Farthest planet in the Solar System

The most distant planet in the Solar System is Neptune, which orbits the Sun at an average distance of 4.498 billion km (2.794 billion miles).

Neptune was discovered by the German astronomer Johann Gottfried Galle on 23 September 1846. He was directed to the relevant sector of the night sky by the calculations made by French astronomer and mathematician Urbain le Verrier. The existence of an eighth planet had been suggested by le Verrier and a British astronomer called John Couch Adams a few years earlier. Both men had noticed that the orbit of the planet Uranus did not match their mathematical models, and hypothesized that it was being perturbed by the gravitational influence of another large planet farther out from the Sun.

In 1930, Neptune lost its record to Pluto, but regained it in 2006 when Pluto was "demoted" to the status of a "dwarf planet". The decision was made following many years of observations, during which time the mass estimate for Pluto was revised down several times. The final straw for Pluto's planet status came in 2005, when astronomers discovered another object (called Eris) that did not meet the International Astronomical Union's definition of a planet, but was nonetheless larger than Pluto.