DID YOU KNOW? Pluto was visited by the New Horizons spacecraft on 14 July 2015, which was launched in 2006 What are dwarf planets? How these diminutive worlds diff er from their more expansive counterparts The big planets’ s their name suggests, dwarf planets little siblings share a lot of similarities with what we Meet the five celestial dwarves of our Solar System that have been discovered so far Awould generally refer to as ‘planets’. Ceres For instance, they orbit the Sun, and possess Ceres is located in the main asteroid belt Sun enough mass to assume an approximate between Mars and Jupiter, and was first Mercury spherical shape. However, it is their discovered in 1801. It is described as an Venus ‘embryonic planet’ due to the gravitational Earth diff erences that serve to draw a dividing line pull from nearby Jupiter preventing it from Mars between them and what astronomers gaining the mass required to complete its consider to be true planets. growth. CERES The most apparent observation is that dwarf planets are much smaller than conventional planets; even smaller than the Jupiter size of Earth’s Moon (although they can have moons themselves). Their size also means Pluto that they are unable to clear their orbital path, Originally designated as the ninth planet in Saturn as is the case with Pluto, which is restricted by the Solar System, Pluto was reclassified – controversially according to some – as a both the orbit of its larger neighbour Neptune dwarf planet in 2006 due to its failing to and assorted objects in the Kuiper Belt. The meet the added criteria of being able to clear planets of the Solar System are able to clear a its own orbital path. In spite of its size, Pluto has five moons: Charon, Hydra, Nix, Kerberos path around the Sun for their orbit, while and Styx. dwarf planets are unable to do this. Uranus Currently, there are fi ve recognised dwarf planets in our Solar System: Ceres, Eris, Makemake, Haumea and Pluto, which was controversially reclassifi ed in 2006, having Eris originally been recognised as the ninth planet Named after the Greek goddess of discord and strife, the climate on Eris is aptly in the Solar System. Distinguishing them even turbulent. Due to its distance from the Sun, Neptune further is the International Astronomical its atmosphere often collapses and Union’s (IAU) designation of dwarf planets that subsequently freezes. Moreover, its orbit is similarly erratic, crossing the path of Pluto orbit the Sun beyond Neptune as ‘plutoids‘, in and nearly intersecting with Neptune’s on an honour of the former planet. orbit of the Sun that takes Eris 557 years. PLUTO

NASA’s Dawn spacecraft recently found evidence of organic material HUAMEA on Ceres, indicated by the red areas on the image Makemake Discovered in 2005, it was Makemake – along with the uncovering of Eris two years MAKEMAKE before – that prompted the IAU to reconsider the classification of planets in the first place. This dwarf planet takes its name from the god of fertility in Rapanui mythology, which originated with the native people of Easter Island.

ERIS

Haumea Haumea’s ellipsoid shape means that it only just meets dwarf planet criteria. Its unusual shape is due to the rapid rotational spin that it possesses. It is thought that this rotation is likely due to a collision, as Haumea is situated within the Kuiper Belt.

Dwarf planet sizes shown relative to Earth’s Moon WIKII©

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