Dwarf Planets

Overview

A dwarf planet orbits the Sun and is massive enough to be nearly round under its own gravity, but has not cleared its orbital neighbourhood. Five are formally recognised by the International Astronomical Union, and more candidates exist among the trans-Neptunian population.

• The category was created in 2006 alongside the formal definition of 'planet'.
• Ceres is in the asteroid belt; the other four recognised dwarf planets are beyond Neptune.
• Roundness is hard to confirm at great distance, which is why the recognised list is short and conservative.
• Only two have been visited: Ceres by Dawn and Pluto by New Horizons.

What the category is for

The dwarf planet class exists because the Solar System turned out to contain many bodies large enough to be round but small enough that they do not dominate their orbital zone. Rather than expand the planet list indefinitely as such objects were discovered, the IAU created a category for them in 2006.

Five are currently recognised: Ceres, Pluto, Eris, Haumea and Makemake. Several other trans-Neptunian objects are plausible candidates — Gonggong, Quaoar and Sedna among them — but confirming hydrostatic equilibrium for a faint, distant, poorly resolved body is difficult, and the IAU has not extended the list.

Ceres: the one in the asteroid belt

Ceres is the largest object in the main asteroid belt and contains a substantial fraction of the belt's total mass. Discovered in 1801, it was initially counted as a planet, then reclassified as an asteroid as more belt objects were found — a historical precedent that closely parallels what happened to Pluto two centuries later.

The Dawn spacecraft orbited Ceres from 2015 and found bright deposits in Occator crater identified as salts, together with evidence for a substantial water-ice component and for geologically recent brine activity. Ceres is now studied as a small ice-rich world rather than as a large rock.

Pluto and the trans-Neptunian dwarf planets

New Horizons' 2015 flyby transformed Pluto from a point of light into a geologically complex world: nitrogen-ice plains with convective cells and no impact craters, indicating an actively resurfacing terrain; water-ice mountains; and a layered haze in its thin nitrogen atmosphere. Its largest moon, Charon, is so large relative to Pluto that the pair orbit a barycentre outside Pluto's surface.

Eris is comparable in size to Pluto and more massive, and its discovery in 2005 was the immediate catalyst for the reclassification debate. Haumea is markedly elongated by rapid rotation and has a ring; Makemake is a bright, methane-rich body. All three remain unvisited, and what is known about them comes from occultations, photometry and spectroscopy rather than from close observation.

Why the list may stay short

Establishing that a distant object is in hydrostatic equilibrium requires knowing its shape and internal structure, which for objects tens of astronomical units away is often not possible with current data. Stellar occultations — timing how a body blocks a background star from multiple sites — provide the sharpest size and shape constraints available and have been decisive for several candidates.

The conservative approach means the recognised list understates how many round bodies orbit beyond Neptune. That is a limitation of measurement, not a claim that others do not exist, and Asteria Star records candidates as candidates rather than promoting them.

In-depth, individual pages in this category.

• Pluto

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  The most famous dwarf planet, in the Kuiper Belt.

• Ceres

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  The largest body in the asteroid belt.

• Eris

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  The distant dwarf planet that reshaped the definitions.

• Haumea

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  The fast-spinning, egg-shaped dwarf planet.

• Makemake

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  A large, bright dwarf planet of the Kuiper Belt.

Continue in the data

Catalogues, hubs, and reference pages that hold the underlying records for this topic.

• Solar SystemDwarf planets alongside planets and small bodies.
• AsteroidsThe main belt population, including Ceres.
• Small-body missionsDawn, New Horizons, and other visiting spacecraft.

Frequently asked

How many dwarf planets are there?

Five are formally recognised by the IAU: Ceres, Pluto, Eris, Haumea and Makemake. Several other trans-Neptunian objects are strong candidates, but confirming that a faint, distant body is round enough to qualify is observationally difficult, so the official list is deliberately conservative and probably an undercount.

Is Ceres an asteroid or a dwarf planet?

Both descriptions apply. It is the largest object in the main asteroid belt and is catalogued as a minor planet, and it also meets the IAU's dwarf planet criteria. The categories overlap because one describes location and population and the other describes size and shape.

What did New Horizons find at Pluto?

A far more active world than expected: vast nitrogen-ice plains with convective cells and no impact craters, indicating ongoing resurfacing; mountains of water ice; evidence of past or present cryovolcanism; and a layered atmospheric haze. For a body that small and that far from the Sun, sustained geological activity was a genuine surprise.