The planet K2-18b is tidally locked to its star, meaning that one side of the planet always faces the star and the other side always faces away from it. This extreme temperature difference between the two sides of the planet could lead to a number of different surface conditions, including oceans of water, oceans of lava, or even a completely different surface composition that we have not yet seen in our own solar system.

Some scientists believe that K2-18b could be covered in oceans of water. This is because the planet is located in the habitable zone of its star, meaning that it receives just the right amount of heat to allow for liquid water to exist on its surface. Additionally, the planet's atmosphere is thought to be relatively thick, which could help to trap heat and keep the planet warm enough for liquid water to exist.

Other scientists believe that K2-18b could be covered in oceans of lava. This is because the planet is very close to its star, which could cause the surface of the planet to be extremely hot. Additionally, the planet's atmosphere is thought to be relatively thin, which could allow heat from the star to reach the surface more easily and cause the planet to become very hot.

It is also possible that K2-18b has a completely different surface composition than we have yet seen in our own solar system. This is because the planet is located in a different star system, and it is possible that the conditions in that system are very different from the conditions in our own solar system.

Ultimately, the question of whether K2-18b is covered in oceans of water or lava is still up for debate. Further research is needed to determine the planet's surface composition and to learn more about the conditions in the K2-18 system.