A rare image of tiny Pluto with its moon, Charon
Overnight an entire planet has vanished—with the stroke of a pen.
At a meeting in Prague, the International Astronomical Union has redefined the term 'planet'. Under this new definition Pluto is no longer considered a planet. Instead it and two other objects will be called 'dwarf planets'.
This is not the first time such historic change has happened. Unlike now, in ancient times the Sun and Moon were called planets. The invention of the telescope later led to the planets Uranus and Neptune being discovered. In the 1800s the asteroid Ceres was briefly considered as a planet, then demoted. Finally the 1930s discovery of Pluto led to one last change in the list of planets—until now.
Long considered an oddball among planets, Pluto is much smaller and orbits the Sun at a different angle, in a much less circular path. In recent years many objects have also been found in its neighbourhood, called the Kuiper Belt. In 2003 a Kuiper Belt object (nicknamed Xena) even larger than Pluto was discovered. The question was asked whether Xena should be the tenth planet, or Pluto downgraded.
According to the IAU definition, a planet is an object orbiting the Sun that is large enough to be round, that isn’t a moon and that is alone in its orbit. Dwarf planets share only the first two of these properties.
Because it exists amongst the Kuiper Belt, Pluto isn’t a planet, but it, Ceres in the asteroid belt, and Xena, are the first dwarf planets to be identified. It is expected that more objects will be labelled as dwarf planets in the future, but the number of planets will remain at eight.