section_path: Physical properties
section_name: Physical properties
paragraph_text: Physical properties
  
  K2-18b has a radius of 2.610 ± 0.087 R, a mass of 8.63 ± 1.35 M, and orbits its star in 33 days. From Earth, it can be seen passing in front of the star. The planet is most likely tidally locked to the star, although considering its orbital eccentricity, a spin-orbit resonance like Mercury is also possible.
  
  The density of K2-18b is about 2.67 +0.52 −0.47 g/cm 3 — intermediate between that of Earth and Neptune — implying that the planet has a hydrogen -rich envelope. The planet may either be rocky with a thick envelope or have a Neptune-like composition. A pure water planet with a thin atmosphere is less likely. 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. Presumably, planets with intermediary radii cannot hold their atmospheres against the tendency of their own energy output and the stellar radiation to drive atmospheric escape. (Planets with even smaller radii are known as super-Earths and those with larger radii as sub-Neptunes.)
  
  The planet may have taken a few million years to form. Tidal heating is unlikely. Internal heating may increase temperatures at large depths, but is unlikely to significantly affect the surface temperature. If an ocean exists, it is probably underlain by a high-pressure ice layer on top of a rocky core, which might destabilize the planet's climate by preventing material flows between the core and the ocean.