By David Warner 
Our universe is filled with wonders, and for the first time in history, we have a tool powerful enough to peek into the atmospheres of planets many light-years away. This tool is the James Webb Space Telescope (JWST). In its quest to understand the cosmos, JWST has been studying a particularly interesting planet called K2-18b. This distant world has become famous because scientists have found strong hints of a very special gas in its air: dimethyl sulfide, or DMS.
This discovery, announced in 2023 and studied intensely into 2025, has caused a huge wave of excitement in the scientific community. Why? Because here on our home planet, Earth, DMS is almost exclusively produced by living things. Finding it on a distant planet, one that also seems to have a liquid water ocean, is one of the most compelling clues we have ever found in the search for life beyond our solar system.
Of course, this is not a confirmation of “aliens.” Science is a careful, step-by-step process. But this discovery has opened a brand new chapter in astrobiology, the study of life in the universe. It moves the question from “Is there life out there?” to “What are the specific signs of life we should be looking for?” So, what exactly is this mysterious gas, and why is its potential presence causing so much debate?
Dimethyl sulfide, or DMS, is a simple chemical molecule. Its name might sound complex, but it just describes its parts. It has sulfur, which is a common element, attached to two “methyl groups,” which are made of carbon and hydrogen. You can think of it as a tiny building block of gas.
Here on Earth, DMS is best known for its very distinct smell. If you have ever been to the ocean and taken a deep breath of sea air, that slightly sulfuric, “oceany” smell is, in large part, thanks to DMS. Some people also compare it to the smell of cooked cabbage or sweet corn. It is an organosulfur compound, meaning it is a chemical that contains both carbon and sulfur, which are two of the most essential ingredients for life as we know it.
This gas is very volatile, which means it turns from a liquid into a gas very easily. This is important because it allows it to escape from the ocean and build up in the atmosphere, where a telescope like JWST could potentially detect it. On Earth, DMS plays a huge role in our planet’s systems. It is a key part of the global sulfur cycle, moving sulfur from the oceans to the land. When it enters the atmosphere, the gas particles can even help clouds form by giving water vapor something to stick to. This simple gas, made by the smallest of creatures, helps shape our entire planet’s climate.
This is the most important question, and it is the key to understanding why the K2-18b discovery matters so much. Here on Earth, the vast, overwhelming majority of DMS—we are talking over 95 percent—is produced by life. It is a true biosignature, a sign of biological processes.
The story starts in our planet’s oceans with phytoplankton. These are microscopic, plant-like organisms that float in the water and get their energy from the sun, just like plants on land. They are the base of the entire marine food web. These tiny creatures, along with other marine life like bacteria and algae, produce a chemical called dimethylsulfoniopropionate (DMSP). They use DMSP for various reasons, perhaps to protect themselves from the salty water, as an antioxidant, or to deter predators.
When these phytoplankton are stressed or eaten—for example, when tiny animals called zooplankton munch on them—the DMSP is released into the water. Bacteria in the ocean then get to work, “eating” this DMSP and breaking it down. As they digest it, one of the waste products they release is dimethyl sulfide gas. This gas then bubbles up from the ocean’s surface and enters the atmosphere.
While tiny amounts of DMS can be released from some soils or plants on land, the marine biological process is the engine that pumps this gas into our air. No known large-scale geological process on Earth, like a volcano, produces DMS in any significant quantity. It is fundamentally a biological product. This is why finding it anywhere else is such a thrilling possibility. It suggests that a similar process, involving life in a liquid ocean, could be happening on another world.
The planet at the center of this story is called K2-18b. It is an exoplanet, which simply means it is a planet that orbits a star outside of our solar system. It is located about 120 light-years away from Earth, which is relatively close in cosmic terms but still incredibly far. A light-year is the distance light travels in a year, so we are seeing this planet as it was 120 years ago.
K2-18b was first discovered in 2015 by the Kepler Space Telescope, which is why it has a “K2” in its name. But it was JWST that gave us the stunning new details. This planet is very different from Earth. It is much larger, classifying it as a “sub-Neptune” or “mini-Neptune.” It is about 2.6 times the radius of Earth and has about 8.6 times the mass. This means it is somewhere between a rocky planet like ours and an ice giant like Neptune.
