How Will We (Most Likely) Discover Alien Life?

The most plausible path to the first detection of extraterrestrial life may not involve radio messages or spacecraft—it's likely to come from “alien sunsets,” the faint fingerprints living chemistry leaves on starlight as a planet passes in front of its star. That’s the promise behind recent observations of K2-18b, a nearby mini-Neptune-type exoplanet whose atmosphere has become a high-stakes target for the James Webb Space Telescope (JWST). Even if K2-18b ultimately proves lifeless, the observing method and the atmospheric chemistry it can test are exactly what future life searches will rely on.

K2-18b orbits the dim red M-dwarf K2-18a about 124 light-years away. Kepler data show the planet is roughly 8.6 times Earth’s mass and about 2.6 times Earth’s diameter, placing it in the “super-Earth/mini-Neptune” gray zone. Its orbit sits near the star’s habitable zone, making liquid water possible in principle, but its low density suggests it’s not a bare rock world. Instead, it likely has a rocky interior plus a lighter, puffier outer layer—either a large ocean, a thick hydrogen-rich atmosphere, or both.

Hubble’s 2019 observations initially raised hopes for water. By comparing starlight that filters through the planet’s limb during transits to starlight from times when the planet isn’t in the way, researchers reported absorption features consistent with water vapor. That finding was tentative because Hubble’s infrared reach is limited, and the same spectral region can be contaminated by other molecules.

JWST’s September results tightened the picture. Using NIRISS and NIRCam, astronomers detected methane and carbon dioxide, but found no convincing evidence of water in the upper atmosphere. The earlier “water” signal now looks more likely to be methane. At the same time, JWST reported a possible hint of dimethyl sulfide (DMS)—a sulfur- and methane-based molecule associated with biological byproducts on Earth, including bacterial and phytoplankton metabolism. The catch is statistical: the DMS signal sits at about the 1-sigma level, meaning there’s roughly a 32% chance it could be a random fluctuation. That’s not enough to claim life, but it’s enough to justify more scrutiny.

The excitement doesn’t stop at chemistry. Atmospheric models that match the observed methane and carbon dioxide abundances often point to a “Hycean” scenario: a planet-wide ocean beneath a hydrogen-dominated atmosphere. In such worlds, water vapor may not reach the upper atmosphere because it condenses and rains out deeper down, which fits JWST’s lack of detected water aloft. Models also struggle with other expected molecules—like ammonia—adding to the uncertainty and underscoring how much depends on which atmospheric structure is correct.

Whether K2-18b hosts life remains unresolved, and the physical environment may be hostile. Deep pressures could push water beyond its critical point or force it into ice layers, potentially blocking nutrient exchange and plate tectonics—key ingredients for a stable biosphere. Still, the next step is straightforward: more JWST observations, especially with MIRI, which can access mid-infrared wavelengths needed to separate methane from DMS cleanly. If DMS survives that test, researchers will then have to eliminate non-biological explanations. Either way, K2-18b is becoming a proving ground for the first real biosignature hunts—using the subtle chemistry of starlight rather than dramatic signals from afar.