The researchers report that the planet might be rich in microbial life, but they want to make it clear that they do not want to claim that living organisms have been found, but only a possible biosignature indication of some biological process. The discovery has to be viewed with caution-and more observations are needed.
Still, they were very excited. “These are the first hints of an alien world that is probably inhabited,” said Nikku Madhusudhan, an astrophysicist at the University of Cambridge’s Institute of Astronomy and lead author of the study published in the Astrophysical Journal Letters.
“This is a transformational moment in the search for life beyond the solar system, where we have demonstrated that it is possible to detect biosignatures in potentially habitable planets with current facilities. We have entered the era of observational astrobiology,” Madhusudhan said.
He pointed out that different initiatives search for life in our solar system and several claims of arenas conducive to life on Mars, Venus, and several icy moons.
K2-18b is 8.6 times as massive as Earth, with a diameter about 2.6 times that of the planet.
It orbits in a “habitable zone,” a distance from the red dwarf where liquid water, an essential ingredient for life, could exist on the surface of a planet. This star is smaller and less luminous than our sun, located about 124 light-years away from Earth in the constellation Leo. A light-year is defined as the distance light can naturally cover in a year, that is, 5.9 trillion miles (9.5 trillion km). One other planet around the same star has been detected.
A ‘HYCEAN WORLD’
Since the 1990s, roughly 5,800 planets beyond our solar system, termed exoplanets, have been discovered. Scientifically speaking, hycean worlds– exoplanets covered by an ocean of liquid water, which is presumably habitable for microorganisms, set in a hydrogen-rich atmosphere hypothesized.
The first observations of Webb in 2021 and operational instruments in 2022 identified methane and carbon dioxide in K2-18 b’s atmosphere, the first time carbon-based molecules had been discovered in the atmosphere of an exoplanet in a star’s habitable zone.
Madhusudhan noted: “Currently, the only scenario that explains all the data from JWST so far, including past and present observations, is that K2-18 b is a hycean world with lots of life. However, we need to be open and continue exploring [other] scenarios.”
Madhusudhan said, “Hycean worlds, if they do exist, talk of microbial life, perhaps similar to what we see in the oceans of the Earth”. The oceans of these worlds are predicted to be warmer than those of the Earth. As for the question of possible multicellular or intelligent life, we won’t be able to answer that at this stage. The baseline assumption is of simple microbial life.”
It has an illustration of a Hycean world exoplanet with a liquid ocean under a hydrogen-dominant atmosphere,
that moves around a red dwarf star. According to Gonzalez et al. (2023), based on the observation of the James Webb Space Telescope, the exoplanet K2-18 b might just fit the description. This illustration has been acquired from Reuters dated April 16, 2025. A. Smith, N. Madhusudhan/University of Cambridge/Handout via REUTERS.
It is DMS and DMDS, both predicted to be exoplanet biosignatures, from the same chemical family. Webb found one or the other, or both, in the atmosphere of the planet at a 99.7% confidence level, or 0.3% chance of being a mere statistical fluke.
The gases exist in atmospheric concentrations appreciably above 10 ppmv.
Madhusudhan said they are “thousands of times higher than their sources on Earth and cannot be explained without biological activity as per current understandings.”
Scientists who did not take part in the study cautioned.
“The rich data from K2-18 b make it a tantalizing world,” commented Christopher Glein, principal scientist at the Space Science Division of the Southwest Research Institute in Texas. These latest data are a valuable contribution to our understanding. Yet, we must be very careful to test the data as thoroughly as possible. I look forward to seeing additional, independent work on the data analysis starting as soon as next week.”
TRANSIT METHOD
K2-18 b is classified as a sub-Neptune, a category of planet whose diameter is larger than Earth’s but smaller than Neptune’s, our solar system’s smallest gas planet.
To determine what is in the atmosphere of a particular exoplanet, astronomers use the so-called “transit method,” in which they look at measurements of a star’s light. At the same time, the planet passes in front of it, creating the view from Earth. Webb detects the reduced brightness of the star as the planet transits, and a small amount of starlight passes through the planetary atmosphere before it gets detected by a telescope. This is how the gases found in the planet’s atmosphere are known.
Webb’s earlier observations of this world produced a probable DMS signal. It acquired this most recent data using volume spectroscopy and a new wavelength window.
Madhusudhan could not have put it better: “The Holy Grail of exoplanet science is to find a signature of life on an Earth-like planet beyond our solar system.” Our species has been wondering for thousands of years whether or not “we are alone” in the universe, and now, it is within just a couple of years of detecting a possible alien life form on a hycean world, Madhusudhan said.
But, still, Madhusudhan urged caution.
“First, we need to repeat the observations two to three times to make sure the signal we are seeing is robust and to increase the detection significance to a level at which the odds of a statistical fluke are below roughly one in a million,” Madhusudhan said.
“Second, we need much more theoretical and experimental work to evaluate whether such a DMS or DMDS can be produced in a planetary atmosphere resembling K2-18 b by a possible other abiotic mechanism (not biological). Though suggested in previous studies as robust biosignatures even for K2-18 b, we should keep our minds broad and pursue other options,” Madhusudhan said.
Thus, “big if” that the observations are attributed to life, and it is “not in anyone’s interest to claim prematurely life has been detected,” Madhusudhan remarked.