What Is a Dwarf Planet?
A dwarf planet is round, orbits the Sun, and is NOT a moon — but unlike a full planet, it hasn't cleared its neighbourhood of other debris. Five are officially recognised, and AstroSat plots every one of them on its real, tilted orbit.
The five recognised dwarf planets
Ceres
The only dwarf planet in the inner solar system, orbiting in the asteroid belt between Mars and Jupiter. It was classified as a planet when found in 1801, then an asteroid, then finally a dwarf planet in 2006.
Pluto
The best known, demoted from ninth planet in 2006. It has five moons of its own — Charon is over half Pluto's size, big enough that the pair orbits a point outside Pluto entirely.
Haumea
Spins so fast — once every 3.9 hours — that it has been stretched into an egg shape, and carries a thin ring of its own, discovered in 2017.
Makemake
One of the brightest objects in the Kuiper Belt after Pluto, with a single known moon so dark by comparison it's nicknamed "MK 2".
Eris
Slightly more massive than Pluto despite being a touch smaller — the discovery that forced astronomers to define "planet" properly in the first place, and directly caused Pluto's reclassification.
The definition dates to 2006, when the International Astronomical Union split "round things that orbit the Sun" into two classes: a planet has enough gravity to have cleared its orbital neighbourhood of comparable-sized debris, while a dwarf planet hasn't — Ceres shares the asteroid belt with millions of smaller rocks, and Pluto shares the Kuiper Belt with countless icy neighbours. Size alone doesn't decide it: Eris is more massive than Pluto and still counts as a dwarf planet under the same rule.
Frequently asked questions
Why isn't Pluto a planet anymore?
Because it hasn't cleared its orbital neighbourhood of other Kuiper Belt objects — the third of the IAU's three 2006 criteria for a planet. It still meets the other two (round, orbits the Sun), which is exactly what makes it a dwarf planet rather than something else.
How many dwarf planets are there?
Five are officially recognised by the IAU: Ceres, Pluto, Haumea, Makemake and Eris. Dozens more candidates are suspected in the Kuiper Belt but haven't been confirmed round enough yet.
Is Ceres a dwarf planet or an asteroid?
Both labels are used, but since 2006 it's officially a dwarf planet — the only one small and close enough to also carry its older name, the largest object in the asteroid belt.
Which dwarf planet is the biggest?
Pluto and Eris are close: Eris is slightly more massive, but Pluto has a very marginally larger diameter. Both dwarf every other confirmed dwarf planet by a wide margin.