What Is a Dwarf Planet? The IAU Definition Explained

The IAU definition, test by test

The IAU put the rule into Resolution 5A. In the resolution's own wording, a dwarf planet is a celestial body that is in orbit around the Sun, has enough mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape, has not cleared the neighborhood around its orbit, and is not a satellite. NASA's explainer lists three tests for a full planet: orbit the Sun, be nearly round, and clear the neighborhood. A dwarf planet passes the first two and fails the third. The fourth item in the resolution, that a dwarf planet is not a satellite, keeps moons off the list.

Here is what each test means in plain terms.

Orbit the Sun. The body must circle the Sun itself. Earth's Moon fails this test because it circles Earth.

Be nearly round. Gravity pulls everything toward a body's center. Once a body has enough mass, gravity overcomes the strength of its rock and ice and pulls it into a ball. Scientists call this shape hydrostatic equilibrium, which means the shape a body takes when it behaves like a fluid under its own pull. Small asteroids stay lumpy because their gravity is too weak to do this.

Have not cleared the neighborhood. NASA describes a planet as a body that sweeps up or scatters away its immediate neighbors, or holds them in stable orbits. The Planetary Society puts it another way: a planet is far more massive than anything else near its orbit, while a dwarf planet is not. The IAU did not give a number for this test, which is one reason astronomers still argue about it.

How a dwarf planet differs from a planet

The table sorts the categories that the 2006 resolution creates or relies on.

Size is not what separates a planet from a dwarf planet. Pluto is smaller than Mercury, and NASA notes it is also smaller than some moons of other planets. What matters is the third test. A footnote to the resolution names the eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Each one dominates its own orbit. Pluto does not. It moves through the Kuiper Belt, a doughnut-shaped region of icy bodies beyond Neptune, where many objects share its space. The full story of how that cost Pluto its planet label is in why Pluto is a dwarf planet.

The Planetary Society's explainer describes planets and dwarf planets as two distinct classes of objects. In everyday speech, many people hear "dwarf planet" as "small planet," and that confusion comes back in the myths section below.

How a dwarf planet differs from an asteroid

The 2006 resolution gave the leftover group a name: all other objects orbiting the Sun, except satellites, are Small Solar-System Bodies. That group includes the many asteroids and comets that are too small for gravity to shape into a ball.

Ceres is the case that blurs the line. It sits in the main asteroid belt between Mars and Jupiter, and NASA calls it the largest object in that belt. It was found in 1801, it was first counted as a planet, and it was then reclassified as the first of a new class of objects called asteroids. In 2006 it became a dwarf planet. NASA describes it as the only dwarf planet in the inner Solar System. So Ceres is both a member of the asteroid belt and a dwarf planet, and the Ceres guide covers what NASA's Dawn spacecraft found when it orbited there. For a plain profile of the other group, see what asteroids are and where they live.

Why the rule exists

The IAU adopted its written definition of "planet" in 2006, and two developments pushed it to act. Starting in the early 1990s, astronomers found many icy bodies in the Kuiper Belt. Then, in 2005, a team announced an object, later named Eris, that was close to Pluto in size. NASA says that discovery prompted the IAU to address how planets are classified, and the Planetary Society adds that Eris's size was at first overestimated, which helped set off the debate. The details are in the Eris article.

If Pluto stayed a planet and Eris joined it, the planet count could keep rising with every new find. The IAU says there may be many more dwarf planets, perhaps more than a hundred, still waiting to be discovered. The fix was to define a planet by what it does to its neighborhood and to give Pluto-like bodies their own category. How many such worlds exist today is the subject of how many dwarf planets there are.

The rule has critics. NASA notes that some astronomers disagreed with the decision. The Planetary Society describes proposals that would drop the neighborhood test and define planets by their internal physics instead. The IAU's FAQ states that the IAU has not changed the definition of a planet in the Solar System since 2006, so the 2006 wording is still the official one.

Common mistakes and myths

• Myth: a dwarf planet is just a small planet. The word describes a different category, not a shrunken version. A dwarf planet fails the neighborhood test, and the Planetary Society calls planets and dwarf planets two distinct classes.
• Myth: Pluto is the only dwarf planet. NASA lists five: Ceres, Pluto, Haumea, Makemake and Eris, as of November 2026.
• Myth: every dwarf planet is a demoted planet. Only Ceres (in the 1800s) and Pluto (long counted as the ninth planet) held that title. Eris was widely called a possible tenth planet in 2005, but no official body gave it that status.
• Myth: all dwarf planets live in the Kuiper Belt. Ceres sits in the asteroid belt, and NASA says Eris travels on a path that extends well beyond the Kuiper Belt.
• Myth: a moon can be a dwarf planet. Charon is about half Pluto's size, but it orbits Pluto, and the Planetary Society describes it as technically a moon. The "not a satellite" rule excludes it.

Frequently asked questions

How many tests does a dwarf planet have to meet?

NASA lists three tests for a planet, and a dwarf planet passes the first two (orbit the Sun, be nearly round) but fails the third (clear the neighborhood). The official IAU wording has four parts, because it also says a dwarf planet cannot be a satellite.

Is Pluto the biggest dwarf planet?

NASA's 2015 New Horizons results say Pluto, at about 2,370 km (1,473 miles), is larger than all other known objects beyond Neptune's orbit. NASA's dwarf planet page describes Eris as nearly the same size as Pluto, so the two are close.

Who decides what counts as a dwarf planet?

The IAU. Its members voted on the definition at the 2006 General Assembly in Prague, and NASA follows the IAU list of five.

Has the definition changed since 2006?

No. The IAU's FAQ says the IAU has not changed the definition of a planet in the Solar System since 2006, although some scientists have proposed other definitions.