Alien plankton, oceans & 33 day years… Everything we know about ‘super earth’ K2-18b that is ‘teeming with life’
SOME 124 light years from our world lies planet “Super Earth” where everything is said to be covered by vast ocean and a year lasts just 33 days.
Top scientists claim to have found the best-ever-seen signal of biological activity on K2-18b – an exoplanet more than twice the size of our Earth where extraterrestrial life possibly exists.
Top alien-hunting astronomers at the University of Cambridge say they’re now 99.7% confident they’ve spotted a possible biosignature on the planet that’s “teeming with life”.
Almost 2.5x the size of Earth, K2-18b has a 33-day orbit and receives almost the same amount of starlight as our planet receives from the Sun.
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.
The planet is being studied as a potential habitable world that has a temperature close to -2 degrees Celsius.
And yesterday, astronomers picked out a molecule called dimethyl sulfide (DMS) in the planet’s atmosphere, claiming it to be the best-ever-seen signal of biological activity outside of our solar system.
On Earth, it’s produced by the tiny algae that live in our seas.
These “phytoplankton” are microscopic and can’t be seen with the human eye.
But in large numbers, they show up as coloured patches on water.
Scientists believe similar alien lifeforms might be churning it out on K2-18b.
Lead scientist Dr Nikku Madhusudhan at the University of Cambridge told The Sun: “This is a revolutionary moment. It’s the first time humanity has seen potential biosignatures on a habitable planet.
“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.”
While an unknown chemical process may be the source of these molecules in K2-18b’s atmosphere, the results are the strongest evidence yet that life may exist on a planet outside our solar system.
“It is in no one’s interest to claim prematurely that we have detected life,” Dr Madhusudhan added.
Astronomers used data from the James Webb Space Telescope to spot fingerprints of two molecules: dimethyl sulphide (DMS) and/or dimethyl disulphide (DMDS) in the planet’s atmosphere.
And the results are so convincing that there’s just a “0.3% probability they occurred by chance”.
It’s all taking place on a so-called Hycean world – a planet with a watery ocean and a thick hydrogen atmosphere.
Hycean comes from combining “hydrogen” and “ocean” together.
“If it is indeed a Hycean world, the planet would be covered in oceans,” Dr said told The Sun.
“We currently don’t know what the temperature of those oceans would be.
“But we expect it to be slightly warmer than Earth.
“On Earth, extremophiles have been known to survive at very high temperatures, so that is not itself an issue.”
We can’t directly see K2-18b because of how close it is to the host star.
So scientists have to use a genius technique to work out what’s going on there.
They watch the star that K2-18b is orbiting, and wait for the planet to pass across its face.
By analysing the star’s light during this “transit”, JWST can pick up the drop in the star’s brightness.
A small fraction of the starlight passes through the planet’s atmosphere.
And the absorption of this starlight in the planet’s atmosphere leaves “imprints” that astronomers can use to work out which gases are present.
Not only did Madhusudhan’s team spot DMS – a biosignature – in the atmosphere, but they also found it in huge quantities.
Normally, DMS and DMDS are found in Earth’s atmosphere at rates of about one part per billion.
But on K2-18b, levels are thousands of times higher, more than 10 parts per million.
“Earlier theoretical work had predicted that high levels of sulfur-based gases like DMS and DMDS are possible on Hycean worlds,” the Cambridge professor explained.
“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.”
He added: “This has been one of the most fundamental questions that we as a species have asked for thousands of years.”
“Everyone would agree that at some point in their lives, they would look at the night sky and ask: are we alone?”
What is a Hycean world? The story of K2-18b so far
Here's what you need to know...
A Hycean world is a type of exoplanet – or planet outside of our solar system.
Specifically, it’s a planet that has both a liquid water ocean and a hydrogen-rich atmosphere.
That’s where the name comes from: Hycean is “hydrogen” and “ocean” combined.
It was coined as a term in 2021 by astronomers at the University of Cambridge.
Scientists think they’re a top location to hunt for alien life.
They expect life on Hycean worlds to be aquatic – that means no land mammals like here on Earth.
One of the best candidates for life is K2-18b, because it appears to have both a Hycean atmosphere and biosignatures, or signs of life.
The planet is 120 lightyears away from Earth, orbiting the red dwarf start K2-18.
It’s about 2.6 times bigger than Earth and has a 33-day orbit.
K2-18b gets about the same amount of sunlight from its star as Earth receives from the Sun.
It was first discovered in 2015, and water vapour was found in the atmosphere in 2019.
Then in 2023, the James Webb Space Telescope picked up carbon dioxide and methane in the atmosphere.
Scientists later predicted that we might find potential biosignatures linked to life in the water ocean they believe to be on the planet.
And in April 2025, scientists were finally able to say with 99.7% certainty the presence of a biosignature: a type of molecule called DMS, which is produced by marine phytoplankton here on Earth.
It might mean that similar lifeforms exist on K2-18b too.
But there’s still more work to be done.
The certainty level is currently three-sigma, which is 99.7%.
Madhusudhan wants to get more analysis time to reach the coveted five-sigma, which means there’s just a 0.00006% probability the results occurred by chance.
“The observation was about eight hours of JWST time,” Madhusudhan told us.
He said they’d need two or three more transits to be certain, which is 16 to 24 hours.
“We have to formally apply for JWST time and there will be a selection process,” said Madhusudhan, speaking to The Sun.
“And if they’re convinced by the case, it’ll be accepted and then the observations will be made.
“But these observations are non-trivial to make. They don’t happen very frequently, especially with JWST.
“There are only around four windows of observation every year. So if these observations are approved, this would happen potentially over the next year.”
It’s also possible that the cause of the DMS is non-biological, but Madhusudhan says there’s no known mechanism for this right now.