In September 2023, the James Webb Space Telescope detected methane and carbon dioxide in the atmosphere of K2-18 b, a planet 124 light-years away. A deeper analysis published in 2025, combining four additional JWST transit observations with the original data, revealed that the planet likely has a water-rich interior. K2-18 b sits in a category that doesn’t exist in our solar system — a “sub-Neptune” about 8.6 times Earth’s mass and 2.6 times its radius. Too big to be rocky, too small to be a gas giant. And sub-Neptunes are the most common type of planet in our galaxy.
The atmospheric chemistry is what makes K2-18 b so intriguing. JWST found methane and carbon dioxide but very little ammonia — a signature consistent with a theoretical model called a “Hycean world,” a portmanteau of hydrogen and ocean. The idea is a planet with a thick hydrogen atmosphere sitting atop a global liquid water ocean with a rocky core beneath. Chemical reactions between ocean and atmosphere would deplete ammonia, which matches the data. But the initial analysis also reported a tentative detection of dimethyl sulfide (DMS), a compound produced almost exclusively by marine phytoplankton on Earth. If confirmed, it would be extraordinarily difficult to explain without biology.
The scientific community is appropriately cautious. A 2024 paper in The Astrophysical Journal Letters argued the same JWST data can be explained by a gas-rich mini-Neptune with no habitable surface at all. Three independent teams analyzing the expanded dataset reached partially overlapping but distinct conclusions. The data sits at the edge of JWST’s detection capabilities for this type of analysis, and more observations are planned to settle the DMS question.
JWST reads atmospheric composition through spectroscopy — when a planet transits its star, starlight filters through the atmosphere, and different molecules absorb different wavelengths like a chemical barcode. The telescope has characterized dozens of exoplanet atmospheres since 2022, but sub-Neptunes remain the hardest targets. Each transit observation is like increasing the exposure time on a camera. The real game-changer ahead is studying truly rocky planets like the TRAPPIST-1 system, which has seven Earth-sized worlds, three in the habitable zone. Two years ago, we couldn’t even detect molecules in a sub-Neptune’s atmosphere. Now we’re debating whether one has an ocean. The pace of discovery is staggering.
Potentially. K2-18 b sits in a strange category that doesn’t exist in our solar system. It’s about 8.6 times the mass of Earth and 2.6 times Earth’s radius. That makes it too big to be a rocky planet like Earth but too small to be a gas giant like Neptune. Scientists call these “sub-Neptunes,” and they’re actually the most common type of planet in our galaxy. We just don’t have one nearby to study.
Not one. Which is why K2-18 b is so valuable. It orbits in the habitable zone of its star, a red dwarf called K2-18, meaning it receives enough energy that liquid water could exist on its surface. The JWST observations detected methane and carbon dioxide in its atmosphere but found very little ammonia. That chemical signature is significant.
Because it’s consistent with a specific theoretical model called a Hycean world. “Hycean” is a portmanteau of hydrogen and ocean. The idea is a planet with a thick hydrogen-rich atmosphere sitting on top of a global liquid water ocean, with a rocky core beneath that. In this scenario, chemical reactions between the ocean and atmosphere would deplete ammonia, which is what the data shows.
A July 2025 paper on arXiv used four additional JWST transit observations combined with the original data to confirm the water-rich interior interpretation. The team applied Bayesian analysis and found that the atmospheric composition strongly favors a scenario where K2-18 b has substantial amounts of water, either as a deep ocean or distributed throughout its interior.
Here’s where it gets controversial. The initial 2023 JWST analysis reported a tentative detection of dimethyl sulfide, or DMS. On Earth, DMS is produced almost exclusively by marine phytoplankton. It’s the compound that gives the ocean its characteristic smell. If confirmed on K2-18 b, it would be hard to explain without biological activity.
It is. And the scientific community has pushed back. A 2024 paper in The Astrophysical Journal Letters argued that the JWST observations of K2-18 b can be explained by a gas-rich mini-Neptune with no habitable surface at all. Under this model, the planet is more like a warm, small gas ball than an ocean world. The atmospheric chemistry that looks like habitability markers could just be the natural result of high-pressure hydrogen chemistry.
That’s the honest state of the science right now. It’s not that anyone is wrong. The data from JWST, while incredible, sits at the edge of the telescope’s detection capabilities for this type of analysis. An August 2025 report from NASA described the situation as three independent teams analyzing the same expanded dataset and reaching partially overlapping but distinct conclusions.
