Dwarf planets and Pluto

7 min read

Key facts

Officially recognized (IAU)	5: Ceres, Pluto, Eris, Haumea, Makemake
Definition adopted	August 2006, IAU General Assembly in Prague
Largest	Pluto, about 1,477 miles across (Eris is nearly as large)
Closest to the Sun	Ceres, in the asteroid belt, about 2.8 AU
Pluto's average distance	About 3.7 billion miles (39.5 AU)
Pluto's year	About 248 Earth years
Pluto's moons	5; the largest, Charon, is about half Pluto's width
Missions	New Horizons (Pluto flyby, 2015); Dawn (Ceres orbit, 2015–2018)

In short

In 2006 the IAU defined a planet as a body that orbits the Sun, is round and has cleared its orbital neighborhood.
A dwarf planet meets the first two conditions but not the third; Pluto shares its zone with many icy Kuiper Belt objects.
New Horizons flew past Pluto in July 2015 and found mountains of water ice and a vast nitrogen-ice plain.
Dawn orbited Ceres and found bright salt deposits left by briny water from below the surface.
Eris, Haumea and Makemake orbit beyond Neptune; many more candidates likely wait to be confirmed.

What is a dwarf planet?

For decades the word planet had no formal definition. That changed in August 2006, when the International Astronomical Union (IAU), the body that names objects in space, voted on three conditions. 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 of other sizable objects.

A dwarf planet meets the first two conditions but not the third. It is round and orbits the Sun, yet it shares its path with many other bodies. It also must not be a moon of another planet.

The new category grew out of discoveries in the outer Solar System. In 2005 astronomers found Eris, a distant world that seemed about as large as Pluto. Either it would become the tenth planet, or a clearer rule was needed.

Pluto: from planet to dwarf planet

Clyde Tombaugh discovered Pluto in 1930 at Lowell Observatory in Arizona, and for 76 years it was counted as the ninth planet. But Pluto is smaller than our Moon, about 1,477 miles across, and it lies in the Kuiper Belt, a wide ring of icy bodies beyond Neptune. It has not cleared that zone, so under the 2006 rule it became a dwarf planet.

The change was not a demotion of Pluto's interest. Many scientists still argue about the definition, and some prefer to call any round world a planet. What is not in doubt is that Pluto is a complex, active world.

New Horizons at Pluto

NASA's New Horizons spacecraft launched in January 2006 and flew past Pluto on July 14, 2015, after a journey of more than nine years. It was the first close look at the system, and it changed how scientists see small icy worlds.

The images showed a heart-shaped region whose western lobe, Sputnik Planitia, is a huge plain of frozen nitrogen with almost no craters, a sign that it is young and still slowly churning. Around it rise mountains of water ice several miles high. Pluto also has a thin, hazy blue atmosphere.

Its largest moon, Charon, is so big compared with Pluto that the two orbit a point in the space between them. After Pluto, New Horizons flew past the Kuiper Belt object Arrokoth in 2019 and, as of 2026, is still traveling outward.

Ceres: the dwarf planet next door

Ceres is the only dwarf planet in the inner Solar System. It lies in the asteroid belt between Mars and Jupiter and is about 590 miles across, the largest object in the belt. When Giuseppe Piazzi spotted it in 1801, it was first called a planet, then an asteroid, and since 2006 a dwarf planet.

NASA's Dawn mission orbited Ceres from 2015 to 2018. It found that the bright spots in Occator Crater are deposits of salts, mostly sodium carbonate, left behind when salty water seeped up and evaporated. Ceres may still hold pockets of brine deep below its crust, making it a place of interest in the search for water in the Solar System.

Eris, Haumea and Makemake

Eris, discovered in 2005, is almost exactly Pluto's size but a little more massive. Its stretched orbit takes it out to nearly 100 AU (an AU is the average Earth–Sun distance) and around the Sun about once every 557 years. It has one known moon, Dysnomia.

Haumea spins so quickly, once in about four hours, that it has stretched into an egg shape. It is the only dwarf planet known to have a ring, and it has two small moons. Makemake, a little smaller than Pluto, has a reddish surface coated with frozen methane and one known moon.

Astronomers suspect there are dozens or even hundreds more dwarf planets in the outer Solar System. Candidates such as Gonggong, Quaoar and Sedna are large, but the IAU has not formally added them, partly because it is hard to confirm their shapes from so far away.

Can I see a dwarf planet?

Most dwarf planets are far too faint for the naked eye. Ceres is the exception that comes closest: at its best it reaches about magnitude 7, so it can be picked up with binoculars from a dark site if you use a finder chart for that week.

Pluto is about magnitude 14, which takes a telescope of at least 8 inches aperture, a very dark sky and patience. It looks like any other faint star, so observers sketch or photograph the field on two nights and look for the dot that moved. Eris, Haumea and Makemake are fainter still and are mostly targets for astrophotographers.

A free planetarium app or a monthly sky guide will show where Ceres and Pluto are; both drift slowly against the background stars over weeks.