Remember back in the 19th Century when the only options for moving objects in the heavens were "moons," "comets," "shooting stars," and "planets"?

Something interesting happened: Ceres was discovered in 1801. Great, there was another planet in the solar system. And there was no problem calling Ceres a planet. Heck, Neptune hadn’t been found yet. Nothing’s wrong with having 8 planets and then Pallas was found in 1802. And then Juno and Vesta in 1807.

In 1802, William Herschel (who found Uranus) suggested that these new objects shouldn’t be called planets because, unlike every other planet, the most powerful telescopes of the time couldn’t resolve their disks. Since they were little points of light but not stationary like stars, he suggested the word, “asteroid” (star-like) for these new objects, and the world ignored the suggestion for decades. They were mostly called planets.

Then in 1845, a rush of new aster-…planets were found. By 1868, there were over 100 planets discovered in what we now call the “asteroid belt,” and 1,000 were found by 1921.

This crowded belt clearly wasn’t filled with anything like Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, or Neptune. Herschel’s term “asteroid” was revived and applied to those objects, stripping them of their planet status.

The asteroids just were not like other planets, so they weren’t called planets anymore.

In 1930, Pluto was found. After Charon was found (1970), astronomers realized Pluto was TINY, smaller than Earth’s moon. (Today, Pluto is only the 17th largest object in the solar system.) It was alone on the fringes of the solar system. It had a moon (now known to have 5). It was big enough to be round. So why not call Pluto a planet?

But then the Kuiper Belt started gaining other occupants in the 1990s-2010s. It gained objects as massive as Pluto, like Eris, Haumea, and Makemake. As of 2018, there are 528 numbered and 2000 unnumbered trans-Neptunian objects filling Pluto’s region of space.

So, no, Pluto no longer rates the “planet” title. It’s just one of the thousands of objects in a belt of similar and smaller objects.