K2-18b facts for kids
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| Discovery | |
|---|---|
| Discovery site | Kepler space telescope |
| Discovery date | 2015 |
|
Detection method
|
Transit |
| Orbital characteristics | |
| 0.15910+0.00046 −0.00047 au 21,380,000 km |
|
| Eccentricity | 0.09+0.12 −0.09 |
| 32.940045±0.000100 d | |
| 354.3+46.4 −33.8° |
|
| Star | K2-18 |
| Physical characteristics | |
|
Mean radius
|
2.610±0.087 R⊕ |
| Mass | 8.63±1.35 M⊕ |
|
Mean density
|
2.67+0.52 −0.47 g/cm3 |
| 12.43+2.17 −2.07 m/s2 |
|
| Temperature | 265 ± 5 K (−8 ± 5 °C) |
K2-18b is an amazing exoplanet, which means it's a planet outside our Solar System. It orbits a red dwarf star called K2-18, located about 38 parsecs (or 124 light-years) from Earth. This planet is much bigger than Earth, about 2.6 times its size.
K2-18b takes about 33 days to orbit its star. What's really cool is that it's in the star's habitable zone. This is the area where temperatures might be just right for liquid water to exist on a planet's surface. K2-18b gets a similar amount of sunlight as Earth does from our Sun.
Scientists first found K2-18b in 2015 using the Kepler space telescope. Later, the powerful James Webb Space Telescope (JWST) looked at it closely to study its atmosphere. In 2019, scientists found water vapour in its atmosphere, which got everyone excited!
Then, in 2023, the JWST found carbon dioxide and methane there too. Some scientists think K2-18b might be a "water world" with a thick, hydrogen-rich atmosphere. Others believe it's more like a "mini-Neptune," a gas planet similar to Uranus or Neptune, but smaller.
In 2025, there was news that dimethyl sulfide (DMS) might be in K2-18b's atmosphere. DMS is a chemical that could be a biomarker, meaning it might be a sign of life. On Earth, DMS is mostly made by tiny ocean creatures called phytoplankton. The amount found on K2-18b was said to be 20 times more than on Earth, which suggested it was being constantly made. However, some scientists say DMS can also be made without life.
Contents
The Star K2-18
K2-18 is a red dwarf star, which is smaller and cooler than our Sun. It's found in the Leo constellation. This star is about 2.4 billion years old. It's too dim to see with your eyes from Earth.
K2-18 has another planet, K2-18c, which orbits closer to the star than K2-18b. Red dwarf stars like K2-18 are very common. Scientists think up to 80% of them might have planets in their habitable zones. This makes them great places to look for other worlds!
What K2-18b is Like
K2-18b is about 2.6 times the radius of Earth and has a mass about 8.6 times that of Earth. It orbits its star every 33 days. From Earth, we can see it pass in front of its star, which helps scientists study it.
The planet is probably tidally locked to its star. This means one side always faces the star, like how the Moon always shows the same side to Earth.
K2-18b's density is somewhere between Earth's and Neptune's. This suggests it has a thick layer of hydrogen gas around it. It might be a rocky planet with a very thick atmosphere, or it could be more like a small Neptune, made mostly of gas and ice. A planet made purely of water with a thin atmosphere is less likely.
Scientists have noticed that planets with sizes between 1.5 and 2 times Earth's radius are rare. This is called the "radius valley". It's thought that planets in this size range might struggle to hold onto their atmospheres. K2-18b is larger than this "valley," so it can keep its thick atmosphere.
Could it have an Ocean?
It's hard to tell if K2-18b has a liquid water ocean from far away. Sometimes, at very high temperatures and pressures, liquids and gases can mix together, so there isn't a clear ocean surface.
Early observations suggested that K2-18b might have a "supercritical" water layer, where water acts like both a liquid and a gas. More recent observations from the JWST were first thought to mean there's a clear boundary between a liquid ocean and the atmosphere. However, scientists are still debating this. Some ideas suggest that a liquid ocean might need life to explain the amounts of certain gases found.
