Most kids your age have grown up knowing Pluto isn't a planet. But your parents might still remember the day it stopped being one. For 76 years — from when it was first discovered in 1930 until 24 August 2006 — Pluto was officially the ninth planet of our solar system. Then it wasn't. Why?
In 2006, astronomers wrote a new official definition of a "planet." A space object now has to do three things: orbit the Sun, be round, and clear its lane of other rocks. Pluto fails the third rule. Its lane — the Kuiper Belt — is still full of icy debris. So Pluto became a dwarf planet instead.
The Story Behind the Decision
Pluto wasn't kicked out of the planet club. It moved into a new one. Here's why.
The Eris Discovery That Changed Everything
In 2005, an astronomer named Mike Brown spotted something out beyond Pluto that looked even bigger. He named it Eris, after the Greek goddess of strife. Eris turned out to be almost the same size as Pluto — and slightly heavier. Suddenly astronomers had a problem. If Pluto was the ninth planet, then Eris had to be the tenth. And what about all the other Pluto-sized icy worlds people were starting to find out beyond Neptune? Were there going to be 12 planets? 20? 50?
Something had to change. Either the solar system was about to get a lot more crowded, or the word "planet" needed a tidier definition.
The 2006 Prague Vote
On 24 August 2006, about 400 astronomers from around the world met in Prague at a gathering called the International Astronomical Union (the IAU for short). They voted on a brand new official definition of what a planet is. The new definition had three rules.
The Three Rules to Be a Planet
To be called a planet under the new IAU definition, a space object has to do all three of these things:
- 1. Orbit the Sun. The object has to go around the Sun, not around another planet. (A moon orbits a planet, not the Sun, so a moon can never be a planet.)
- 2. Be round. The object has to be big enough that its own gravity has squashed it into a ball shape. Small asteroids are lumpy. Planets are round.
- 3. Have cleared its orbit. The object has to be the boss of its lane around the Sun — either pushing other rocks out of the way or pulling them in as moons. There can't be lots of other rocks the same size still hanging around.
Earth passes all three. So do Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto passes the first two. The third one is where it fails.
Imagine a giant highway around the Sun. Earth has its own clean lane — it has scooped up or pushed away all the other rocks that used to share its path. Pluto's lane is more like a busy market street, full of millions of icy rocks all moving along together. Pluto is the biggest one in its lane, but it isn't in charge.
The Rule Pluto Fails — Clearing Its Orbit
Pluto lives in a region called the Kuiper Belt — a giant doughnut-shaped ring of frozen worlds, rocks and comets out beyond Neptune. The Kuiper Belt is huge and crowded. Astronomers have already found over 2,000 objects there, and they think millions more are waiting to be discovered.
Pluto is one of the biggest objects in the Kuiper Belt, but it shares its lane with many others. Its gravity isn't strong enough to push them out or pull them in. So under the IAU's third rule, Pluto fails. It hasn't "cleared its orbit." Read more about the Kuiper Belt.
So What Is Pluto Now? A Dwarf Planet
The 2006 vote didn't just demote Pluto. It created a whole new category for it — the dwarf planet. A dwarf planet is a space object that orbits the Sun and is round, but hasn't cleared its orbit. There are now five officially recognised dwarf planets:
- Pluto — in the Kuiper Belt
- Eris — even further out than Pluto, and slightly heavier
- Haumea — an egg-shaped dwarf with rings
- Makemake — named after a creator god from Easter Island
- Ceres — the only one closer in, in the asteroid belt between Mars and Jupiter
Astronomers think there could be hundreds more dwarf planets out there, still waiting to be found. Meet the other dwarf planets.
Not Everyone Agrees
Plenty of astronomers think the 2006 vote got it wrong — and they say so loudly.
Many scientists argue that round = planet should be enough. They point out that under the IAU's third rule, even Earth doesn't fully clear its orbit. Earth shares its lane with countless tiny asteroids and the Moon. If Earth gets a free pass for being big and round, they argue, Pluto should too.
The loudest voice is Alan Stern, the lead scientist of NASA's New Horizons mission, the spacecraft that flew past Pluto in 2015. He has never accepted the 2006 vote and still calls Pluto a planet.
Hear From Pluto Itself
"I didn't change. The rules did."
Whatever you decide to call it, the world hasn't moved.
Why You Should Still Care About Pluto
Whatever you call it, Pluto is one of the most fascinating worlds in our solar system. It has a heart-shaped plain of frozen nitrogen, mountains made of water-ice, ice volcanoes, a blue sky, and five moons. It also has the most exciting backstory of any object out there — the only world in the solar system to ever have had its planet status taken away.
And in many ways, Pluto's reclassification opened the door to a deeper understanding of the outer solar system. It made astronomers think more carefully about what the word "planet" really means. The New Horizons mission flew past Pluto in 2015 and showed us just how much there is to learn about it. Whatever the IAU says, Pluto is still a place worth visiting in your imagination.
Pluto isn't a planet, but it isn't a nothing either. Read about Pluto's surface, the five dwarf planets, or jump back to the main Pluto facts page.
Frequently Asked Questions
Why isn't Pluto a planet anymore?
Pluto isn't a planet because it doesn't meet all three of the rules that the International Astronomical Union (IAU) set in 2006. It orbits the Sun (rule 1, ✅) and is round (rule 2, ✅), but it hasn't cleared its orbit of other rocks (rule 3, ❌). Pluto shares its lane in space with millions of icy objects in the Kuiper Belt.
What is a dwarf planet?
A dwarf planet is a space object that orbits the Sun and is big enough to be round, but hasn't cleared the area around its orbit of other rocks. There are five officially recognised dwarf planets: Pluto, Eris, Haumea, Makemake and Ceres.
When did Pluto stop being a planet?
Pluto was reclassified as a dwarf planet on 24 August 2006, after astronomers at the International Astronomical Union (IAU) meeting in Prague voted on a new definition of what a planet is. The vote was triggered partly by the discovery of Eris, which is about the same size as Pluto.
Is Pluto bigger than Eris?
Pluto is slightly larger than Eris in diameter (2,377 km vs 2,326 km), but Eris is heavier. When astronomers discovered Eris in 2005, they thought it was bigger than Pluto — and that helped trigger the 2006 vote that reclassified Pluto.
Could Pluto become a planet again?
Yes, in theory. The IAU could vote on a new definition at any future meeting. Many astronomers, including New Horizons mission lead Alan Stern, still argue that Pluto should be called a planet. For now, though, Pluto is officially a dwarf planet.