A team of astronomers announced on April 16, 2025, that they found evidence for an unexpected atmospheric gas while studying a planet around another star. On Earth, that gas – called dimethyl sulfide – is mostly produced by living organisms.
In April 2024, the James Webb Space Telescope observed the host star of the planet K2-18b for nearly six hours. During this time, the orbiting planet passed in front of the star, allowing starlight to filter through its atmosphere and carry the fingerprints of atmospheric molecules to the telescope.
By comparing those fingerprints to 20 different molecules they expected to observe in the atmosphere, the astronomers concluded that the most probable match was a gas that, on Earth, is a good indicator of life.
K2-18b, a mysterious world, provides a fascinating backdrop for this discovery. The planet’s name means it is the first planet in the 18th planetary system found by the extended NASA Kepler mission, K2. Astronomers assign the “b” label to the first planet in the system, not “a,” to avoid possible confusion with the star.
Located a little over 120 light-years from Earth, K2-18b is practically in our galactic backyard. Although astronomers know very little about K2-18b, it is confirmed to be very unlike Earth. It is about eight times more massive than Earth, with a volume that is about 18 times larger. This means it’s only about half as dense as Earth. In other words, it must have a lot of water, which isn’t very dense, or a very big atmosphere, which is even less dense.
Astronomers speculate that this world could either be a smaller version of our solar system’s ice giant Neptune, termed a mini-Neptune, or perhaps a rocky planet with no water but a massive hydrogen atmosphere, called a gas dwarf.
Another concept, recently proposed by University of Cambridge astronomer Nikku Madhusudhan, suggests that K2-18b may be a “hycean world.” This term refers to hydrogen-over-ocean, where astronomers predict planets with global oceans many times deeper than Earth’s oceans, without continents. These oceans are covered by massive hydrogen atmospheres that are thousands of miles high.
Though hycean worlds are still theoretical, models predicting their behavior align with limited data collected on K2-18b by JWST and other telescopes.
This sets the stage for an exciting prospect. Mini-Neptunes and gas dwarfs are unlikely to be hospitable for life because they probably don’t possess liquid water, and their interior surfaces endure enormous pressures. But a hycean planet could have a large and likely temperate ocean, leading to tantalizing questions: could the oceans of hycean worlds be habitable or even inhabited?
In 2023, Madhusudhan and his colleagues used the James Webb Space Telescope’s short-wavelength infrared camera to study starlight filtering through K2-18b’s atmosphere for the first time. Their findings indicated the presence of two simple carbon-bearing molecules – carbon monoxide and methane – while the planet’s upper atmosphere was found to lack water vapor.
This composition supported, but did not prove, the idea that K2-18b could be a hycean world. In such a world, water would be trapped deeper in the warmer atmosphere, closer to the oceans than the upper atmosphere observed by JWST.
Intriguingly, the data also displayed an additional, very weak signal corresponding to a gas called dimethyl sulfide, or DMS. On Earth, DMS is primarily produced by marine algae and has very few, if any, nonbiological sources.
This signal generated excitement: a potential detection of a biological gas on a planet that may feature a massive ocean. However, scientists had mixed reactions to this initial announcement. While the findings were thrilling, some astronomers pointed out the weakness of the DMS signal and the uncertain nature of K2-18b as a hycean world.
To address these concerns, Madhusudhan’s team turned JWST back toward K2-18b a year later. For this second set of observations, they utilized a different camera on JWST that focused on another range of wavelengths. The results, announced on April 16, 2025, reinforced their earlier findings.
The new data exhibited a stronger, yet still relatively weak, signal they attribute to DMS or a very similar molecule. The fact that the DMS signal appeared again in another camera during a separate observation strengthened their interpretation.
Madhusudhan’s team also offered a detailed analysis of the uncertainties surrounding the data and its interpretation. In real-life measurements, uncertainties are unavoidable; however, their analysis concluded that these uncertainties are unlikely to account for the signal present in the data, further bolstering the DMS interpretation.
As an astronomer, I find this analysis exciting.
Does this mean that scientists have discovered life on another world? Perhaps, but certainty remains elusive.
First, does K2-18b actually possess an ocean beneath its thick atmosphere? This needs to be tested.
Second, can the signal detected in two cameras and two years apart genuinely be attributed to dimethyl sulfide? Scientists will require more sensitive measurements and observations of the planet’s atmosphere to ascertain this.
Third, if it is indeed DMS, does this guarantee the presence of life? This may be the toughest question. Life itself is not detectable with existing technology, and astronomers must evaluate and eliminate all other potential sources to build confidence in this possibility.
The new measurements may lead researchers toward a landmark discovery. However, essential uncertainties persist. Astrobiologists require a deeper understanding of K2-18b and comparable worlds before they can confidently affirm the presence of DMS and its interpretation as a sign of life.
Scientists globally are already scrutinizing the published study and endeavoring to conduct new tests of the findings—independent verification lies at the core of scientific inquiry.
Moving forward, K2-18b will be a significant target for JWST, the world’s most sensitive telescope. The telescope may soon observe other potential hycean worlds to check for similar signals in their atmospheres.
With additional data, these preliminary conclusions may not withstand the test of time. But for now, the prospect that astronomers may have detected gases emitted by an alien ecosystem within a dark, blue-hued alien ocean remains an incredibly fascinating possibility.
image source from：https://theconversation.com/scientists-found-a-potential-sign-of-life-on-a-distant-planet-an-astronomer-explains-why-many-are-still-skeptical-254900
