‘Dwarf Planet’

A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to planetary geologists is that, being possibly differentiated and geologically active bodies, they may display planetary geology, an expectation borne out by the Dawn mission to Ceres and the New Horizons mission to Pluto in 2015. Counts of the number of dwarf planets among known bodies of the Solar System range from 5-and-counting to over 120. Apart from Sedna, the largest of these candidates have either been visited by spacecraft or have at least one known moon, which allows their masses and thus an estimate of their densities to be determined. Mass and density in turn can be fit into geophysical models in an attempt to determine the nature of these worlds.