At first glance, the planet K2-18b sounds like just another set of numbers. It’s about 125 light-years from Earth. It orbits a cool, faint star about once a month. And it’s more than twice the diameter of Earth, and about nine times Earth’s mass. Yet those numbers give scientists a profile of a remarkable world. It may be covered by liquid water, have a thick hydrogen atmosphere, and be at a comfortable distance from its star. That could make the planet a good place to hunt for life. K2-18b is a possible example of a type of planet known as “hycean” — a combination of hydrogen and ocean. Such worlds are bigger and heavier than Earth, but smaller than Neptune, the solar system’s next-largest planet. Astronomers have discovered only a few candidate worlds, but a recent study says they could be quite common. K2-18b resides in its star’s “habitable zone” — the region where temperatures are just right for water on the surface. Astronomers have even discovered water in its atmosphere. But th