What makes K2-18b so special is its location and its potential environment. It orbits its star, a cool red dwarf, in the “habitable zone.” This is the “Goldilocks” region where it is not too hot and not too cold for liquid water to potentially exist on the planet’s surface. And based on JWST’s data, K2-18b seems to be a perfect candidate for a “Hycean” world. This is a relatively new and theoretical type of planet. The name “Hycean” comes from the words “hydrogen” and “ocean.” Scientists believe this type of world has a vast, deep ocean of liquid water, but unlike Earth, it is covered by a very thick, hydrogen-rich atmosphere. This is a very different kind of habitable world than anything we have in our solar system.
Detecting a specific gas 120 light-years away sounds like science fiction, but it is possible thanks to an amazing technique called transit spectroscopy. The James Webb Space Telescope is a master at this. It works by waiting for the exoplanet, K2-18b, to pass in front of its host star from our point of view. This event is called a “transit.”
As the planet transits, a tiny, tiny fraction of the starlight shines through the outer edges of the planet’s atmosphere. This is the key moment. The light from the star travels through the planet’s air before it continues its journey to JWST’s giant mirrors. Different gas molecules in that atmosphere will absorb very specific colors, or wavelengths, of the starlight. Every chemical, whether it is methane, carbon dioxide, or dimethyl sulfide, has a unique “fingerprint” or “barcode” of light that it absorbs.
JWST’s advanced instruments, like its spectrometers, capture this light and spread it out into a rainbow, just like a prism. Scientists can then analyze this spectrum of light and see which “colors” are missing or dimmer than they should be. These missing bars in the light’s barcode tell them exactly which chemicals are present in the planet’s atmosphere. It is an incredibly delicate measurement. Scientists are essentially decoding the chemical recipe of a world they can never visit, just by analyzing the starlight that has passed through its air.
The potential detection of DMS was not the only thing JWST found. The telescope’s observations revealed a rich chemical cocktail in K2-18b’s air, which is what builds the case for it being a Hycean world. The two most significant findings, besides DMS, were the clear and unambiguous presence of methane (CH4) and carbon dioxide (CO2).
Finding both of these gases together is very important. In a hydrogen-rich atmosphere like the one K2-18b seems to have, these gases would be expected if there is a global liquid water ocean. On Earth, methane is also a strong biosignature, produced by many living organisms. However, it can also be produced by geological processes, so finding it alone is not proof of life.
Just as important as what JWST did find is what it did not find. The data showed a distinct lack of ammonia (NH3). This is a crucial clue. In a hydrogen-rich environment, basic chemistry predicts that ammonia should be very common. So, why is it missing? The leading theory is that the ammonia is being dissolved into a massive liquid water ocean below the atmosphere. Water is very good at dissolving ammonia, which would effectively “hide” it from JWST’s view. Therefore, the combination of methane, carbon dioxide, and a lack of ammonia strongly points to the existence of a “Hycean” planet: a water world under a hydrogen sky.
This is where all the pieces of the puzzle come together. We have a planet, K2-18b, that orbits in the habitable zone. We have strong evidence (methane, CO2, and no ammonia) that this planet has a huge liquid water ocean. And now, in the atmosphere of this exact same planet, we have a tentative detection of dimethyl sulfide.
As we discussed, on Earth, DMS is almost exclusively a waste product from marine life. We do not know of any common geological process that can create it in large amounts. So, when we find a possible water world that also has a possible sign of a gas made by life in water, it becomes the most exciting hint of extraterrestrial life we have ever found.
It is important to be very clear: this is not proof. It is an indicator. It is a “potential biosignature.” But it is a very good one. For decades, scientists have debated what the best chemical sign of life would be. Many pointed to oxygen, but we now know that non-living processes on other types of planets could create oxygen. DMS, however, is considered a much more robust biosignature. It is a complex molecule that is “hard” to make without the specific machinery of biology. Finding it, even as a possibility, is a landmark moment. It shows that our new telescope can, for the first time, actually search for these specific, life-indicating molecules.
This is the question every good scientist immediately asks. Science is built on skepticism, and before anyone can claim there is life, they must rule out every other possible explanation. This is the “abiotic” (non-living) challenge, and it falls into two main categories.
First, is the signal even real? The initial detection of DMS was what scientists call “less robust” than the clear signals for methane and CO2. This means the signal was weak and noisy. It is possible it is a “false positive”—just a blip in the data, an error from the instrument, or a misinterpretation of a different chemical that happens to absorb light in a similar way. The science team that made the discovery was very careful to state that the DMS detection was only a “possibility” and required further validation.
