An artist’s concept of what the faraway planet TOI-270 d may look like. A new study suggests these types of planets may be hiding more water than they let on.
Illustration Credit: Courtesy of NASA
Scientific Frontline: Extended "At a Glance" Summary: Sub-Neptune Exoplanet Atmospheres
The Core Concept: The most common type of planet in the galaxy, known as mini- or sub-Neptunes, may harbor significantly more water than previously estimated by concealing it deep beneath thick, hydrogen-rich atmospheres.
Key Distinction/Mechanism: Unlike previous working assumptions that planetary atmospheres are evenly mixed like a "well-shaken cocktail," new simulations demonstrate that water can sink below lighter hydrogen in cold or water-abundant environments, effectively hiding it from the James Webb Space Telescope's spectroscopic sensors.
Major Frameworks/Components:
- Spectroscopic Extrapolation: Using starlight filtered through an exoplanet's atmosphere to deduce its surface and internal composition.
- Water-Hydrogen Demixing: The physical and chemical conditions under which water separates from hydrogen, sinking toward the planet's interior due to its higher density.
- Supercritical Fluids: The theoretical behavior of water under the extreme pressure and temperature conditions deep within planetary interiors.
- Planetary Modeling: The integration of telescope data, chemical laws, and physics to simulate internal planetary environments when direct observation is impossible.
Branch of Science: Astronomy, Astrophysics, Planetary Science, and Astrochemistry.
Future Application: These findings will refine how astronomers interpret atmospheric data from next-generation telescopes, guiding more accurate simulations of planetary formation and internal compositions.
Why It Matters: Understanding the precise distribution and abundance of water in sub-Neptunes is critical for determining how these ubiquitous planets form and for advancing the broader search for habitable worlds.
The planets that appear most common in the universe could have a lot of water—but it could be hiding where telescopes cannot detect it, according to a new study led by scientists at the University of Chicago.
Their study examines a vast, mysterious class of worlds known as mini- or sub-Neptunes, which are slightly smaller than Neptune. They are the most common type of planet cataloged in the galaxy, yet they have no equivalent in our solar system; therefore, scientists must build detailed simulations to understand what these planets actually look like.
According to the new analysis, these worlds may have more water than previously thought: water could sink deep inside the planets, where it cannot be seen even by 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 of 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,” she said.
A Planetary Puzzle
Most of the universe’s planets—and there are millions, if not billions, of them in our Milky Way galaxy—circle faraway stars, much as Earth circles the 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 particular mystery since there is no 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 a mix of rock, gas, and water, but no one knows exactly how that composition plays out for each planet.
In recent years, the working assumption among scientists 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.
However, upon running the numbers, Piaulet-Ghorayeb and the team found that this was not the entire picture.
How to Tell What Planets Are Like
Planets are complicated. Even what we know about Earth still involves guesswork—we have never managed to drill down further than 7.5 miles, barely scratching the crust—and that is when we are standing on the planet itself.
To understand what faraway planets actually look like, scientists must build simulations. These models 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.
However, water behaves very differently under varying 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 heavily on the exact composition of the mixture.
In cold atmospheres, or when water is highly 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 do not yet have the capacity to confirm or rule out which category planet TOI-270 d falls into, explained study co-author Professor Eliza Kempton; for now, these types of planets exist in a gray area.
The Key Ingredient for Life
TOI-270 d and planets like it are not very likely to be hospitable to life on Earth. If they have water, it would be under the immense pressures and temperatures of thick, heavy atmospheres.
However, a better grasp of the physics and chemistry of these planets will boost our understanding of how planets form more generally, which is essential for any search for habitable worlds.
This is particularly true for the mechanisms that would produce and maintain water, a key ingredient for life as we know it.
Unfortunately, according to study co-author Associate Professor Leslie Rogers, 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.”
Funding: Funding: NASA, Brinson Foundation, Suzuki Postdoctoral Fellowship.
Published in journal: The Astrophysical Journal
Title: A Window for Water-hydrogen Demixing on Warm Metal-rich Sub-Neptunes
Authors: Caroline Piaulet-Ghorayeb, Daniel P. Thorngren, Eliza M.-R. Kempton, Justin Lipper, Leslie Rogers, Fernanda Correa Horta, and Shi Lin Sun
Source/Credit: University of Chicago | Louise Lerner
Edited by: Scientific Frontline
Reference Number: astr071326_01
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