What Is a Dwarf Planet?

The International Astronomical Union (IAU) created the dwarf planet category in 2006, famously reclassifying Pluto from planet to dwarf planet. The key difference from a full planet is that a dwarf planet has not gravitationally dominated its orbital zone — it shares its orbital neighbourhood with other objects of comparable size. Five bodies are officially recognised as dwarf planets: Pluto, Eris, Haumea, Makemake (all in the Kuiper Belt), and Ceres (in the asteroid belt between Mars and Jupiter). Pluto, with a diameter of 2,377 km, is smaller than Earth Moon but has a complex surface with nitrogen ice glaciers, mountains of water ice, and a thin nitrogen atmosphere. Eris, slightly more massive than Pluto, was the discovery that triggered the IAU reclassification debate. Scientists estimate there may be dozens to hundreds of additional dwarf planets awaiting discovery in the outer solar system.