Exoplanets orbiting M dwarfs present a valuable opportunity for their detection and atmospheric characterisation .
This is evident from recent inferences of H _ { 2 } O in such atmospheres , including that of the habitable-zone exoplanet K2-18b .
With a bulk density between Earth and Neptune , K2-18b may be expected to possess a H/He envelope .
However , the extent of such an envelope and the thermodynamic conditions of the interior remain unexplored .
In the present work , we investigate the atmospheric and interior properties of K2-18b based on its bulk properties and its atmospheric transmission spectrum .
We constrain the atmosphere to be H _ { 2 } -rich with a H _ { 2 } O volume mixing ratio of 0.02 - 14.8 % , consistent with previous studies , and find a depletion of CH _ { 4 } and NH _ { 3 } , indicating chemical disequilibrium .
We do not conclusively detect clouds/hazes in the observable atmosphere .
We use the bulk parameters and retrieved atmospheric properties to constrain the internal structure and thermodynamic conditions in the planet .
The constraints on the interior allow multiple scenarios between rocky worlds with massive H/He envelopes and water worlds with thin envelopes .
We constrain the mass fraction of the H/He envelope to be \lesssim 6 % ; spanning \lesssim 10 ^ { -5 } for a predominantly water world to \sim 6 % for a pure iron interior .
The thermodynamic conditions at the surface of the H _ { 2 } O layer range from the super-critical to liquid phases , with a range of solutions allowing for habitable conditions on K2-18b .
Our results demonstrate that the potential for habitable conditions is not necessarily restricted to Earth-like rocky exoplanets .