Neptuneis the eighth and farthestplanetfrom theSunin theSolar System. It is the fourth-largest planet by diameter and the third-largest by mass. Among thegiant planetsin the Solar System, Neptune is the most dense. Neptune is 17 times the mass ofEarthand is slightly more massive than its near-twinUranus, which is 15 times the mass of Earth, and not as dense as Neptune.Neptune orbits the Sun at an average distance of 30.1astronomical units(4.50×109 km). Named after theRoman god of the sea. Neptune is not visible to the unaided eye and is the only planet found by mathematical prediction rather than byempirical observation. Unexpected changes in the orbit of Uranus ledAlexis Bouvardto deduce that its orbit was subject togravitationalperturbationby an unknown planet. Neptune was subsequently observed with a telescope on 23 September 1846byJohann Gallewithin adegreeof the position predicted byUrbain Le Verrier. Its largest moon,Triton, was discovered shortly thereafter, though none of the planet's remaining 14moonswere 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 byVoyager 2, when it flew by the planet on 25 August 1989.The advent ofHubble Space Telescopeand largeground-based telescopeswithadaptive opticshas allowed for more-detailed observations.