Recent computational experiments conducted by scientists from UCLA and Princeton have revealed new insights into the atmospheric composition of young planets. The study, published in The Astrophysical Journal Letters, indicates that newly formed planets, particularly those between Earth and Neptune in size, possess atmospheres made up of a homogeneous mixture of hydrogen and water. Over time, as these planets cool down, the hydrogen and water begin to separate.
The research suggests that this separation could generate significant heat deep within these worlds and alter their atmospheric composition over billions of years. This finding has implications for understanding potentially habitable exoplanets with hydrogen atmospheres above water oceans.
“Planets can be extremely hot when they are born,” explained Akash Gupta, the study’s first author. As these planets age, “clouds begin to form as water condenses out,” leading to a separation where heavier water sinks while lighter hydrogen rises.
The research also provides insight into why Uranus emits less heat than Neptune despite their similar sizes. “Rainout of water may have so far occurred to a greater extent in Neptune than in Uranus,” Gupta noted.
Hilke Schlichting, co-author of the study, highlighted that if hydrogen and water are substantially mixed throughout a planet’s interior, it could lead to structural differences from standard models used in planetary science.
The study utilized quantum mechanical molecular dynamics simulations on supercomputers at UCLA and Princeton due to the extreme temperatures and pressures involved being difficult to replicate in laboratory settings. Lars Stixrude, another co-author from UCLA, emphasized the novelty of these findings: “We usually think of basic physics and chemistry as being known already… But when it comes to the deep insides of planets, we just don’t know.”
This research was funded by NASA, the National Science Foundation, the Heising-Simons Foundation, and Princeton University. It offers a framework for identifying planetary systems with potential water-rich exoplanets or atmospheres where hydrogen and water are completely mixed.
