super-Earth

Our Solar System has a curious gap: between Earth, the largest rocky world, and Neptune, the smallest gas-and-ice giant, there is nothing. Yet across the galaxy, planets that fall right into that gap turn out to be among the most common of all. A super-Earth is a planet larger and heavier than Earth but smaller than the ice giants Uranus and Neptune — a whole category of world that, oddly, our own Solar System lacks entirely.

Roughly speaking, super-Earths span from a bit bigger than Earth up to about 1.5 to 2 times its radius, and from a couple to perhaps ten Earth masses. The name is about size, not whether the planet is hospitable; a super-Earth can be a temperate rocky ball, a sweltering lava world, or something with a thick atmosphere. There is an important divide near 1.5 to 1.6 Earth radii: below it, planets tend to be dense and rocky like a scaled-up Earth; above it, they tend to be wrapped in thick gas envelopes and shade into a different class. Pinning down which a given planet is takes both its radius (from transits) and its mass (from radial velocity or timing) to get its density.

Super-Earths matter because they are abundant — surveys suggest a large fraction of stars host one or more — so they are central to the question of how common potentially habitable worlds are. Because they are bigger than Earth, they are also somewhat easier to detect and study, making them prime targets for measuring atmospheres. A caveat worth keeping: a super-Earth is not simply a bigger, friendlier Earth. Greater mass can mean crushing gravity, a thick suffocating atmosphere, or vigorous geology, and we are still learning what these common worlds are actually like.