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Dwarf Planets: Ceres, Eris and the Rest

A dwarf planet orbits the Sun, is massive enough to be nearly round, is not a moon, and has not cleared its orbital neighbourhood. The International Astronomical Union (IAU) recognises five: Ceres, Pluto, Haumea, Makemake and Eris.

What is a dwarf planet?

The category was created in 2006 by the International Astronomical Union (IAU). A dwarf planet is a body that:

1. orbits the Sun;
2. has enough mass for its own gravity to pull it into a nearly round shape (hydrostatic equilibrium);
3. has not cleared the neighbourhood around its orbit;
4. is not a moon of another body.

The third point is what separates dwarf planets from the eight full planets. Earth or Jupiter dominates its orbital zone, while a dwarf planet shares its space with many other objects. So “dwarf” does not simply mean small. It describes a difference in orbital dominance.

Dwarf planets also differ from ordinary asteroids and comets. Most small bodies are irregular because their gravity is too weak to round them. Dwarf planets are big enough to have become round. For comets, see Comets.

The five recognised dwarf planets

The IAU currently recognises five.

Ceres. The only one in the inner Solar System, it lies in the main asteroid belt between Mars and Jupiter. Giuseppe Piazzi discovered it in 1801. It was first counted as a planet, later as an asteroid. It is roughly 940 km across. NASA’s Dawn spacecraft orbited it from 2015 to 2018 and revealed bright salt deposits on its surface.

Pluto. Found in 1930 and counted as the ninth planet until 2006. NASA’s New Horizons flew past in 2015, revealing mountains, icy plains and a thin atmosphere. See Pluto and Why Pluto is not a planet.

Eris. Discovered in 2005. It is close to Pluto in diameter but somewhat more massive. Its discovery sharpened the question of what a planet is and helped lead to the 2006 decision. It has one known moon, Dysnomia.

Haumea. It spins once in about four hours, which stretches it into an egg-like shape. It has two moons, and a ring was detected around it in 2017.

Makemake. Discovered in 2005, it is one of the brightest objects in the distant outer Solar System. A small moon was found in 2016.

The names Haumea and Makemake were adopted in 2008, drawn from Hawaiian and Rapa Nui traditions respectively.

The Kuiper Belt

The Kuiper Belt is a broad region of icy bodies beyond Neptune, beginning roughly 30 astronomical units from the Sun (one astronomical unit is the average Earth–Sun distance). It resembles the asteroid belt but is much wider and made largely of frozen volatile materials. Some short-period comets are thought to originate there.

Pluto and Makemake are linked to this region, and Haumea is usually counted as part of it. Eris belongs to the scattered disc, which reaches farther out and has more tilted, stretched orbits. These are all trans-Neptunian objects, meaning their average distance from the Sun is greater than Neptune’s.

For other small bodies of the outer Solar System, see Chiron and the Centaurs.

Why the count could change

Five is the number officially recognised, not necessarily the true total. Other large bodies, such as Sedna, Gonggong, Quaoar and Orcus, are often discussed as candidates, but the IAU has not added them to its list.

Several reasons keep the number open:

• Hard observations. These objects are distant and dark, so their sizes and shapes are difficult to measure.
• New discoveries. Telescopes keep finding objects in the outer Solar System.
• No sharp cutoff. “Nearly round” has no exact size or mass threshold, so each object has to be assessed.
• A negotiated definition. Scientists still discuss how best to word the criteria.

The list reflects what we know today. As missions and surveys improve, it may grow, which is simply how science works.

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