on
Distant exoplanets may be hiding water beyond Webb Telescope's reach, study finds
The planets that appear most common in the universe could have a lot of water—but it could be hiding where telescopes can't detect it, according to a new study led by scientists with the University of Chicago.
Their study examines a vast, mysterious class of worlds known as mini- or sub-Neptunes, which are a little smaller than Neptune. They are the most common type of planet we have cataloged around the galaxy, yet they have no equivalent in our solar system, so scientists must build detailed simulations to try to understand what these planets actually look like.
According to the new analysis published in The Astrophysical Journal, these worlds may have more water than previously thought: Water could sink deep inside the planets, where it can't be seen by even the James Webb Space Telescope.
"It's very possible these planets are hiding much more water than their atmospheres let on," said Caroline Piaulet-Ghorayeb, a UChicago postdoctoral researcher and the first author on the study. "It's an interesting question, both because water is so important for life as we know it, and because it signals we have to interpret the data coming in from new, powerful telescopes in a more nuanced way to really know what's going on."
A planetary puzzle
Most of the universe's planets—and there are millions, if not billions, of them out there in our Milky Way galaxy—circle faraway stars, much as we do our sun. These stars far outshine the planets themselves, making such planets difficult to see directly, but scientists have learned to tease out clues about them.
For example, NASA's James Webb Space Telescope can catalog the molecules present on a planet by capturing the starlight that filters through the planet's atmosphere as it crosses in front of its host star.
"The challenge is, how do we extrapolate from what's in the atmosphere to what the surface is like?" said Piaulet-Ghorayeb.
The surprisingly large population of "mini-Neptune" planets has been a mystery in particular, since we don't have an easy comparison in our own solar system: These planets are too dense to be gas giants like Jupiter, but not dense enough to be rocky planets like Earth. They are likely some mix of rock, gas and water, but no one knows exactly how that mix plays out for each planet.
In the last few years, scientists' working assumption has been that because these planets are warm, their molecules would be fairly evenly mixed, like a well-shaken cocktail. That would mean the readings from the atmosphere give a decent approximation of the molecules deeper inside.
But running the numbers, Piaulet-Ghorayeb and the team found that wasn't the entire picture.
How to tell what planets are like
Planets are complicated. Even what we know about Earth still involves guesswork—we've never managed to drill down further than seven and a half miles (12 kilometers), barely a scratch on the crust—and that's when we're standing on the planet itself.
So, to try to understand what faraway planets actually look like, scientists must build simulations. These combine data from telescopes, our knowledge of planetary science and the laws of chemistry and physics.
As a case study, the team used a planet known as TOI-270 d, which circles a star in the constellation Pictor. The Webb telescope had picked up readings in its atmosphere indicating the presence of hydrogen, methane and carbon dioxide, which should be accompanied by abundant water, the scientists said.
But water behaves very differently under different conditions; it could be frozen, gaseous, liquid or even a supercritical fluid, a strange phase that water can reach at extremely high pressures.
The scientists looked more closely into the conditions under which hydrogen and water mix or separate and found that the answer depends closely on the exact composition of the mix.
In cold atmospheres, or when water is very abundant as it is on TOI-270 d, for example, water could sink below the lighter hydrogen—where it is then hidden from the sight of telescopes.
With current techniques, we don't yet have the capacity to confirm or rule out which category the planet TOI-270 d falls into, explained study co-author Eliza Kempton, a professor. For now, these types of planets exist in a gray area.
The key ingredient for life
TOI-270 d and planets like it aren't very likely to be friendly to Earthlings. If they have water, it would be under the immense pressures and temperatures of thick, heavy atmospheres. But a better grasp of the physics and chemistry of these planets will boost our understanding of how planets form more generally—essential for any search for habitable worlds.
This is particularly true for the mechanisms that would produce and maintain water, which is a key ingredient for life as we know it. Unfortunately, according to study co-author Leslie Rogers, an associate professor, water is one of the more difficult molecules to pin down.
"Water has an intermediate density, so it could be mimicked with a mix of rock and gas," she explained. "We're trying to get any constraint we can for this problem."
Publication details
Caroline Piaulet-Ghorayeb et al, A Window for Water-hydrogen Demixing on Warm Metal-rich Sub-Neptunes, The Astrophysical Journal (2026). DOI: 10.3847/1538-4357/ae64ff
Who's behind this story?
BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021. Full profile →
Master's in physics with research experience. Long-time science news enthusiast. Plays key role in Science X's editorial success. Full profile →
Citation: Distant exoplanets may be hiding water beyond Webb Telescope's reach, study finds (2026, July 13) retrieved 13 July 2026 from https://phys.org/news/2026-07-distant-exoplanets-webb-telescope.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.