<Li> ammonium hydrosulfide (NH 4SH) </Li> <Li> methane ice (?) (CH 4 5.75 H O) </Li> <P> Neptune is the eighth and farthest known planet from the Sun in the Solar System . In the Solar System, it is the fourth - largest planet by diameter, the third-most - massive planet, and the densest giant planet . Neptune is 17 times the mass of Earth and is slightly more massive than its near - twin Uranus, which is 15 times the mass of Earth and slightly larger than Neptune . Neptune orbits the Sun once every 164.8 years at an average distance of 30.1 astronomical units (4.50 × 10 km). It is named after the Roman god of the sea and has the astronomical symbol ♆, a stylised version of the god Neptune's trident . </P> <P> Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation . Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet . Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the position predicted by Urbain Le Verrier . Its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining known 13 moons were located telescopically until the 20th century . The planet's distance from Earth gives it a very small apparent size, making it challenging to study with Earth - based telescopes . Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989 . The advent of the Hubble Space Telescope and large ground - based telescopes with adaptive optics has recently allowed for additional detailed observations from afar . </P>

How long it takes for neptune to orbit the sun