Eris
A distant dwarf planet about as large as Pluto and more massive, whose discovery led to the definition of planet.
NASA, ESA, CSA, STScI; J. DePasquale, A. Koekemoer, A. Pagan (STScI) (CC BY 4.0)
The world that made astronomers decide what a planet is, and the reason Pluto was reclassified.
Eris was discovered in 2005. Its highly eccentric, steeply inclined orbit takes it from about 38 to 98 au from the Sun, and one orbit takes about 560 years.
It is slightly more massive than Pluto. Finding a body that size beyond Neptune pushed the International Astronomical Union to define the word planet in 2006. Its moon Dysnomia made it possible to measure its mass.
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