The k2-18b planet has become one of the most intriguing exoplanets in recent years. Not because we have found life there, but because scientists have detected traces that may indicate biological processes. What makes K2-18b so special? And how far along are we really in the search for life? In this blog, we dive deep into the facts
What is the K2-18b planet?
K2-18b is an exoplanet at about 124 light years distance from Earth, located in the constellation Leo. The planet was discovered in 2015 during the second phase of NASA's Kepler mission (the K2 mission).
It orbits a relatively cool red dwarf star, K2-18, and is located within the habitable zone - the area around a star where liquid water could exist.
Key properties of K2-18b:
- Beam: about 2.5 times the size of Earth.
- Mass: about 8.6 times heavier than Earth.
- Job period: one year lasts only 33 earth days.
- Distance to his star: about 0.142 astronomical units (AE) (For comparison, the earth is 1 AU from the sun).
- Tidal lock: Probably yes - one side always facing the star.
Due to its short orbital period and the red dwarf's weak radiation, K2-18b remains cool enough to potentially hold water, despite its proximity to its star.
What makes this planet extra special is its unknown nature: it is between a super-Earth and a mini-Neptune in terms of size. We do not know of any comparable object in our own solar system, so scientists are not yet quite sure whether it is an ocean world, a gas giant or something in between.
What do experts say about the possible biosignatures on K2-18b?
On April 16, 2025, a team of astronomers, led by Nikku Madhusudhan of the University of Cambridge, published new results in which they claimed dimethyl sulfide (DMS) detected in the atmosphere of K2-18b.
DMS on Earth comes almost exclusively from living organisms such as plankton, so the discovery caused much excitement. Some media outlets even headlined that possible extraterrestrial life had been found.
National Geographic, however, spoke with ten independent experts and their reactions were divided:
- Skeptics (such as Clara Sousa-Silva and Ryan MacDonald) point to the likelihood of false-positive results. They argue that the 20-molecule comparison leaves room for error, and that the ‘three sigma’ reliability may have been enhanced by selective data processing (“statistical hacking”).
- Cautious optimists (like Laura Kreidberg) consider the research carefully conducted, but stress that scientific caution remains necessary.
- Others point out that DMS has also been found in dead comets (such as on comet 67P), meaning that it is not necessarily produced by life.
In short, it is an indication, but far from definitive proof.
Is K2-18b really habitable?
Although K2-18b is in the habitable zone, this does not automatically mean it is a habitable world.
Several scenarios are circulating:
- According to some models, K2-18b would be a Hycean World could be, with a deep ocean beneath a thin hydrogen atmosphere - a favorable environment for microbial life.
- However, other researchers, who reanalyzed earlier data, found no evidence for carbon dioxide (CO₂) - an essential ingredient for a stable, livable atmosphere. Without CO₂, it would be more of a inhospitable gas giant are.
- There are also models in which the ‘ocean’ of K2-18b is not made of water, but of magma would exist due to extremely high temperatures.
In addition, K2-18b 2.6 times larger and almost nine times as heavy as the Earth. As a result, the surface pressure, if any, may be extremely high - possibly unsuitable for life as we know it.
Research leader Madhusudhan remains optimistic: “There are still open questions, but they do not exclude life.”
Why does this discovery remain important?
Even if no direct evidence of life is found, the discovery around K2-18b remains revolutionary.
Thirty years ago we didn't even know exoplanets existed; today we analyze their atmospheres in search of molecules that may indicate life.
As philosopher of science Peter Vickers put it:
“If we find signs of life on one of the first habitable planets we study, then it is likely that life occurs everywhere.”
Or as astrobiologist Michael Wong says:
“We are only at the beginning. But what a privilege that we can ask these questions at all.”
Why is K2-18b so much talked about?
In 2019, the Hubble Space Telescope found water vapor in the atmosphere of K2-18b. This was extraordinary, but not decisive. Recent observations with the James Webb Space Telescope (JWST). have now demonstrated even more important molecules: methane and carbon dioxide. Both may indicate biological processes.
Moreover, there is weak evidence for dimethyl sulfide (DMS), a molecule produced on Earth almost exclusively by living organisms, particularly plankton in oceans.
Important to realize: this is not proof of life. For example, methane can also be created by non-biological processes.
What is a Hycean World?
One hypothesis is that K2-18b is a so-called Hycean World is: a planet with a thin hydrogen-rich atmosphere and a huge ocean below. In such an environment, microbial life and photochemical processes similar to those on Earth would be possible.
A Hycean World would strongly indicate biological activity in the presence of high concentrations of methane (as well as relatively low concentrations of other gases such as ammonia). The detection of DMS supports this idea, but the observations are not yet certain enough.
How reliable are the measurements?
Scientists use a so-called ‘sigma score’ to indicate reliability:
- 2 sigma: about 95% for sure (similar to 1 in 20 chance of wrong conclusion).
- 3 sigma: about 99.7% for sure (1 in 1000 chance).
The DMS detection at K2-18b now sits at about 2-3 sigma. This is exciting, but not yet convincing enough for firm claims. More observations with the JWST are planned to confirm or refute the data.
Why are biomarkers alone not enough for the K2-18b planet?
Even if methane and DMS are present, that does not automatically mean there is life. There may be unknown geological or chemical processes producing these molecules. As one of the scientists involved stated: “There is no molecule that by itself proves life,” he said.”
Therefore, follow-up research remains crucial. Science looks for patterns: combinations of molecules that together are difficult to explain without biological activity.
How can you yourself contribute to exoplanet research?
Although K2-18b itself is not visible with amateur telescopes, you can observe stars that may harbor exoplanets. Smart telescopes such as the DWARF 3 make this accessible:
- Automatic tracking and stacking allow you to measure variations in starlight.
- Some stars show small dips in brightness as planets move past them (transits).
- With patience and good equipment, you can even contribute to citizen science projects.
Check out the capabilities of the DWARF 3 smart telescope for beginners.
Summary: hope, but no proof yet
The k2-18b planet offers a fascinating glimpse into the complexity of the search for extraterrestrial life. The presence of molecules such as water vapor, methane and possibly dimethyl sulfide makes K2-18b a prime candidate. But as always in science: extraordinary claims require extraordinary evidence.
With new observations and better technology, the truth is getting closer step by step.
Keep looking at the stars, because the universe is far from giving away all its secrets.
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