K2-18 b
Sub-Neptune · An ocean world?
K2-18 b is a planet bigger than Earth but smaller than Neptune, in its star’s habitable zone. The James Webb Space Telescope found methane and carbon dioxide in its atmosphere, and some scientists think it could be covered by a deep ocean.
- 124 light-years awayEven light – the fastest thing there is – needs 124 years to get here.
- ~2.6 × EarthAbout 2.6 times as wide as Earth.
- ~8.6 × EarthMass
Take a closer look
The missing kind of planet
Planets between the size of Earth and Neptune turn out to be among the most common in our galaxy – yet our Solar System has none. That makes K2-18 b a precious chance to study a kind of world we can’t visit at home and still don’t fully understand.
A mystery recipe
For its size, K2-18 b is too light to be solid rock like Earth, but too heavy to be mostly gas like Neptune. So it must be built from some mix of rock, water and gas. Scientists are still arguing about which recipe is right.
Once every 33 days
Webb can only sniff K2-18 b’s air when the planet passes in front of its star, which happens just once every 33 days and lasts a few hours. Each pass gives a little more starlight-filtered data, so scientists need many passes to be sure what they are seeing.
How we found out
- 2015
Found by NASA’s K2 mission (Kepler’s second life).
- 2019
Hubble detects water vapour in its atmosphere.
- 2023
Webb detects methane and carbon dioxide, which fits the idea of an ocean world under a hydrogen-rich sky.
- 2025
One team reports a possible hint of a gas that, on Earth, is made by living things. Many other scientists say the evidence is weak. The debate continues!
How would we know if a planet has life?
Scientists look for “biosignatures”: combinations of gases that are hard to make without life, like oxygen together with methane. But they must rule out every other explanation first.
See it yourself
Only with large telescopes, in the constellation Leo.
From the encyclopedia
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