Atmosphere and Climate
Observations show that K2-18b's atmosphere is mostly hydrogen. The James Webb Space Telescope found that methane and carbon dioxide each make up about 1% of the atmosphere. There's also some water vapour, but not a lot, possibly because of a "cold trap" that freezes water higher up.
The atmosphere of K2-18b probably looks a lot like the atmospheres of Uranus and Neptune in our own Solar System. Scientists haven't found much evidence of hazes, but there might be water clouds. These clouds would likely be made of ice, but liquid water clouds are also possible.
How the Atmosphere Changes
The star K2-18 sends out strong UV radiation and X-rays. These can heat the upper atmosphere of K2-18b and cause hydrogen to escape into space. This process is slow, so the planet won't lose its entire atmosphere during its lifetime. Scientists are looking for signs of this escaping hydrogen.
Different Ideas
Studying exoplanet atmospheres is tricky, and sometimes different scientists have different ideas about what the data means. For example, some have suggested that the water vapor signals might be from the star itself, not the planet. Others have proposed that methane might be a bigger part of the atmosphere than first thought, or that the water signal could actually be due to methane.
Climate Models
Scientists use climate models (computer simulations) to guess what K2-18b's climate might be like. These models help them understand how the atmosphere moves and what kind of weather the planet might have.
- Some models show that K2-18b, if it's tidally locked, would have mild temperature changes across its surface. Winds would move air from the day side to the night side.
- Clouds might form, especially where the star's light hits the planet directly. If it rains, the rain might evaporate before it reaches the surface.
- Other models suggest that the planet could have strong winds, like jet streams, high above its equator.
- Models also predict that a layer of sulfur haze could form in the atmosphere. This haze would make it much harder for scientists to study the gases below it.
Can K2-18b Support Life?
K2-18b gets about the same amount of energy from its star as Earth gets from the Sun. It's right inside or very close to the habitable zone. This means its temperature, without a thick atmosphere, would be similar to Earth's.
However, whether the planet is truly habitable depends on its atmosphere. The deeper parts of its atmosphere might be too hot for life. But layers with water could have temperatures and pressures suitable for life to develop.
Scientists know that some tiny living things on Earth can survive in hydrogen-rich atmospheres. This shows that hydrogen itself isn't a problem for life. But finding life on K2-18b would need different "biosignatures" (signs of life) than what we look for on Earth. The James Webb Space Telescope might be able to find some of these signs with more observations.
How K2-18b Was Found
The Kepler space telescope first discovered K2-18b in 2015. Its existence was later confirmed by other telescopes, like the Spitzer Space Telescope. Scientists made sure that the signals they saw were from a planet and not from other things like binary stars.
K2-18b has helped scientists define a new type of planet called a "hycean planet". These planets are thought to have lots of liquid water and a thick hydrogen atmosphere. Before K2-18b, scientists thought such planets would be too hot for life. But K2-18b suggests they might be cool enough to have liquid water oceans, which are important for life. The thick hydrogen atmosphere could even help keep them warm enough. K2-18b is probably the most famous "hycean planet."
In 2024, a podcast from the Planetary Society talked about the JWST's observations of K2-18b. Scientists are still gathering more data to confirm what they've found. In April 2025, the team that first reported DMS suggested they had stronger evidence for it. They even put out a press release saying it was "strongest hints yet of biological activity outside the solar system." However, NASA and other scientists quickly pointed out that finding one possible sign of life doesn't mean life has been discovered. They also reminded everyone that DMS can be made without living things. Scientists expect to get even more clear answers in the next year or two.
Images for kids
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Artist's impression of K2-18b orbiting its star.
See also
- Extraterrestrial liquid water
- Habitability of red dwarf systems
- List of potentially habitable exoplanets
- Planetary habitability
