What is k218b? Planet where scientists say alien life may be present
SCIENTISTS have hailed the “enormous” discovery that extraterrestrials could be living on k218b — an exoplanet 124 lightyears from Earth.
Cambridge University astronomers are 99.7% confident they have uncovered evidence of life there. Here’s everything you need to know.
What is K2-18b?
K2-18b is a distant exoplanet — planets outside of our solar system — orbiting a red dwarf star named K2-18.
It is located approximately 124 light-years from Earth in the constellation Leo.
K2-18b was first discovered in 2015, quickly becoming the focus of immense interest due to its position within the “habitable zone”.
The habitable zone is the region around a star where conditions might allow for liquid water to exist on a planet’s surface.
K2-18b is classified as a “super-Earth” or “sub-Neptune”, with a mass about 8.6 times greater than Earth and a radius roughly 2.6 times larger.
Its density suggests it could have a thick hydrogen-rich atmosphere, possibly enveloping a rocky or icy core.
Alien life?
In April 2025, observations using the James Webb Space Telescope, which launched on Christmas Day in 2021, revealed compelling chemical signatures in K2-18b’s atmosphere.
Scientists detected signals of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) — with researchers 99.7% certain of their presence.
On Earth, DMS is produced almost exclusively by living organisms, particularly marine phytoplankton—the tiny algae that live in our seas.
The detection of these molecules, especially DMS, has led researchers to suggest that K2-18b may possess an ocean beneath its hydrogen-rich atmosphere, potentially “teeming” with microbial life.
This is considered the “strongest evidence to date” for the possible presence of alien life.
This comes after the Hubble Space telescope seemingly detected water vapour in K2-18b’s atmosphere in 2019, with scientists then declaring it “the most habitable known world”.
In 2023, this water signal was shown to be methane in later observations made by Professor Nikku Madhusudhan of Cambridge University.
The concentration of these molecules in K2-18b’s atmosphere is reportedly thousands of times higher than on Earth, further fuelling speculation about biological activity.
However, scientists caution that these findings are not definitive proof of life, but rather considered as a reliable biosignature — a chemical indicator associated with biological processes.
Is K2-18b habitable?
There are a number of reasons why researchers believe K2-18b could sustain life, including:
- Habitable zone: K2-18b orbits within its star’s habitable zone, where temperatures could allow for liquid water.
- Atmospheric composition: The detection of methane and possibly DMS and DMDS suggests a complex atmosphere which could support life.
- Ocean world hypothesis: While this view is contested, the planet could be covered by a deep ocean and shielded by a thick hydrogen-rich atmosphere — conditions possibly suitable for marine life.
Next steps
While the discovery is groundbreaking, researchers stress the need for further observations to confirm these findings and rule out non-biological explanations for the detected molecules.
The next few years are expected to bring more data, which could either strengthen the case for life or offer alternative interpretations.
Professor Madhusudhan said: “This is the strongest evidence to date for a biological activity beyond the solar system.
“We are very cautious. We have to question ourselves both on whether the signal is real and what it means.
“Decades from now, we may look back at this point in time and recognise it was when the living universe came within reach.
“This could be the tipping point, where suddenly the fundamental question of whether we’re alone in the universe is one we’re capable of answering.”
The professor also told The Sun: “There is no mechanism in the literature that can explain what we are seeing without life.
“It’s a question humanity has been asking for thousands of years. It’s a shock to the system. It takes time to recover from the enormity of it.”
He added: “Earlier theoretical work had predicted that high levels of sulfur-based gases like DMS and DMDS are possible on Hycean worlds.
“And now we’ve observed it, in line with what was predicted.
“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.”
A Hycean world is a type of exoplanet that has a liquid water ocean and a hydrogen-rich atmosphere.
The word “Hycean” is a combination of “hydrogen” and “ocean”.
In 2021, the term was coined by astronomers at the University of Cambridge.