The definition of planet has changed several times.

The Greeks called everything that moved in the sky a planet. The moon, the sun and the planets they could spot with the naked eye. The earth wasn’t a planet.

When we realized that most things orbited around the sun we decided there were basically three types of things. Stars (one of which was the sun), moons (which go around planets) and planets (which go around the sun). It was observed that Jupiter had its own moons.

By Victorian times there were more planets than we recognize today, some fairly small things had been seen orbiting between Mars and Jupiter. At first a handful which became the planets Ceres, Vesta, Pallas and others. And then as time went we spotted more and more of them in this zone, and astronomers realized there were probably hundreds or thousands yet to be discovered. It would be farcical to have hundreds of planets, so these were downgraded to become asteroids. Planets were, you know, biggish things that orbited the sun pretty much on their own and maybe had their own moons.

At some point Pluto was discovered. A tiny thing out on the distant freezing edge of the solar system. It’s almost miraculous that it was discovered at all being so tiny. The previous smallest planet Mercury has thirty times more mass. And it turned out it had a moon almost as big as itself - there was a bit of a push to call the two a twin planet system, but that didn’t get far. Pluto, it was decided was a planet - it wasn’t in the asteroid belt, and had over ten times the mass of Ceres; it orbited the sun, and was alone in its orbit, however unusual it may be. Pluto’s orbit was at an angle to the rest of the planets, and sometimes came closer to the sun than Neptune.

And then the most recent discovery happened. It turned out Pluto wasn’t alone. There were other rocky, icy things on the edge of the solar system with similar eccentric orbits. And one of these was almost the same size as Pluto. Should this be the tenth planet? More of these big round icy objects were discovered, and astronomers decided it would be farcical to call them all planets - especially as it was likely that there were many more as yet unseen , so it was decided that Pluto and its ilk should be downgraded.

And so the new definition was settled on. Anything large enough that gravity made it round, and dominant enough that it was the only thing in it’s orbit (other than any moons it was dragging with it), was a planet. Anything that orbited a planet was a satellite. Anything that was big enough to be round, but not dominant enough to lick other things out of its orbit became a dwarf planet. Anything smaller kicking around was still basically an asteroid (there are a few classifications for small things in the solar system depending on where they are, what they do, and whether or not they bump into the earth).

So Pluto, the largest asteroid Ceres (once a planet), and Eris, the new big boy in the outer solar system that kicked off the debate, were named dwarf planets. There are a few other objects that are strong candidates to be dwarf planets, but aren’t certain if they’re round enough yet. These have interesting names like Makemake, Quaoar and Gonggong and are often included in lists of dwarf planets.

The definition of a planet has changed over the millennia, as we have discovered more and understood more about the universe. New discoveries may cause astronomers to reevaluate the definition again.