The Planet Neptune, number 8 in a set of 9 prints featuring the planets of our Solar System. Neptune is the eighth and farthest planet from the Sun in the Solar System. The planet is named after the Roman god of the sea, Neptune. Discovered on September 23, 1846,[1] Neptune was the first planet found by mathematical prediction rather than by empirical observation. Because of its great distance from the Sun, Neptune's outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching ?218 C (55 K).