Astronomers reveal how clouds shape the hidden interiors of the galaxy’s most common planets

Sub-Neptunes are the most common type of planet in our galaxy. Bigger than Earth but smaller than Neptune, these worlds remain deeply mysterious because scientists still do not know what they are made of. What astronomers really want to figure out is their interior composition. They could be rocky planets wrapped in hydrogen-rich atmospheres, or they could be volatile-rich worlds loaded with water or carbon-bearing molecules.

One promising piece of this puzzle lies with the James Webb Space Telescope (JWST), which is powerful enough to study the atmospheres of these far-off planets. But JWST only sees their upper atmosphere. The challenge lies in connecting the atmosphere we can see to the interior we cannot. Clouds make it harder still.

We know that if clouds form high up in a planet’s atmosphere, they can hide information about its chemical makeup, making it difficult to understand these planets. But astronomers at Arizona State University (ASU) have found a surprising relationship between the clouds in the atmosphere and the conditions deep in the interior.

The findings were published in the Astrophysical Journal Letters. The study, led by 51 Pegasi b Postdoctoral Fellow Sagnick Mukherjee in ASU’s School of Earth and Space Exploration, shows that clouds forming in the atmospheres of sub-Neptunes are not just a nuisance to observers. They are actively shaping the physical conditions deep inside these planets in ways that could fundamentally change how scientists interpret what JWST sees.

A warm blanket around other worlds

Using detailed computer models, Mukherjee and the research team found that clouds composed of vaporized rocks and salts can form deep in the atmospheres of sub-Neptunes. Once there, they act like a thermal blanket, trapping heat trying to escape from the planet’s interior and driving temperatures far higher than they would otherwise be.

For the sub-Neptunes the team modeled, accounting for these clouds, raises temperatures in the deeper atmosphere by more than 1,000°C (1,800°F), while cooling the upper, shallower layers. That drastic temperature shift reaches all the way down to the boundary between the atmosphere and the underlying interior.

“Among the sub-Neptunes currently being studied with JWST, we were amazed to find that cloud-driven heating can raise the temperature at the planet’s atmosphere-interior boundary by roughly over 1,400 to 2,600 degrees Celsius,” said lead author Sagnick Mukherjee.

For two planets in particular, GJ 1214 b and TOI-1231 b, that added heat is enough to push the rocky interface past its melting point. Without clouds in the model, the rock at that boundary would stay solid.

“For some of the planets we modeled, that extra heat is enough to melt the planet’s surface, creating a magma ocean,” said co-author Matthew Nixon, a 51 Pegasi b Postdoctoral Fellow at ASU.

Magma oceans change everything

A magma ocean is not just a curious geological question. Like volcanoes on Earth, an exposed magma ocean can actively exchange gases between a planet’s rocky interior and its outer atmosphere. The study found that cloud-driven heating significantly changes the nature of that exchange.

The added warmth increases the release of gases such as oxygen, silicon hydride and silicon monoxide from the rocky interior into the atmosphere. Meanwhile, other gases, including methane, water vapor and ammonia, get preferentially absorbed into the molten magma. The result is an atmosphere chemically “polluted” by events deep underground, and the extent of that pollution depends directly on how cloudy the atmosphere is.

This creates a serious challenge for interpreting JWST data. The chemistry that the telescope detects in a sub-Neptune’s atmosphere may not accurately reflect the planet’s true bulk composition. The clouds and the magma oceans they help create could be distorting that signal.

“Interpreting JWST observations of sub-Neptunes is particularly challenging due to the complex relationship between the atmosphere and interior,” said co-author Luis Welbanks, assistant professor in ASU’s School of Earth and Space Exploration. “This work takes us one step closer to answering the question of what these mysterious worlds are made of.”

More than a muffled signal

Clouds have long been treated by astronomers primarily as an obstacle. They blur and mute the spectral signatures that reveal what distant exoplanets are made of, making observations harder to read.

Beyond muffling signals, clouds drive heating and cooling patterns that reshape the interior structure of sub-Neptunes and influence how these planets contract and cool over millions to billions of years, affecting their present-day sizes and internal heat.

Sub-Neptunes are drawing considerable interest as potential candidates for habitability under the right conditions. Evaluating that potential requires a clear picture of their surfaces and internal structures. Because their atmospheres shape what astronomers can infer about these deeper properties, this research highlights how understanding the role of sub-Neptune clouds is a fundamental part of understanding the planets themselves.

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Sadie Harley

BSc Life Sciences & Ecology. Microbiology lab background with pharmaceutical news experience in oil, gas, and renewable industries.

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Andrew Zinin

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