The Former Ninth Planet - Why Isn't Pluto Considered a Planet Anymore?

The clearest image we have of Pluto, the former ninth planet.

As of August 24, 2026, it will have been 20 years since Pluto was officially reclassified from planet to dwarf planet. While there are those who argue that Pluto didn’t deserve to be “demoted,” the history of Pluto and our perceptions of it paints a very different picture. So why isn’t Pluto considered a planet anymore?

Planet X

To understand why Pluto has gone through so much confusion with its classification, we have to rewind all the way back to when we first discovered it in 1930. Ever since the 1840s, astronomers had been confused by the orbit of Uranus. Its orbit defied standard gravitational predictions, implying there had to be an unknown massive object beyond its orbit with gravity enough to distort Uranus’ path. Eventually, Neptune was discovered, but even Neptune wasn’t enough to fully explain the perturbations in Uranus’ orbit. This led to theories of a Planet X that must have existed even further from the Sun than Neptune.

After years of searching, in 1930, it was announced that Planet X had been discovered. The planet was quickly named Pluto. At the time of its discovery, Pluto was estimated to have a mass seven times that of Earth, but soon after its discovery, that was reduced to about the same mass as Earth. This was already a big red flag, as it wouldn’t explain the orbit of Uranus as expected. By 2006, we had refined our calculations of Pluto’s mass all the way down to 0.218% the mass of Earth.

A size comparison of Earth, the Moon, and Pluto. Pluto is significantly smaller than even the Moon.

Controversies

By the early 2000s, it was generally agreed that Pluto could not be the predicted Planet X. In fact, in the 1990s, closer observation of Neptune revealed that the planet had a slightly smaller mass than we thought, which actually completely eliminated the need for a Planet X to explain Uranus’ orbit. This added fuel to the question that was already on everyone’s mind. Is Pluto even a planet at all?

It’s not just that Pluto was smaller than expected that put its classification into question, it was the discovery of other, similar-sized objects. In the 1990s, just at the same time as Pluto’s status was really coming into question, other objects of similar size to Pluto were discovered in the same region of the Solar System, the Kuiper Belt. Having so many planets, all of them being so small, in the same region of space didn’t seem right. These new objects were classified as Kuiper Belt Objects, rather than planets. If Pluto had been discovered at the same time as them, it would have been considered a KBO as well, from the very beginning. But, as Pluto was discovered first and called a planet, it got grandfathered in and kept its status.

Debate grew stronger and stronger as the years went by on whether or not Pluto should be reclassified. The issue became completely unavoidable in 2005 with the discovery of Eris, an object more massive than Pluto. No longer was Pluto able to coast on its legacy. If Pluto was to be considered a planet, then Eris would have to be as well. But if Eris was going to be considered a KBO, then the less massive Pluto being a planet would make no sense. Something had to give.

The Definition of a Planet

Part of the problem up until this point was that we didn’t have a proper definition for what a planet was. For most of human history, planets were just the lights we saw in the sky that weren’t stars, so no technical definition was needed. The only way to solve the Pluto controversy was to come up with a definition once and for all. So, astronomers from around the world debated on what that definition would be. In August of 2006, the International Astronomical Union voted to finalize a definition. The definition they agreed on had three criteria. For an object to be considered a planet, it had to meet all three.

The first criterion states that a planet must orbit a star. This one is simple enough. The Sun is a star and Pluto does indeed orbit the Sun, so it passes that.

The second criterion says that a planet must have achieved hydrostatic equilibrium and taken on a round or nearly round shape. This is essentially a minimum mass requirement for a planet. Over time, gravity likes to make things round. Since gravity acts from the center of an object and gets weaker the further from the center you get, being round means that the entire surface of the object is being acted upon by gravity equally. Essentially, everything is balanced. This is why larger objects in space, like planets and stars, are always round, while smaller objects like asteroids tend to have lumpy and uneven shapes. Pluto is round, so it does meet the second criterion.

The third and final criterion states that a planet must clear its orbital path. Essentially, this means that a planet has to clean up after itself. In the early Solar System, there was a lot of debris floating about. And, for example, much of it ended up in the path of the Earth’s orbit. But, as the Earth orbited the Sun, Earth’s gravity would redirect the path of the debris and fling it far away, never to return. Over time, the Earth has completely cleared its path of nearly all debris. Meanwhile, with Pluto, it sits in the Kuiper Belt, which is an extremely messy part of the Solar System. It just doesn’t have enough gravity to clean up its area, therefore it is not considered to be the gravitationally dominant object in its orbit, and it fails to meet the criterion.

As Pluto does not meet all three criteria, it is instead considered to be a dwarf planet. There is nothing personal about this classification. Pluto wasn’t so much demoted as it was given a more fitting title. If we wanted to be scientifically consistent while also keeping Pluto classified as a planet, then the other dwarf planets would have to be classified the same. With what we’ve found so far, that would shoot the total number of planets in our Solar System up to 17. But astronomers believe hundreds, if not thousands, of additional dwarf planets are waiting to be found beyond the orbit of Pluto, and surely not all of them should be considered planets.