Many astronomers believe that Pluto was an escaped moon of Neptune, knocked out of orbit by its largest current moon, Triton.
This hypothesis has been heavily criticised because Pluto never comes near Neptune in its orbit.
The discovery of Pluto made headlines across the globe, and its name was proposed by Venetia Burney, an eleven-year-old schoolgirl in Oxford, England.
Named after the Roman god of the underworld, its name is appropriate for such a presumably dark and cold world.
We know very little about Pluto because no spacecraft has gone near it, and its small size and great distance make observation difficult.
Pluto is 3.6 billion miles (5.8 billion km) from Earth.
It would take more than 45 million Earths placed side by side to cover the distance between the two planets.
Like Neptune, Pluto was discovered as a result of mathematical computation.
Its orbit is decidedly irregular, and Pluto’s origins are debated, in the scientific community, but even more so in the media.