JWST’s data has been variously interpreted as indicating a water ocean planet with a hydrogen-rich atmosphere, and a gas-rich mini-Neptune.

K2-18b has been studied as a potential habitable world that, temperature aside, more closely resembles a gas planet like Uranus or Neptune than Earth.

[21] Planets with radii of about 1.5–2 R🜨 are unexpectedly rare relative to their expected occurrence rate, a phenomenon known as the radius valley.

[27] At temperatures exceeding the critical point, liquids and gases stop being different phases and there is no longer a separation between an ocean and the atmosphere.

[37][38] Another paper suggests that a liquid water ocean model requires the presence of a biosphere in order to produce sufficient amount of methane.

[39] The presence of water vapour is likely[40] but with uncertainty,[41] as James Webb Space Telescope observations indicating concentrations of less than 0.1%;[42] this may be due to the JWST seeing a dry stratosphere[33] as the atmosphere is thought to have an efficient cold trap.

[54] Most computer models expect that a temperature inversion will form at high elevation, yielding a stratosphere.

[59] Observations of decreases of Lyman alpha radiation emissions during transits of the planet may show the presence of such an exosphere; this discovery requires confirmation.

[64] Among the climate modelling efforts made on K2-18b are: Incoming stellar radiation amounts to 1368+114−107 W/m2, similar to the average insolation Earth receives.

[84] According to Madhusudhan et al., several of these markers could be detected by the James Webb Space Telescope after a reasonable number of observations.

[58] Analyses of the transits ruled out that they were caused by unseen companion stars,[88] by multiple planets or systematic errors of the observations.

Planets with such compositions were previously thought to be too hot to be habitable; findings at K2-18b instead suggest that they might be cold enough to harbour liquid water oceans conducive to life.

The strong greenhouse effect of the hydrogen envelope might allow them to remain habitable even at low instellation rates.

[95][96] Some scientists have voiced concerns about the statistical significance of the DMS signal at K2-18b, and how its putative detection might be understood in an environment different than Earth.

[99] A podcast on the Planetary Society's website in January 2024 featured NASA astrophysicist Knicole Colón describing some of the scientific results from the observations of K2-18b by JWST.