Second, let’s assume the DMS is really there. Could a non-living process be making it? This is the bigger question. K2-18b is not Earth. It is a massive planet with crushing gravity, a hydrogen-rich atmosphere, and potentially a very different kind of geology. Perhaps there are strange, unknown photochemical reactions happening in its upper atmosphere, where the starlight zaps chemicals and combines them in weird ways. Or maybe there are unknown geological processes, like massive undersea volcanoes in a hydrogen-rich ocean, that can produce DMS under high-pressure conditions we have never seen before. We simply do not know what is “normal” for a Hycean planet, because we have never studied one up close.
The discovery of potential DMS on K2-18b is not an endpoint; it is the starting pistol for a new race. The work is just beginning, and the next steps are already underway. The first and most important step is confirmation. Scientists are using the James Webb Space Telescope again to get a second, longer look at K2-Example.
They will stare at the planet for many more hours as it transits its star, collecting more light and better data. Specifically, they are using JWST’s MIRI (Mid-Infrared Instrument), which sees a different wavelength of light than the instruments used for the first detection. If MIRI also sees the DMS “fingerprint,” the case will become much, much stronger. If it does not, the initial finding was likely a false positive.
Scientists will also be looking for other related sulfur chemicals. If life is really there and “breathing” sulfur, it should produce other sulfur-based gases, not just DMS. Finding a whole family of these related chemicals would be far more convincing than finding just one. Back on Earth, teams of scientists are running complex computer models. They are building virtual “K2-18b” planets on their computers, trying to find any possible non-living chemical pathway, no matter how strange, that could produce DMS without life. If all their models fail to make DMS, but the “life” model works, the biological explanation becomes the best and simplest answer.
The potential discovery of dimethyl sulfide on K2-18b has electrified the world of astronomy. We have a distant, massive planet that appears to be a “Hycean” water world. In its atmosphere, our most powerful telescope has caught a whiff of a gas that, on our planet, is a distinct signature of life.
It is absolutely crucial to remember that this is not proof of aliens. It is a tantalizing clue, a “maybe” that requires much more investigation. The signal might be noise, or the gas might be produced by strange, non-living geology on a world vastly different from our own.
But for the first time, we are no longer just guessing. We are actively collecting data. The James Webb Space Telescope is not just a telescope; it is a tool for chemical investigation on an interstellar scale. We are living in an era where we can finally test the atmospheres of other worlds for the chemical byproducts of life. Regardless of whether K2-18b turns out to be sterile or teeming with life, this discovery has shown us that the search has truly begun.
If it is confirmed that this gas is real and that no non-living process can explain it, what would it mean to finally know that we are not the only life in this vast, dark universe?
That distinct “smell of the sea” is largely caused by dimethyl sulfide (DMS). This gas is released by marine bacteria as they break down a chemical produced by tiny ocean plants called phytoplankton.
K2-18b is about 120 light-years away from Earth. A light-year is the distance light travels in one year, so it is incredibly far, and we are seeing the planet as it was 120 years ago.
No. Scientists did not find life or aliens. They found a possible hint of a gas called dimethyl sulfide (DMS), which on Earth is produced by life. This is an exciting clue, but it is not proof of life.
A Hycean (pronounced HY-see-an) planet is a theoretical type of exoplanet. The name comes from “hydrogen” and “ocean.” It is thought to be a large planet, bigger than Earth, with a deep liquid water ocean covering its surface and a thick atmosphere rich in hydrogen.
On Earth, oxygen is made by life, but scientists know of non-living (abiotic) ways it could be made on other planets. DMS is considered a stronger biosignature because it is a more complex molecule, and on Earth, it is almost exclusively made by life. We do not know of any common non-living process that creates it in large amounts.
The James Webb Space Telescope (JWST) made this discovery. It is the most powerful space telescope ever built and is specially designed to study the atmospheres of exoplanets by analyzing the infrared light that passes through them.
K2-18b is a “sub-Neptune” planet. It is much larger and more massive than Earth but smaller than ice giants like Neptune. It has about 2.6 times the radius of Earth and 8.6 times the mass.
JWST found clear and strong evidence of methane (CH4) and carbon dioxide (CO2). It also found a significant lack of ammonia (NH3). This specific chemical mix is what strongly suggests K2-18b may be a “Hycean” world with a large liquid water ocean.
This is the method used to study an exoplanet’s atmosphere. When the planet passes (transits) in front of its star, starlight shines through the planet’s thin atmosphere. Scientists use telescopes to “read” that light and see which colors are absorbed by chemicals, revealing what the atmosphere is made of.
It will likely take more time, possibly years, to be certain. Scientists are already conducting follow-up observations with JWST to gather more data. They need to confirm the signal is real and then work to rule out every possible non-living source for the gas.