What Is a Dwarf Planet? The IAU Definition, Pluto’s Demotion, and the 5 Official Members

A dwarf planet is a celestial body that orbits the Sun, is massive enough for gravity to pull it into a roughly spherical shape, but has not gravitationally cleared the other debris from its orbital path around the Sun. The International Astronomical Union (IAU) established this three-part definition in August 2006, creating a new planetary category that reclassified Pluto and formally recognized four other objects. As of 2024, the IAU lists five official dwarf planets: Pluto, Eris, Ceres, Haumea, and Makemake.

That third criterion, “clearing the neighborhood,” is the one that changed everything. Planets like Earth and Jupiter have swept their orbital zones clean over billions of years, either absorbing nearby objects or flinging them out through gravity. Pluto shares its orbital zone in the Kuiper Belt with thousands of other icy bodies, which is precisely why it lost full planet status. The debate over whether the 2006 vote was scientifically sound has never fully quieted down.

The IAU 2006 Definition: Three Criteria That Must All Be Met

Before 2006, there was no formal scientific definition of a planet. The solar system had nine planets by convention, not by rule. That changed when the IAU, the body of professional astronomers that names and classifies objects in space, convened in Prague and passed Resolution B5 on August 24, 2006.

Under that resolution, a full planet must:

1. Orbit the Sun.
2. Have sufficient mass for self-gravity to overcome rigid body forces and assume a hydrostatic equilibrium shape (nearly round).
3. Have cleared the neighborhood around its orbit.

A dwarf planet satisfies the first two criteria but fails the third. The word “dwarf” is not diminutive here; it describes a structural reality about orbital dominance. Earth’s gravity so thoroughly dominates its orbital zone that every object in the vicinity either orbits Earth as a moon, got absorbed, or was ejected long ago. Pluto’s zone, by contrast, is crowded with Kuiper Belt Objects (KBOs), and Pluto simply does not have the gravitational reach to change that.

Hydrostatic equilibrium is the key phrase in criterion two. It means that gravity is strong enough to compress a body into a sphere (or something close to a sphere) rather than leaving it as a lumpy, irregular rock. This threshold typically requires a diameter of roughly 400 km for icy bodies, though the exact cutoff depends on composition and density. Asteroids and comets generally fall well below this threshold, which is why they are neither planets nor dwarf planets.

One more term worth knowing: plutoid. The IAU introduced this label in 2008 for dwarf planets that orbit beyond Neptune (trans-Neptunian objects). Pluto, Eris, Haumea, and Makemake are all plutoids. Ceres is not, because it orbits in the asteroid belt between Mars and Jupiter.

Why Pluto Was Reclassified in 2006

Pluto was discovered in 1930 by Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona. For 76 years it was the ninth planet. Its demotion in 2006 had less to do with Pluto changing and more to do with astronomers finding objects comparable to, or larger than, Pluto in the outer solar system.

The immediate catalyst was Eris, discovered in 2005 by a team led by Mike Brown at Caltech. Eris appeared slightly larger than Pluto at the time of discovery (later measurements from the New Horizons flyby in 2015 revised Pluto’s diameter upward to about 2,377 km; current estimates put Eris at approximately 2,326 km, making them nearly identical in size). If astronomers were going to count Pluto as a planet, they had to count Eris too, and there were dozens more Kuiper Belt objects of similar size waiting to be found. The alternative was to redefine what a planet actually is.

The Prague vote resolved this by adding the orbital-clearing criterion, which Pluto clearly cannot meet. Of the roughly 2,412 IAU members who participated in the 2006 General Assembly, 424 voted for the new definition. That is a relatively small fraction of the full IAU membership, a fact critics have pointed to ever since. Alan Stern, the principal investigator for NASA’s New Horizons mission to Pluto, was among the loudest objectors, arguing that the “clearing the neighborhood” criterion is poorly defined and would technically disqualify Earth or Jupiter if placed in Pluto’s orbital position.

