Ceres
The largest object in the asteroid belt and the only dwarf planet in the inner solar system.
NASA, ESA, CSA, STScI; J. DePasquale, A. Koekemoer, A. Pagan (STScI) (CC BY 4.0)
First a planet, then an asteroid, now a dwarf planet: Ceres has changed category more often than any other world.
Ceres was discovered by Giuseppe Piazzi in 1801 and was first counted as a planet, then an asteroid, and since 2006 a dwarf planet. It is by far the most massive object in the belt.
NASA’s Dawn spacecraft orbited Ceres from 2015 to 2018 and found bright deposits of salts in Occator crater, evidence of briny water beneath the surface.
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