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Hycean Planets

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 the combination of water and thick atmosphere could mean that such worlds could be “habitable” even if they were far outside that zone.

Scientists plan to study K2-18b and other candidates in detail. They’ll try to confirm the thickness and composition of the atmosphere. Eventually, they’ll look for “biomarkers” — combinations of compounds that, here on Earth, are produced by life.

More about exoplanets tomorrow.

Script by Damond Benningfield

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