A dwarf planet is a celestial body that orbits the sun, has enough mass for gravity to pull it into a roughly round shape, but has not cleared the neighborhood around its orbit. That last part is what separates dwarf planets from the eight full planets. Our solar system currently has five officially recognized dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris.
What Makes a Dwarf Planet Different From a Planet
The International Astronomical Union (IAU) established the dwarf planet category in 2006, and the definition comes down to one key distinction: clearing the orbit. A full planet has enough gravitational dominance to sweep up, scatter, or absorb smaller objects in its orbital path over time. Earth, Jupiter, and the other planets have done this. Dwarf planets have not. They share their orbital neighborhoods with lots of other debris.
Pluto, for example, orbits within the Kuiper Belt, a vast region of icy objects beyond Neptune. It’s one of thousands of bodies in that zone, and it hasn’t gravitationally dominated the area the way Neptune dominates its own orbit. The same is true for Eris, Haumea, and Makemake, which also live in or beyond the Kuiper Belt. Ceres sits in the asteroid belt between Mars and Jupiter, surrounded by millions of smaller rocks it hasn’t cleared away.
Size alone doesn’t determine the classification. A dwarf planet can be fairly large, but if it hasn’t cleared its orbit, it stays in the dwarf category. And a dwarf planet is not a moon. It orbits the sun directly, not another planet.
The Five Recognized Dwarf Planets
Ceres
Ceres is the closest dwarf planet to the sun and the only one in the inner solar system. It sits in the asteroid belt between Mars and Jupiter, and it’s also the largest object in that belt. Italian astronomer Giuseppe Piazzi first spotted it in 1801, making it the first asteroid belt object ever discovered. At roughly 940 kilometers across, Ceres is far smaller than the other dwarf planets, but its round shape and direct solar orbit qualify it for the category. NASA’s Dawn spacecraft orbited Ceres from 2015 to 2018, revealing bright salt deposits on its surface and evidence of a subsurface ocean.
Pluto
Pluto is the most famous dwarf planet, largely because it spent 76 years classified as the solar system’s ninth planet before the IAU reclassified it in 2006. It orbits the sun at an average distance of 39.5 AU (about 5.9 billion kilometers) and takes nearly 248 Earth years to complete one trip around the sun. Pluto is roughly 2,370 kilometers in diameter and has five known moons. The largest, Charon, is so big relative to Pluto that the two bodies orbit a shared center of gravity between them, leading some astronomers to describe them as a double system. NASA’s New Horizons flyby in 2015 revealed a surprisingly complex world with nitrogen glaciers, mountain ranges made of water ice, and a thin atmosphere.
Haumea
Haumea is one of the most unusual objects in the solar system. It spins so fast, completing a full rotation every four hours, that its gravity can’t pull it into a sphere. Instead, it’s stretched into an elongated shape, roughly the proportions of a football. Discovered in 2003, Haumea orbits at an average distance of about 43 AU and takes nearly 284 Earth years to go around the sun. Scientists believe a massive collision billions of years ago set off its rapid spin and created its two small moons. Haumea also has a ring, making it one of the few known objects beyond the giant planets to possess one.
Makemake
Makemake, discovered in 2005, is the second-brightest object in the Kuiper Belt after Pluto. It orbits at about 45.5 AU from the sun, with an orbital period of just over 306 Earth years. At roughly 1,500 kilometers in diameter, it’s about two-thirds the size of Pluto. Its surface has a reddish tint, likely from organic molecules created by solar radiation interacting with surface ices. Makemake has one known moon, discovered in 2015.
Eris
Eris is the most massive known dwarf planet, about 1.27 times the mass of Pluto, and it’s the reason the dwarf planet category exists. When astronomers discovered Eris in 2003 and realized it was more massive than Pluto, it forced the question: is Eris a tenth planet, or is Pluto not really a planet? The IAU chose the second option. Eris orbits far from the sun, averaging nearly 68 AU, and takes about 559 Earth years to complete one orbit. Its surface is coated with methane ice, and it has one known moon called Dysnomia. Eris and Pluto have strikingly similar densities, suggesting they’re made of similar mixtures of rock and ice.
Dwarf Planet Candidates
The IAU has only officially recognized five dwarf planets, but dozens of other objects likely qualify. Sedna, Gonggong, and Quaoar are three of the strongest candidates. All three are large enough to be spherical, and observations from the James Webb Space Telescope have revealed ices and organic compounds on their surfaces, hinting at complex geology. They’re somewhat smaller than the methane-rich worlds like Pluto and Eris but still big enough that their own gravity has pulled them into round shapes.
Other candidates include Orcus, Salacia, and 2002 MS4. The reason these haven’t been officially designated is largely practical: confirming a dwarf planet requires enough observational data to verify its shape and orbit, and the IAU has been slow to act on new classifications. Some astronomers estimate the Kuiper Belt and more distant regions could contain over a hundred dwarf planets, most of them too far away and too dim to study in detail with current technology.
How Big Are Dwarf Planets
Dwarf planets span a wide range of sizes. Eris and Pluto are the largest, both around 2,300 to 2,370 kilometers in diameter, roughly two-thirds the width of Earth’s moon. Makemake comes in around 1,500 kilometers. Haumea’s elongated shape makes a single diameter measurement misleading: it stretches about 980 kilometers along its longest axis but only 500 kilometers along its shortest. Ceres, the smallest of the five, is about 940 kilometers across.
For comparison, Earth’s moon is 3,474 kilometers in diameter, so even the largest dwarf planets are noticeably smaller. Mercury, the smallest full planet, is about 4,880 kilometers across, more than twice the diameter of Pluto.
Where Dwarf Planets Orbit
Four of the five recognized dwarf planets orbit in or beyond the Kuiper Belt, a doughnut-shaped region of icy bodies that extends from roughly Neptune’s orbit (30 AU) out to about 50 AU, with scattered objects reaching much farther. Eris, at nearly 68 AU on average, spends most of its orbit well beyond the main Kuiper Belt in what’s called the scattered disk.
Ceres is the outlier. It orbits at about 2.8 AU from the sun, nestled in the asteroid belt. This makes it far more accessible to spacecraft and gives it a much shorter year: just 4.6 Earth years compared to Pluto’s 248 or Eris’s 559. The enormous distances of the Kuiper Belt dwarf planets are a major reason we know so little about most of them. Even traveling at over 58,000 kilometers per hour, New Horizons took nine and a half years to reach Pluto.