K2-18

system

K2-18 is a red dwarf 124 light-years away in Leo, host to K2-18 b — a habitable-zone sub-Neptune at the centre of the most-watched question in exoplanet science: whether its atmosphere holds water, and possibly a chemical fingerprint of life.

Open in 3D viewSee K2-18 in the live, interactive 3D map

Key facts

Type

Red dwarf + 2 planets

Distance

124 light-years

Planets

b (transit, HZ) + c (RV, hot)

Why famous

Water vapour + contested DMS hint (on b)

Science

Classification

Type: Single red-dwarf system
Central star: K2-18 — M2.5V red dwarf, ~3457 K, 0.36 M☉

Planets

Transiting (HZ): K2-18 b · the famous biosignature sub-Neptune
Radial-velocity (inner): K2-18 c — shown, disputed

Distance

Distance: 124 light-years · in Leo, the Lion

How we know

Note: K2-18 b is confirmed and transiting, so its size and density are measured (NASA Exoplanet Archive, checked 2026-07-07). Water vapour (2019) is a real detection; the 2023 hint of dimethyl sulphide (a possible life gas) is CONTESTED and unconfirmed — no claim about life is established. The inner world K2-18 c is a radial-velocity detection (minimum mass, no transit → no measured size): its 9-day signal was disputed as stellar activity in 2018 but independently recovered in 2019 and 2022, though the Archive still flags it controversial — so it IS shown, plainly labelled, as the hot inner sibling (not the biosignature planet). Placed at its real position, 124 light-years away.

Discovery & history

Name: A designation from NASA's K2 mission (the repurposed Kepler telescope), the 18th planet-hosting star it confirmed; the star has no proper name.

K2-18 b was found transiting by NASA's K2 mission in 2015. In 2019 Hubble detected water vapour in its atmosphere — the first for a habitable-zone planet — and from 2023 JWST reported carbon-bearing molecules and a contested hint of dimethyl sulphide (DMS), a gas made only by life on Earth. The claim remains hotly debated. A second planet, K2-18 c, was detected by radial velocity in 2017 — challenged as stellar activity in 2018, then independently recovered (2019, 2022), though the NASA Exoplanet Archive still flags it controversial.

Images

Artist's impression of the habitable-zone sub-Neptune K2-18 b, its star and a sibling planet — an artist's concept, not a photograph. — NASA, ESA, CSA, and Joseph Olmsted (STScI) / M. Kornmesser (public domain) · Wikimedia Commons

Illustration of the sub-Neptune K2-18 b built from the science data — an artist's concept, not a photograph. — NASA, CSA, ESA, J. Olmsted (STScI) (public domain) · Wikimedia Commons

Images from open archives · credit shown per image