Not at all. Another fascinating target is TOI-270 d, a temperate sub-Neptune about 73 light-years away. JWST detected methane, carbon dioxide, and water vapor in its atmosphere. Like K2-18 b, multiple models can explain the observations, and the debate about whether it’s a mini-Neptune or a Hycean world is ongoing.
Spectroscopy. When an exoplanet transits in front of its host star, a tiny fraction of the starlight passes through the planet’s atmosphere before reaching us. Different molecules absorb different wavelengths of light. JWST’s infrared instruments are sensitive enough to detect these absorption signatures in the filtered starlight, essentially reading the chemical fingerprint of an atmosphere from over a hundred light-years away.
That’s a good way to think about it. Each molecule has a unique barcode. Water absorbs at certain infrared wavelengths, methane at others, carbon dioxide at others. JWST reads those barcodes with enough precision to tell you not just what’s there but roughly how much of it there is.
JWST has characterized the atmospheres of dozens of exoplanets since becoming operational in 2022. But the sub-Neptune category, the ones most relevant to habitability questions, remains the hardest. The signal is incredibly faint. K2-18 b required multiple transit observations stacked together to get a clear reading. Each transit gives you a slightly better picture, like increasing the exposure time on a camera.
More observations of K2-18 b are planned to settle the DMS question and pin down the atmospheric composition more precisely. But the real game-changer will be looking at truly rocky planets in habitable zones. Planets like the TRAPPIST-1 system, which has seven Earth-sized planets, three in the habitable zone. JWST is already studying those, though detecting atmospheres on Earth-sized planets is significantly harder than on larger sub-Neptunes.
Thinner atmospheres and less surface area to filter starlight through. The signal gets proportionally weaker. For TRAPPIST-1 planets, it might take years of accumulated data to confirm or deny the presence of an atmosphere. But that’s the beauty of JWST. It was built to last at least 20 years. Time is on its side.
You’d need multiple independent lines of evidence. Confirmed water vapor in the atmosphere. Temperature measurements consistent with liquid water. The absence of atmospheric features that would rule out surface conditions suitable for life. And ideally, a biosignature gas like oxygen or DMS that can’t be easily explained by non-biological chemistry. We’re not there yet for any planet. But two years ago, we couldn’t even detect molecules in a sub-Neptune’s atmosphere. The pace of progress is staggering.
We went from “we don’t even know if exoplanets have atmospheres” to “we’re debating whether this one has an ocean” in about two years.
And in two more years, we might be having a very different conversation. Sources and papers are all linked below.
- arXiv - “A water-rich interior in the temperate sub-Neptune K2-18 b revealed by JWST” (2025) - https://arxiv.org/html/2507.12622v1
- The Astrophysical Journal Letters - “JWST Observations of K2-18b Can Be Explained by a Gas-rich Mini-Neptune with No Habitable Surface” (2024) - https://ui.adsabs.harvard.edu/abs/2024ApJ…963L…7W/abstract
- Big Think - “The evidence for biosignatures on K2-18b is flimsy, at best” - https://bigthink.com/starts-with-a-bang/evidence-biosignatures-k2-18b-flimsy/
- Astronomy.com - “Signs of life on K2-18 b revisited in new NASA study” (2025) - https://www.astronomy.com/science/new-study-revisits-signs-of-life-on-k2-18-b/
- The Planetary Society - “Revisiting K2-18 b: JWST finds a new lead in the search for life” (2025) - https://www.planetary.org/planetary-radio/2025-jwst-new-lead-in-search-for-life
- NASA - “Webb Discovers Methane, Carbon Dioxide in Atmosphere of K2-18 b” - https://www.nasa.gov/universe/exoplanets/webb-discovers-methane-carbon-dioxide-in-atmosphere-of-k2-18-b/
When a planet passes in front of its star, a sliver of starlight filters through the planet’s atmosphere. Different molecules absorb different wavelengths. JWST reads those absorption patterns like a barcode, telling us what’s in an atmosphere from over a hundred light-years away.
K2-18 b is what scientists call a sub-Neptune. Too big to be rocky like Earth, too small to be a gas giant. And here’s the wild part: sub-Neptunes are the most common type of planet in the galaxy.
Frequently Asked Questions
Has water been found on an exoplanet?
Yes, JWST has detected water vapor in the atmospheres of several exoplanets, including some rocky worlds in habitable zones. The detection of water on rocky planets is significant because liquid water is considered a key prerequisite for life as we know it.
Could there be life on exoplanets with water?
Water is necessary but not sufficient for life. A habitable exoplanet also needs a stable atmosphere, appropriate temperature range, and energy sources. However, the discovery of water on rocky exoplanets dramatically increases the estimated probability of extraterrestrial life in our galaxy.
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