Promising Candidates

Promising candidates for hycean worlds are identified based on atmospheric observations with the James Webb Space Telescope. These are planets for which the observed atmospheric composition is consistent with theoretical predictions for a hycean world.

K2-18 b

K2-18 b is the first habitable-zone exoplanet identified as a promising candidate for being a hycean world. Transmission spectroscopy with the JWST NIRSpec and NIRISS instruments in the 1-5 micron range led to the first detections of  CH₄ and CO₂ (Madhusudhan et al. 2023b) in a habitable-zone exoplanet. These detections along with non-detections of  NH₃ and CO, are consistent with predictions for a hycean planet (e.g. Madhusudhan et al. 2023a, Cooke & Madhusudhan 2024). The spectra also suggest a tentative inference of dimethyl sulfide (DMS), a potential biosignature that on Earth is known to be produced uniquely by life. However, more observations are needed to robustly confirm its presence and possible origin.

STAR NAME: K2-18

RA and DEC

11h30m14.43s +07d35m16.19s

Distance

38.03 pc

V magnitude

13.48

Star Mass

0.50 Solar Mass

Star Radius

0.44 Solar Radii

Spectral Type/Temperature

M2.5V / 3457K

Rotation Period

39 Days

planets in system

2

PLANET NAME: K2-18 b

Planet Mass

8.63 Earth Mass

Planet Radius

2.61 Earth Radii

Planet Gravity

1.26 Earth Gravity

Orbital Period

32.94 Days

Equilibrium Temperature

255

Molecules Discovered

COsub>2, CHsub>4, and tentatively Dimethyl Sulphide (DMS)

JWST DATA

TOI-270 d

The temperate sub-Neptune TOI-270 d was the second promising candidate for a hycean world identified with JWST observations (Holmberg & Madhusudhan 2024). The transmission spectrum of the planet reveals distinct absorption features from CH₄ and CO₂, similar to K2-18 b. Furthermore, the detections of those species and non-detections of NH₃ and CO are suggestive of a hycean world, with a potential ocean surface. Tentative evidence for H₂O and CS₂ is also observed, though at lower significance. The inferred atmospheric properties are consistent with predictions for hycean planets. However, the somewhat higher temperature on TOI-270 d compared to K2-18 b means that it could be a dark hycean world, with habitable oceans on the night side but the dayside being too hot to be habitable.

STAR NAME: TOI-270

RA and DEC

04h33m39.86s -51d57m26.62s

Distance

22.48 pc

V magnitude

12.60

Star Mass

0.39 Solar Mass

Star Radius

0.38 Solar Radii

Spectral Type/Temperature

M3V / 3506

Rotation Period

140-150 Days

planets in system

3

PLANET NAME: TOI-270 d

Planet Mass

4.78 Earth Mass

Planet Radius

2.13 Earth Radii

Planet Gravity

1.05 Earth Gravity

Orbital Period

11.38 Days

Equilibrium Temperature

354

Molecules Discovered

CO₂, CH₄, tentative H₂O, CS₂, C₂H₆

JWST DATA