Dwarf Planet

A dwarf planet is a celestial body that is in orbit around the Sun and has enough mass to be roughly round in shape.

A dwarf planet is a celestial body that only meets two out of the three formal criteria to be considered a planet. These criteria were decided upon by the International Astronomical Union (IAU) in August 2006. Firstly, for a celestial body to be classified as a dwarf planet, it has to orbit our Sun — meaning it must exist within our Solar System. Secondly, it must have enough mass that it has formed into an approximately round shape under its own gravity (in contrast to, for example, asteroids, which have irregular shapes). However, unlike a planet, a dwarf planet does not have enough gravitational pull to have cleared its neighbourhood of any other celestial objects of significant mass (except for any of its satellites, such as a moon). Pluto was reclassified from a planet to a dwarf planet in 2006 because it is not gravitationally dominant in its neighbourhood. Pluto does, however, have the distinction of being the first ever recognised dwarf planet. There are now five known celestial bodies in our Solar System that qualify as dwarf planets: Pluto, Eris, Ceres, Makemake and Haumea.

Hubble has supported the study of various dwarf planets and their moons. For example, the telescope has helped to enhance our understanding of the Pluto system; Hubble observations in 2006 uncovered two small moons orbiting Pluto, Nix and Hydra, followed by the discoveries of two more moons in 2011 (Kerberos) and 2012 (Styx). In 2015, scientists gathered all available Hubble data on the four outer moons of Pluto to analyse the system in more depth than ever before. You can learn more about Hubble’s study of the Pluto system in this Hubblecast video.

In 2016, Hubble captured an image of the dwarf planet Eris and its satellite Dysnomia. Eris is the most massive and second-largest known dwarf planet in the Solar System. Also in 2016, Hubble revealed the first moon ever discovered around the dwarf planet Makemake.