By Sykes criteria, the smallest known planet in the solar system would be Ceres, the largest and most massive object in the asteroid belt. Ceres is less than half the diameter of Pluto.

Sykes, director of the Planetary Science Institute is talking about planets across the universe in general, about Pluto and other small
planets in particular, [...]

Applying a similar test to the planet Mercury, astronomer Jean-Luc Margot of Cornell U and his colleagues have determined that Mercury is raw or soft-boiled and has a fluid yolk, sorry, core.

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