The roster of “dwarf planets” keeps growing. But it’s not official – there’s no league office to tell us who’s on the roster and who’s not. Various groups keep their own lists, but they don’t agree on which objects belong.

The dwarf-planet category was formalized a couple of decades ago. Astronomers had discovered some new Pluto-like objects beyond the orbit of Neptune. They had to decide whether to add those objects to the roster of planets, or to put them in a new category. So in 2006, the International Astronomical Union voted to create the “dwarf planet” designation.

A dwarf planet was defined as a body that’s large enough for its gravity to pull it into a rounded shape, but not large enough to clear its orbit of other bodies. The initial list included Pluto and three other distant objects, plus Ceres, the largest asteroid.

Since then, astronomers have discovered thousands more objects in the realm of Pluto and beyond. Most of them are fairly small. But some are larger. Because they’re so far away, though, it can be tough to figure out an exact size and mass. So that makes it harder to decide whether some of these bodies are dwarf planets, or just big comets or asteroids.

Today, most planetary scientists agree on a core list of about 10 dwarf planets. Another dozen or so are considered good candidates. And many more are possibilities – including a recently discovered one that we’ll talk about tomorrow.

Script by Damond Benningfield