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Mystery #2Tier 2space / planets

What exactly is a dwarf planet?

If Pluto isn't a planet, and it isn't a moon, and it isn't a comet, then what is it? Astronomers gave it and a small club of other round little worlds a name of their own: dwarf planet. There are five officially recognised so far, with curious names like Eris, Ceres, Makemake and Haumea. But what makes a world a dwarf planet rather than a full planet?

The dwarf planet Ceres, round and cratered, photographed by NASA's Dawn spacecraft, with bright pale spots in one crater.
Photo: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA · Public domain (NASA) · via Wikimedia Commons

Your guess

A dwarf planet is round like a planet, but it isn't called a planet. What do you think is the one big difference?

There's no single magic word to type, have a go in your own words, then reveal the answer to see how close you got.

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