As astronomers have discovered more planets in other star systems, they’ve gotten more surprises. One of the biggest is a class of planets unlike anything in the solar system. “Super-Earths” are heavier than our own planet, and a little bigger. And they’re quite common. Current catalogs list hundreds of them.

In our own solar system, there are two classes of planets: small and rocky, like Earth, and big and “gassy,” like Jupiter. Super-Earths fall between those extremes. There’s no formal definition for them. But a general definition is any planet between roughly 2 and 10 times the mass of Earth.

Depending on the distance from their parent stars, such worlds could take several forms. They could be mainly rock and metal, like Earth. They could have a massive core that’s surrounded by a thick atmosphere, or that’s topped by global oceans. Or they could have a smaller core surrounded by thick layers of hydrogen and helium gas.

The closest super-Earth may be just four and a quarter light-years away. Proxima Centauri b orbits the nearest star to the Sun. Astronomers haven’t been able to nail down the planet’s mass. It could be about as heavy as Earth. But it could also be a few times heavier, putting it solidly in the super-Earth category.

The planet is far enough from its star that it may be too cold for life. But there’s enough uncertainty about conditions that life can’t be ruled out — a “super” possibility for a super-Earth.

Script by Damond Benningfield