Why Pluto Is a Dwarf Planet

What happened in Prague

In 2006 the IAU held its General Assembly in Prague, and the IAU says more than 2,500 astronomers took part overall. The planet question was put to a vote on August 24. Resolution 5A, the definition of "planet" and "dwarf planet," passed with what the IAU called a great majority. Resolution 6A then applied that definition to Pluto. A proposal for a separate "classical planet" category received 91 votes in favor, with many more against. A proposal to name Pluto-like bodies "Plutonian objects" drew 183 votes for and 186 against.

Only a fraction of the astronomers at the assembly were still in the hall for the last day. The Planetary Society says roughly 500 members were present. The three counts for Resolution 6A add up to 411 ballots.

• 1930: Clyde Tombaugh discovers Pluto, and it becomes known as the ninth planet.
• Early 1990s: astronomers begin finding icy bodies beyond Neptune, in what is now called the Kuiper Belt.
• 2005: a team announces an object similar in size to Pluto, later named Eris.
• January 19, 2006: NASA's New Horizons spacecraft launches toward Pluto.
• August 24, 2006: the IAU votes on the definition of a planet and on Pluto.

The clearing-the-neighborhood test

Resolution 5A says a planet must orbit the Sun, have enough mass for its own gravity to pull it into a nearly round shape, and have cleared the neighborhood around its orbit. A dwarf planet meets the first two conditions but has not cleared its neighborhood, and it is not a moon. NASA describes clearing as sweeping up or scattering away nearby objects, or holding them in stable orbits. The Planetary Society puts it simply: a planet is much more massive than anything else near its orbit.

Pluto's situation is the opposite. It orbits at an average of about 39 AU from the Sun, and its path ranges from 30 AU to 49.3 AU. An AU, or astronomical unit, is the average distance from Earth to the Sun, about 150 million km (93 million miles). That puts Pluto in the Kuiper Belt, a doughnut-shaped region of icy bodies beyond Neptune, where many objects of comparable nature share the space. Pluto does not dominate its zone. It is one large member of a crowd. For more on that region, see the Kuiper Belt and where Pluto lives.

The IAU did not supply a formula for the third test. The Planetary Society points out that the idea depends on distance, so a planet such as Mercury might fail it if it sat far from the Sun. That criticism is fair, and it explains why the debate has not ended.

Why Eris helped force the vote

The 2005 announcement of Eris raised the stakes. It was a Kuiper Belt object close to Pluto in size, and NASA says its discovery prompted the IAU to settle how planets are classified. If Pluto stayed a planet, Eris would have a strong claim to be the tenth, and every similar find would raise the count again. The story of that discovery is in the Eris article, and the formal rules are explained in what a dwarf planet is.

What the new label changed, and what it did not

The IAU's wording is plain. Resolution 6A says Pluto is a dwarf planet by the definition and is recognized as the prototype of a new category of trans-Neptunian objects, which means objects that orbit beyond Neptune. The IAU's FAQ states that Pluto does not meet the criterion for a planet and is instead a dwarf planet.

Everything else about Pluto stayed the same. It still takes about 248 years to circle the Sun. It still has five moons, named Charon, Styx, Nix, Kerberos and Hydra. It still has a thin atmosphere that, according to NASA, expands when Pluto is near the Sun and can freeze and fall as snow when it is far away. The name did not change either. NASA says an 11-year-old girl from Oxford, England, Venetia Burney, suggested "Pluto" after the Roman god of the underworld.

The vote also did not stop exploration. New Horizons launched on January 19, 2006, about seven months before the vote, when Pluto was still counted as the ninth planet. It passed Pluto on July 14, 2015, and the results are in what New Horizons found at Pluto.

Why some scientists still disagree

NASA notes that some astronomers disagreed with the decision, saying the criteria seemed designed to limit the number of planets or were unclear. The Planetary Society lists several criticisms. The phrase "cleared the neighborhood" is not defined in the resolution. The definition applies only to the Solar System and not to exoplanets, the planets that circle other suns. And planetary scientist Mark Sykes argued shortly after the vote that it ignored the intrinsic nature of planets.

Alternatives exist. In 2015, David Grinspoon suggested dropping the neighborhood test, so that a planet would be any gravitationally rounded object orbiting a star. In 2017, Kirby Runyon and coauthors proposed that a planet is any round body of less than stellar mass that has never undergone nuclear fusion, wherever it orbits. The Planetary Society's page notes that proposals of this physical kind have not been formally peer reviewed. The IAU has not adopted either one, and its FAQ says it has not changed the definition since 2006. Meanwhile, the debate has not stopped astronomers from counting worlds that fit the current rule, a tally covered in how many dwarf planets there are.

Common mistakes and myths

• Myth: Pluto lost its status because it is too small. The deciding test was the neighborhood around its orbit. NASA does point out that Pluto is smaller than Mercury and some moons, but size is not one of the IAU's three tests.
• Myth: all astronomers voted. Over 2,500 attended the assembly, but the tallies show far fewer ballots on the final resolutions, about 411 on Resolution 6A.
• Myth: Pluto is now an asteroid. The resolution calls it a dwarf planet and the prototype of a new category of trans-Neptunian objects.
• Myth: the vote changed Pluto. A label is a human decision. Pluto's orbit, size and moons are the same as before.
• Myth: Pluto will be reinstated soon. As of November 2026, the IAU's FAQ still says the definition is unchanged since 2006. Other proposals exist, but they are only proposals.

Frequently asked questions

When did Pluto become a dwarf planet?

On August 24, 2006, when IAU members in Prague passed Resolution 6A, which says Pluto is a dwarf planet.

Was the vote unanimous?

No. Resolution 6A passed 237 to 157, with 17 abstentions. Other proposals at the same assembly failed or drew close, split votes.

Do scientists still study Pluto?

Yes. NASA's New Horizons spacecraft made its flyby in 2015, nine years after the vote. NASA's mission page describes results from it, including a thin atmosphere and a possible ocean of water beneath the ice.

Could Pluto become a planet again?

Only if the IAU changes its definition, and its FAQ says that has not happened since 2006. Scientists have proposed other definitions, but none is official.