JWST atmosphere target
K2-18
JWST detected possible dimethyl sulfide — a potential biosignature, still debated.
- Planets
- 2
- Distance
- 124.0 ly
- Host
- M-type
About K2-18
Discovery
K2-18 b was identified in 2015 by Montet and colleagues in data from Kepler's repurposed K2 mission, transiting a cool M-dwarf about 124 light-years away in Leo.
Why it matters
In 2019 Hubble detected water vapor in K2-18 b's atmosphere — the first habitable-zone sub-Neptune with a confirmed atmospheric molecule. It became the test case for the 'Hycean world' hypothesis: ocean-covered planets under thick hydrogen envelopes.
Current research
Madhusudhan et al. reported tentative JWST detections of dimethyl sulfide (DMS) — a potential biosignature — but follow-up MIRI observations in 2025 placed new constraints on DMS/DMDS, and the biosignature interpretation remains contested.
Comparable to
A sub-Neptune more than twice Earth's radius and roughly nine times its mass — too big to be terrestrial, possibly an ocean world under a hydrogen sky.
System geometry
At a glance
- Hostname
- K2-18
- Spectral type
- M2.5 V
- Distance
- 124.0 ly · 38.03 pc
- Stellar mass
- 0.36 M☉
- Stellar radius
- 0.41 R☉
- Luminosity
- 0.025 L☉
- Effective temp
- 3457 K
- Confirmed planets
- 2
- Habitable zone
- 0.151 – 0.218 AU
Top-down orbital diagram
Orbits to scale within this system. Dashed green = habitable-zone edges.
Planet positions are illustrative (evenly spaced in phase). For live motion see the 3D scene.
The planets
2 confirmed.
Sub-Neptune
K2-18 c
- Orbit
- 0.060 AU
- Period
- 8.96 days
- Radius
- 2.65 R⊕
- Mass
- 7.51 M⊕
- Eq. temperature
- 363 K
- Eccentricity
- 0.470
- Discovered
- 2017 · Radial Velocity
Sub-Neptune
K2-18 b
- Orbit
- 0.143 AU
- Period
- 32.94 days
- Radius
- 2.37 R⊕
- Mass
- 8.92 M⊕
- Eq. temperature
- 284 K
- Eccentricity
- 0.200
- Discovered
- 2015 · Transit
Compared to our Solar System
Each row shows the closest Solar-System analog by radius (log-space). Earth is pinned at the bottom as the constant frame of reference.
| Planet | Radius (R⊕) | Mass (M⊕) | Orbit (AU) | Period (days) | Eq temp (K) | Solar analog |
|---|---|---|---|---|---|---|
| K2-18 c | 2.65 | 7.51 | 0.060 | 8.96 | 363 | Neptune |
| K2-18 b | 2.37 | 8.92 | 0.143 | 32.94 | 284 | Neptune |
| Earth (reference) | 1.00 | 1.00 | 1.000 | 365.25 | 255 | — |
Research status
◇ JWST observation status
1 planet has confirmed JWST observation time across Cycles 1–3.
- K2-18 b
JWST programs include transit spectroscopy, thermal phase curves, and direct imaging coronagraph observations depending on planet class.
Discovery timeline
- 2015
K2-18 b
via Transit
- 2017
K2-18 c
via Radial Velocity
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55 Cancri
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Hosts 55 Cancri e — a lava world JWST detected silicate atmosphere on.
Experience it
See K2-18 in interactive 3D
Fly through the system, click any planet, watch orbits play out at 100× speed.
▶ Launch 3D scene