A dwarf planet is a body orbiting the Sun, massive enough to be spherical but which has not done gravitational housekeeping on its orbit. Pluto, Ceres, Eris, Haumea and Makemake are the five officially recognized.
In August 2006, at the IAU General Assembly in Prague, astronomers had to settle a problem that had been festering for fifteen years. Since the discovery of 1992 QB1 in 1992 and then Eris in 2005 (then nicknamed Xena, slightly more massive than Pluto), it had become untenable to keep Pluto in the 'planet' category without also including Eris, Haumea, Makemake and potentially dozens of other bodies. But broadening the category that far made 'planet' scientifically uninformative.
Resolution 5A, adopted on August 24, 2006, created a three-criterion definition for 'planet': (1) orbits the Sun, (2) has enough mass for hydrostatic equilibrium (approximately spherical), (3) has cleared its orbital neighborhood. A 'dwarf planet' meets the first two but not the third. A 'small Solar System body' (asteroid, comet, small TNO) meets only the first.
The key discriminator is therefore 'ability to clear the neighborhood'. This criterion, proposed by astronomer Alan Stern (ironically, also PI of the New Horizons mission that made Pluto famous), measures a body's gravitational dominance over its environment. Stern himself introduced the quantitative indicator ΛM²/T (M = mass, T = period), which cleanly separates the eight planets (Λ > 1) from the dwarf planets (Λ < 1).
Why was this criterion adopted? Because it reflects a real physical difference. The eight planets each swept their orbital zone during the first 100-500 million years of the Solar System, absorbing or ejecting neighboring planetesimals. Dwarf planets could not, either because they formed in a densely populated zone (Kuiper Belt), or because they are too small, or because they share a resonance with a more massive planet (Pluto with Neptune).
The IAU officially recognizes five dwarf planets (April 2026):
• Ceres. Diameter 940 km. The only dwarf planet in the asteroid belt, at 2.77 AU from the Sun. Period 4.6 years. Discovered on January 1, 1801 by Giuseppe Piazzi in Palermo. Flown by the Dawn mission (NASA, 2015-2018), which revealed salt deposits (sodium chloride) from a subsurface brine.
• Pluto. Diameter 2,377 km. Mean distance 39.5 AU. Period 248 years. Orbit inclined 17° from the ecliptic, highly eccentric (up to 49 AU at aphelion). Discovered on February 18, 1930 by Clyde Tombaugh. Plutonian system with 5 satellites (Charon, Styx, Nix, Kerberos, Hydra). Explored by New Horizons on July 14, 2015.
• Haumea. Equatorial diameter ~1,960 km (but very ellipsoidal: 2,320 × 1,700 × 1,000 km due to its 3.9 h rotation). Distance 43 AU. Two moons (Hi'iaka, Namaka) and a ring discovered in 2017 — the first ring found around a TNO.
• Makemake. Diameter ~1,430 km. Distance 46 AU. One moon (MK2) discovered in 2016.
• Eris. Diameter 2,326 km (slightly smaller than Pluto but 27% more massive). Mean distance 67.8 AU, highly eccentric orbit (38 to 97 AU), in the scattered disc. One moon (Dysnomia). Discovered in 2005 by Mike Brown, Chad Trujillo, David Rabinowitz.
Several other candidates are considered dwarf planets by most astronomers but not formally recognized: Gonggong (~1,230 km, 67 AU average), Quaoar (~1,080 km, 44 AU), Orcus (~910 km, 39 AU), Sedna (~1,000 km, orbit between 76 and 937 AU). Estimates suggest 100-200 dwarf planets may exist in the Kuiper Belt and scattered disc, most still undiscovered.
Dwarf planets distribute across three distinct dynamical zones.
Main-belt dwarf planet. Ceres is the only one of its kind. It accounts for about 25% of the total asteroid belt mass (8.95 × 10²⁰ kg) and may host a subsurface ocean according to Dawn data. Its surface is dark (albedo 9%) with spectacular bright spots in Occator Crater — carbonate and salt deposits exposed by cryovolcanism.
Plutoids. Dwarf planets beyond Neptune's orbit. Pluto, Eris, Haumea, Makemake belong here, as do the candidates Gonggong, Quaoar, Orcus, Sedna. The IAU formalized the sub-term in 2008. Most host thin methane or nitrogen atmospheres (varying with distance from the Sun and geology), varied surface ices, and moons.
Pluto is the archetype: New Horizons revealed an active world, with nitrogen glaciers in Sputnik Planitia, mountain ranges of water ice (Tenzing Montes up to 6 km), a thin blue atmosphere, and seasonal atmosphere-surface exchange.
Dwarf planets with rings. Haumea and Quaoar have confirmed rings (2017 and 2023), proof that ring formation is not reserved for gas giants.
