laws that govern how atoms interact.
The results were surprising: when planets are very young or very close to their stars, their high temperatures keep hydrogen and water completely mixed.
But over time, as the planet cools, the water begins to separate from the hydrogen and sinks deeper into the atmosphere, like rain. This process, called “rainout,” creates a hydrogen-rich outer layer and a water-rich inner layer inside the planet.
This internal “rainfall” could create extra heat inside the planet and change how its atmosphere evolves over billions of years.
“This kind of deep rain could explain why Neptune gives off more heat than Uranus, even though they’re similar in size,” said lead author Akash Gupta, now a fellow at Princeton University. “Neptune might still have water rainout happening deep inside, while Uranus had it happen earlier and now stays cooler.”
The study also helps scientists better understand planets outside our solar system, like K2-18 b and TOI-270 d.
These exoplanets might have thick hydrogen atmospheres over water-rich layers. Depending on their temperatures, hydrogen and water might be fully mixed—or separated into layers, which affects whether they could have oceans or not.
“Understanding how water and hydrogen interact helps us figure out what these planets are really like inside,” said UCLA professor Hilke Schlichting, one of the study’s co-authors. “It helps us predict which ones might be more likely to have water oceans and which are just giant hydrogen-water mixtures.”
The study gives scientists new tools to explore distant worlds and rethink what makes a planet potentially habitable.
