Detected gasesmethane, carbon dioxidethe CO₂ detection has since been questioned[2]
About the image. Drawn as a hycean world: a water ocean under a thick hydrogen haze. That is one proposed interpretation. A gas-rich mini-Neptune without any ocean fits the data too.
2.9×as wide as our Sun looks from Earth
≈ 1×about as much light as Earth gets
The sky from K2-18b
K2-18 would look 2.9× as wide as the Sun does from Earth, and the planet gets about as much light as Earth gets from the Sun.
Its colour comes from its surface temperature of 3 457 K (3 180 °C); the Sun's is about 5 772 K (5 500 °C).
How fast K2-18b spins is unknown, so the length of its day is too.
Apparent size = star radius ÷ orbit; light = size² × (temperature ÷ 5 772 K)⁴. Both from the values on this page. The landscape is imagined.
K2-18b orbits a red dwarf every 33 days, inside the zone where a planet could, in principle, hold liquid water. It is about 2.4 times wider than Earth and nearly nine times as massive: too big to be simply a larger Earth, too small to be a Neptune.
What JWST found
In 2023 the James Webb Space Telescope watched starlight filter through the planet’s atmosphere. The team led by Nikku Madhusudhan reported methane and carbon dioxide, each at roughly one percent. Later observations have cast some doubt on the carbon dioxide. Water, reported earlier from Hubble data, now looks scarce in the upper atmosphere.
The hycean idea
Madhusudhan’s group proposed that K2-18b is a hycean world: a planet-wide ocean beneath a hydrogen-rich atmosphere. It is the prototype of that idea. Other researchers fit the same data with a gas-rich mini-Neptune, or with a magma ocean, and none of these can yet be ruled out.
The life claim
Dimethyl sulfide is made on Earth mostly by marine plankton. A tentative DMS signal was reported in 2024 and a stronger claim followed in 2025. Other teams found the signal could come from other molecules, depends on how the data are processed, has low statistical significance, and that DMS can form without life. As of 2025 the claim is not widely accepted. What is not in doubt is that K2-18b is one of the most closely watched planets there is.
Its orbit, to scale
K2-18b goes around once every 33 days, at 0.1429 AU from its star. The whole view is tiny on Solar System scale: Mercury orbits the Sun about 3 times farther out than the outermost orbit shown here.
K2-18b: equilibrium temperature. Logarithmic scale from 100 K to 10 000 K. Reference values: NASA, IAU.
How long to get there?
K2-18b is about 124 light-years away.
Light299 792 km/s124 years
Breakthrough Starshotplanned light sail, 20% of light speed620 years
Parker Solar Probefastest spacecraft so far, 192 km/s193 616 years
Voyager 1leaving the Solar System, 17 km/s2.2 million years
At Voyager 1's speed the trip would take about 27 334 human lifetimes of 80 years. Bars are on a logarithmic scale. Speeds: NASA, Breakthrough Initiatives.
Your weight on K2-18b
On K2-18b
Gravity there is about 1.59× Earth's, at the planet's measured radius; it may have no solid surface. Calculated from the measured mass and radius.
Gallery
Artist's impressions rendered from the same model as the view at the top of this page.
Timeline
2015Discovered by NASA's K2 mission.
2019Water vapour reported in its atmosphere from Hubble data.
2023JWST finds methane and carbon dioxide; the hycean-world idea gains attention.
2024A tentative dimethyl sulfide signal is reported.
2025A stronger DMS claim follows and is widely disputed.
Questions people ask
Has life been found on K2-18b?
No. A possible signal of dimethyl sulfide, which on Earth comes mostly from plankton, was reported, but other researchers found it weak, dependent on data processing and possible without life. It is not accepted as evidence of life.
Does K2-18b have an ocean?
Possibly. One interpretation is a water ocean under a hydrogen-rich atmosphere. The same data also fit a gas-rich mini-Neptune with no ocean.
How far away is K2-18b?
About 124 light-years. It orbits a red dwarf star.