By Cordy Brown, Science Correspondent
Published: April 17, 2025
In a groundbreaking development, astronomers have announced what they believe to be the strongest evidence yet of potential extraterrestrial life on exoplanet K2-18b, located 124 light-years away in the constellation Leo. Using data from NASA's James Webb Space Telescope (JWST), a team led by the University of Cambridge has detected significant concentrations of chemical compounds in the planet's atmosphere that, on Earth, are exclusively produced by living organisms.
A Promising Candidate for Life
K2-18b, discovered in 2015 by NASA's Kepler mission, is a "super-Earth" exoplanet approximately 2.6 times the diameter and 8.6 times the mass of Earth. Orbiting a cool red dwarf star, the planet resides in the habitable zone—often referred to as the "Goldilocks zone"—where conditions may allow for liquid water, a key ingredient for life as we know it. Previous observations in 2019 by the Hubble Space Telescope suggested the presence of water vapor, sparking interest in K2-18b as a prime candidate for habitability.
Chemical Clues Point to Life
The latest study, published on April 17, 2025, in The Astrophysical Journal Letters, reports the detection of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) in K2-18b's atmosphere. On Earth, these sulfur-based molecules are primarily produced by marine microorganisms, such as phytoplankton. The concentrations of DMS and DMDS detected on K2-18b are estimated to be thousands of times higher than those found in Earth's atmosphere, suggesting a potentially significant biological presence.
"This is the strongest evidence yet there is possibly life out there," said lead researcher Professor Nikku Madhusudhan of Cambridge's Institute of Astronomy. "If we confirm that there is life on K2-18b, it should basically confirm that life is very common in the galaxy."
A Cautious Optimism
While the findings are exciting, the research team and independent scientists urge caution. The detection of DMS and DMDS is reported at a "three-sigma" confidence level, indicating a 99.7% chance the signal is real but falling short of the "five-sigma" threshold required for a definitive discovery. Further observations with JWST are needed to confirm the presence of these molecules and rule out non-biological processes that could produce them.
Some skepticism remains within the scientific community. For instance, Nicolas Wogan from NASA's Ames Research Center suggests K2-18b might be a mini gas giant with no solid surface, potentially inhospitable to life. Others, like Raymond Pierrehumbert from Oxford University, argue the planet could be too hot to support liquid water, proposing oceans of lava instead.
A Hycean World?
The research team hypothesizes that K2-18b is a "Hycean" world—a planet with a hydrogen-rich atmosphere and a vast ocean. This classification, derived from "hydrogen" and "ocean," is supported by earlier JWST observations in 2023 that detected methane and carbon dioxide in the planet's atmosphere, marking the first identification of carbon-based molecules on an exoplanet in a habitable zone. The potential presence of a global ocean teeming with microbial life aligns with the high concentrations of DMS and DMDS observed.
"Given everything we know about this planet, a Hycean world with an ocean that is teeming with life is the scenario that best fits the data we have," Madhusudhan noted.
The Road Ahead
To strengthen their findings, Madhusudhan's team is collaborating with other researchers to investigate whether DMS and DMDS could be produced through non-biological processes. Additional JWST observations, potentially requiring just 16 to 24 hours of telescope time, could elevate the detection to the five-sigma level, providing greater certainty.
The discovery has sparked widespread excitement, with some calling it a "revolutionary moment" in the search for extraterrestrial life. However, as Professor Catherine Heymans, Scotland's Astronomer Royal, cautioned, "Even with perfect data, we can't say for sure that this is of a biological origin on an alien world because loads of strange things happen in the Universe."
As astronomers await further data, the tantalizing possibility of life on K2-18b continues to captivate the scientific community and the public alike, bringing humanity one step closer to answering the age-old question: Are we alone in the universe?
Sources: The Astrophysical Journal Letters, NASA, University of Cambridge