Deep in the outer solar system, billions of miles from Earth, lurks a realm of small, icy worlds called dwarf planets. Astronomers know relatively little about these dim and distant objects, but in recent years, new evidence has revealed that the tiny planets can hold a surprising range of features, from oceans and mountains to canyons, dunes and volcanoes.
Much of what astronomers know about dwarf planets comes from the NASA New Horizons spacecraft’s 2015 Pluto flyby. Pluto thrilled scientists with its towering mountain ranges made of ice. Its surprising geological complexity leaves astronomers eager to see the diversity of similar worlds in the Kuiper Belt — a donut-shaped region beyond Neptune packed with icy space rocks both large and small.
So far, the International Astronomical Union, the astronomy world’s official record-keeper, recognizes just five dwarf planets: Ceres, Eris, Haumea, Makemake and Pluto. But astronomers keep finding new dwarf planet candidates every year.
What is a dwarf planet? According to the IAU, a dwarf planet is a world orbiting the sun that’s big enough for its gravity to make round, but may “orbit in a zone that has many other objects in it.” For example, while planets like Earth are occasionally hit by space rocks, they’ve largely swept the debris out of their orbits. In contrast, a dwarf planet like Ceres orbits in the asteroid belt.