The Five Officially Recognized Dwarf Planets

The IAU currently recognizes five dwarf planets. Several dozen additional candidates exist, but formal recognition requires a proposal, review, and vote by the IAU, a process that moves slowly.

Pluto remains the most studied of the five. NASA’s New Horizons spacecraft flew within 12,500 km of Pluto on July 14, 2015, sending back the first close-up imagery of its surface. Those images revealed mountain ranges made of water ice up to 3,500 meters high, a vast nitrogen ice plain informally called Tombaugh Regio, and a surprisingly complex atmosphere of nitrogen, methane, and carbon monoxide.

Eris is the most massive of the five, roughly 27% more massive than Pluto despite being slightly smaller in diameter. Its surface is one of the most reflective in the solar system, likely due to a thin layer of methane frost. Eris orbits the Sun at an average distance of about 68 astronomical units (AU), placing it far out in the scattered disc region beyond the Kuiper Belt, and its orbital period is approximately 557 Earth years.

Ceres has a different origin story from the other four. It was discovered in 1801 by Giuseppe Piazzi and was considered a planet for several decades before being reclassified as an asteroid when more objects were found in the same belt. The 2006 IAU vote promoted it back to dwarf planet status. NASA’s Dawn spacecraft orbited Ceres from 2015 to 2018, discovering bright spots in Occator Crater that turned out to be salt deposits from ancient brine water that had evaporated at the surface.

Haumea is the strangest of the five. It spins so fast, completing one rotation every 3.9 hours, that centrifugal force has stretched it into a football-like elongated shape. It also has a ring system, confirmed in 2017, making it the only known dwarf planet with rings. Haumea and its moons are thought to be fragments from an ancient collision.

Makemake was discovered in March 2005, just months before Eris, by the same Caltech team led by Mike Brown. Its surface is covered in frozen methane and ethane, giving it a reddish-brown color. A small, dark moon was confirmed in 2016 from Hubble Space Telescope observations.

How Many More Dwarf Planets Are There?

The short answer: many more than five, but the exact number is genuinely unknown. Mike Brown, the astronomer whose discovery of Eris triggered the 2006 reclassification, has argued publicly that there are likely hundreds of dwarf planets in the Kuiper Belt and the scattered disc beyond it. Some estimates from planetary scientists run as high as 10,000 objects large enough to qualify if we could image them clearly.

The problem is detection. Objects in the Kuiper Belt orbit the Sun at distances of 30 to 50 AU. At those distances, even a body with a diameter of 1,000 km reflects too little light for current ground-based telescopes to identify definitively. The Vera C. Rubin Observatory in Chile, which began its ten-year Legacy Survey of Space and Time (LSST) in 2025, is expected to dramatically expand the catalog of trans-Neptunian objects and may formally identify dozens of additional dwarf planet candidates.

Several unofficial candidates are frequently cited in the scientific literature. Sedna, discovered in 2003, has a diameter estimated between 995 and 1,060 km and an extreme orbit that takes it between 76 and 937 AU from the Sun over roughly 11,400 years. Quaoar, confirmed in 2022 to have its own ring system, is estimated at about 1,121 km. Gonggong, Orcus, and Salacia are other objects that many planetary scientists treat informally as dwarf planets despite lacking IAU official status.

The IAU’s formal recognition process requires a submitted proposal, committee review, and a vote at a General Assembly, which meets only every three years. This slow pace means the official list will almost certainly remain an undercount for years to come. Researchers studying the space science beat largely accept that the true population of dwarf planets dwarfs the official five.

The Ongoing Scientific Debate Over Planetary Classification

The 2006 vote settled the administrative question of Pluto’s status, but it did not end the scientific argument. The criticism has two main threads.

