The existence of Neptune was predicted from perturbations observed in the orbital characteristics of the planet Uranus. Two mathematicians, John Couch Adams of England and Jean Joseph Urbain Le Verrier of France, independently predicted its location in 1845 and 1846, respectively. By telescopic sightings from the Berlin Observatory in 1846, Johann Gottfried Galle and Louis Heinrich D'Arrest confirmed Neptune's existence as the eighth planet in the Solar System.
Neptune's equatorial diameter is 5 × 105 km, similar to Uranus's. It is 30 Astronomical Units (1 AU = 1.5 × 108 km) from the sun and has an orbital period of 165 years. Before Voyager 2′s encounter, the planet's rotational rate was believed to be between 17 and 18 hours. The planet's rotational axis is tilted 29° relative to its orbital plane. Neptune radiates 2.8 times more energy than it absorbs from sunlight, a greater energy imbalance than exists for any of the other planets. In addition, its density of 1.64 gm cm−3 is greatest among the giant planets. These two extremes may be linked, an indication of an unusual degree of radiogenic heating within Neptune.