This helps to displace the opinion that Pluto is a planet," says Brian Marsden of the Harvard-Smithsonian Center for Astrophysics, who thinks Pluto is instead the largest Kuiper Belt object found so far.

The size of the new object, named Quaoar, was measured by the Hubble Space Telescope. It is about 1250 kilometres in diameter, making it the largest object found in the Solar System since Pluto itself in 1930.

The half rock, half ice object has been made dark by faint ultraviolet light that over the 4.5 billion year life of the Solar System has slowly turned organic compounds into a dark tar.