Topics Space and the universe

Why is Pluto not a planet?

Pluto is not a planet because in 2006 the International Astronomical Union adopted a formal definition of the word, and Pluto fails one of its three tests. A planet must orbit the Sun, be massive enough to pull itself into a round shape, and have cleared the neighborhood around its orbit. Pluto meets the first two. It does not meet the third, because it shares its region with many other icy bodies. It is now classed as a dwarf planet.

The story is more interesting than a simple demotion. Pluto was found in 1930 and treated as a planet for decades, until astronomers began finding other objects out beyond Neptune. One of them, Eris, turned out to be about the same size as Pluto and somewhat more massive. Either Eris joined the planets, and possibly many more after it, or the definition needed to change. Some planetary scientists still disagree with the 2006 choice, and that debate is part of the story.

An episode on this question would walk through the discovery, the Kuiper Belt, the vote, and what spacecraft have since shown about Pluto itself. bre's hosts are AI, so they can make mistakes, and you can press Talk to ask about any part you want to push on.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. How Pluto became the ninth planetPluto was found in 1930 and counted as a planet for decades. Later measurements showed it was far smaller than first thought, even smaller than our Moon.
  2. The Kuiper Belt changes the pictureFrom the 1990s, astronomers found many icy bodies beyond Neptune. Pluto began to look less like a lone planet and more like the largest of a crowd.
  3. Eris forces the questionEris, found in 2005, is about the same size as Pluto and roughly a quarter more massive. Astronomers had to decide whether it was a tenth planet or whether the word needed a definition.
  4. The 2006 vote and the three testsThe International Astronomical Union voted on a formal definition: orbit the Sun, be round, and clear the orbit. Pluto fails the last test.
  5. What clearing the orbit really meansIt means a body dominates its orbital zone, by absorbing or flinging away nearby objects. Pluto shares its zone with many others, and Neptune's gravity shapes its path.
  6. Why some scientists still disagreeCritics say the orbit test is vague and that a planet should be defined by its own properties. The debate is real and unsettled.
  7. What New Horizons foundThe 2015 flyby revealed mountains of water ice, glaciers and a surprisingly active world. Dwarf planet status says nothing about how interesting it is.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Tess. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    In 2006, members of the International Astronomical Union voted in Prague on what the word planet means, and Pluto didn't make the cut. So the real question is why a vote about a word felt like such a big deal.

  2. TessAI host

    Okay, but real talk. Most of us grew up with nine planets on a poster. Did Pluto change, or did somebody just move the goalposts?

  3. breAI host

    Pluto didn't change at all. What changed was what we knew. For the first time there was a formal definition, and by then we had found a lot of Pluto-like objects out past Neptune.

  4. TessAI host

    So it's a classification problem. If Pluto counts, then what about everything else out there that looks like it?

  5. breAI host

    Exactly. Eris is about Pluto's size and a bit more massive. Count Pluto and you probably have to count Eris, and then you're asking where the list stops.

  6. TessAI host

    And a list with no stopping point is useless to a person trying to learn this stuff. Fine. What are the tests?

  7. breAI host

    Three. Orbit the Sun, be massive enough to pull into a round shape, and clear the neighborhood around your orbit. Pluto passes the first two easily.

  8. TessAI host

    So it's the third one that sinks it. What does clearing a neighborhood even look like?

  9. breAI host

    Okay, here's the part nobody tells you. It's less about being alone and more about dominating, and that's exactly where scientists still argue.

Questions people also ask

When did Pluto stop being a planet?
In August 2006, members of the International Astronomical Union adopted a formal definition of a planet at a meeting in Prague. Under it, Pluto was reclassified as a dwarf planet. It had been counted as the ninth planet since its discovery in 1930.
What is a dwarf planet?
A dwarf planet orbits the Sun and is massive enough to be round, but it has not cleared its orbital neighborhood of other bodies. Pluto, Eris and Ceres, in the asteroid belt, are well-known examples. They are still worlds worth studying.
Is Pluto smaller than the Moon?
Yes. Pluto is smaller across than our Moon. It is the largest known object in the Kuiper Belt by diameter or close to it, but Eris is somewhat more massive, which was part of why the definition was revisited.
Do all scientists agree Pluto is not a planet?
No. Many astronomers accept the 2006 definition, but some planetary scientists argue that the orbit-clearing test is unclear and that planets should be defined by their own physical features. The classification is a human choice, and the disagreement continues.

Related topics

More: all 300 topics, space and the universe, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.