All these objects display extraordinary diversity in densities (1.1 to 2.5 g/cm³), albedos (5 to 85%) and surface compositions — proof that they have had individual geological histories despite modest sizes.
Dwarf planets are challenging but not impossible telescope targets, and space probes have revealed their diversity.
From Earth. Ceres reaches magnitude 6.6 at its brightest — just below naked-eye limit. With 10×50 binoculars, you can track it month after month through the belt. Pluto is tougher: peak magnitude 13.7, requires at least a 200 mm telescope. Eris tops out at magnitude 18.7, out of amateur reach. The other dwarf planets are fainter still.
Space missions. Two missions revolutionized our understanding: Dawn (NASA) orbited Ceres between 2015 and 2018, mapping its surface in high resolution and identifying its potential subsurface ocean. New Horizons (NASA) flew by Pluto on July 14, 2015, transforming a fuzzy dot into a detailed geological world — nitrogen glaciers, layered atmosphere, impact craters and terrains of varied age. No mission has yet visited the other dwarf planets.
Stellar occultations. Particularly powerful for TNOs: when a dwarf planet passes in front of a star, it briefly interrupts the light, allowing precise size measurements and detection of atmospheres or rings. Occultation is how Haumea's ring (January 2017) and Quaoar's ring (2021-2023) were discovered.
James Webb Space Telescope. Since 2022, JWST has been mapping the surface composition of several dwarf planets in infrared, revealing methane and ethane ices, and even traces of potential cryovolcanic activity on Eris.
To visualize the orbits and current positions of dwarf planets in the Solar System, use our 3D Solar System map.
Several categories of bodies are close but distinct.
Planet. The eight classical planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune) meet all three IAU criteria, including orbital clearing. Dwarf planets meet the first two but not the third. There is no continuous gradient: the mass gap between Mercury (smallest planet) and Eris (most massive dwarf planet) is about a factor of 60, and the Λ ratio sharply separates the two categories.
Asteroid / small Solar System body. Rocky main-belt asteroids are not in hydrostatic equilibrium (not massive enough), except for Ceres. Vesta (525 km) is a borderline case: ellipsoidal and only roughly spherical, it is the largest body the IAU does not recognize as a dwarf planet. Pallas and Hygiea are in the same gray zone. The sphericity threshold lies around 400-500 km for rocky bodies, lower (300-400 km) for less dense icy bodies.
Moon / natural satellite. Several large moons (Titan, Ganymede, Callisto, Io, the Moon, Europa, Triton) are larger than Pluto or Ceres. But since they do not orbit the Sun directly, they are not eligible for dwarf-planet status. The IAU regularly discusses a more geophysical classification in which 'dwarf planet' would include large moons — but the current definition remains orbit-based.
Interstellar object. Neither 1I/'Oumuamua nor 2I/Borisov is a dwarf planet — they do not orbit the Sun. They are interstellar visitors on hyperbolic trajectories.
No, according to the official IAU definition adopted on August 24, 2006. Pluto is classified as a 'dwarf planet'. It is spherical and orbits the Sun (criteria 1 and 2), but it has not cleared its orbital neighborhood: it shares its zone with thousands of other Kuiper Belt objects and sits in 2:3 resonance with Neptune. This is a scientific distinction, not a demotion: Pluto remains a fascinating world, as New Horizons showed in 2015. Some astronomers, including Alan Stern, still contest this decision — the debate isn't fully closed, but the IAU definition holds authority.
Five are officially recognized by the IAU (April 2026): Ceres, Pluto, Haumea, Makemake and Eris. Four more are considered dwarf planets by most astronomers but not formally validated: Gonggong, Quaoar, Orcus and Sedna. Models suggest the Kuiper Belt and scattered disc could contain 100 to 200 dwarf planets in total, most still to be discovered. The Vera C. Rubin Observatory (first light 2025) should identify dozens in the coming years and settle this census.
Eris and Pluto vie for first place, and it's very close. Eris measures 2,326 ± 12 km in diameter but is 27% more massive (1.66 × 10²² kg versus 1.30 × 10²² kg for Pluto), making it the most massive dwarf planet. Pluto is slightly larger in diameter (2,377 ± 3 km measured by New Horizons in 2015), so technically the most voluminous. Upshot: Pluto is larger in volume, Eris is heavier in mass. The two have been faithful rivals since 2005.
Because it meets the first two IAU criteria: it orbits the Sun and is in hydrostatic equilibrium (spherical, 940 km across, well above the sphericity threshold for a rocky body). But it has not cleared its orbital zone: it shares the asteroid belt with more than a million other bodies. So it is both an asteroid (numbered 1 Ceres in the minor-planet catalogue) AND a dwarf planet — both statuses coexist. It is, in a sense, the 'Pluto of the asteroid belt'.