The first is practical: the “clearing the neighborhood” criterion is not precisely quantified. Planetary scientist Alan Stern and David Levison formalized a metric called the planetary discriminant (sometimes written as mu, or the Greek letter), which measures a body’s mass relative to the total mass of objects in its orbital zone. Under this metric, the eight recognized planets score far above any threshold, while Pluto and other dwarf planet candidates score far below. But critics argue this still does not make “clearing” a physically meaningful characteristic of a body itself; it is a property of the orbital environment, which depends on where a planet happens to form.

The second criticism concerns the vote itself. Only about 424 of approximately 10,000 IAU members voted on Resolution B5 in Prague. Many planetary scientists were not present or felt the process was rushed. A 2017 paper by Stern and several co-authors proposed a geophysical definition of a planet based purely on hydrostatic equilibrium, which would bring Pluto, Ceres, and potentially dozens of other bodies back under the planetary umbrella. That proposal has not been adopted by the IAU.

For practical purposes, the five IAU-recognized dwarf planets remain the working list. But anyone who tells you the classification question is fully settled is glossing over a real, ongoing disagreement among working astronomers. You can read more about ongoing discoveries in our Space coverage and related planetary science stories in our Environment section, where we cover how astronomical research connects to Earth-based science.

Frequently Asked Questions About Dwarf Planets

Is Pluto still a planet?

No, not under the current IAU definition. Pluto was reclassified as a dwarf planet on August 24, 2006, because it has not cleared the orbital neighborhood around it in the Kuiper Belt. It meets the first two IAU criteria (orbits the Sun, roughly round in shape) but fails the third. The classification remains official, though a minority of planetary scientists continue to contest the definition.

What is the difference between a dwarf planet and a moon?

A dwarf planet orbits the Sun directly. A moon orbits a planet or other body rather than the Sun as its primary gravitational anchor. Pluto’s largest moon, Charon, is about half the diameter of Pluto and is sometimes described as a double dwarf planet system, but Charon does not independently orbit the Sun, so the IAU does not classify it as a dwarf planet.

Why is Ceres considered a dwarf planet and not an asteroid?

Ceres is large enough that its own gravity has pulled it into a spherical shape, satisfying the IAU’s hydrostatic equilibrium criterion. Most asteroids in the same belt are irregularly shaped because they lack sufficient mass for gravity to dominate. Ceres accounts for roughly 35% of the total mass of the entire asteroid belt, setting it apart from the millions of smaller rocky bodies around it.

How many dwarf planets could there be in the solar system?

Current estimates from planetary scientists range from a few hundred to potentially thousands, the vast majority of them undetected in the outer Kuiper Belt and scattered disc beyond 50 AU. The Vera C. Rubin Observatory’s ongoing sky survey is expected to identify many new candidates over its ten-year run, starting in 2025.

What does “clearing the neighborhood” actually mean?

It means a planet has gravitationally dominated its orbital zone over the history of the solar system, either absorbing other objects, capturing them as moons, or ejecting them. A body that has cleared its neighborhood has a high planetary discriminant, a ratio of the body’s mass to the total mass of other objects sharing its orbit. Earth, Jupiter, and the other seven planets score orders of magnitude higher than Pluto on this measure.

Could a dwarf planet ever become a full planet?

In principle, yes, if a dwarf planet somehow absorbed or ejected enough of the competing bodies in its orbital zone to achieve gravitational dominance. In practice, the objects sharing Pluto’s region of the Kuiper Belt number in the thousands, and Pluto lacks the mass to change that on any realistic timescale. Its reclassification reflects where it formed and how the outer solar system evolved, not any deficiency in Pluto itself.

The IAU’s 2006 definition of a dwarf planet requires three conditions: the object orbits the Sun, its mass creates sufficient self-gravity to assume a hydrostatic equilibrium (spherical) shape, and it has not cleared the other objects from its orbital neighborhood. Five bodies currently meet the first two criteria but fail the third: Pluto, Eris, Ceres, Haumea, and Makemake.