Small (smaller than our Moon) and icy – an outer planet, but not Jovian in composition (a puzzle!)

Outer planet Pluto, with potential cliffs, craters and other surface features labeled (right). [NASA/NH]

Erratic orbit, very elliptical, and inclined 17 degrees from the orbital plane

Cold atmosphere (even nitrogen is frozen solid)

Charon: single moon, half as large as Pluto

Could Pluto be an escaped moon of Neptune?

Could Pluto be a leftover building block for planets, that was never assimilated?

Are Pluto and Charon planet and moon, or two trapped planetesimals (building blocks)?

Outer planet Pluto and its largest moon Charon (left). [NASA/NH]