Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical predictions rather than by empirical observation. It is named after the Roman god of the sea and has the astronomical symbol, representing Neptune's trident. The planet orbits the Sun once every 164.8 years at an average distance of 30.1 astronomical units (4.5 billion kilometres 2.8 billion miles). Being composed primarily of gases and liquids, it has no well-defined solid surface. Neptune is denser and physically smaller than Uranus because its greater mass causes more gravitational compression of its atmosphere. It is 17 times the mass of Earth, and slightly more massive than its near-twin Uranus. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. The spacecraft remains on a trajectory heading towards the red star Aldebaran (in the constellation Taurus) and is expected to pass by it in about 2,000,000 years.ġ984: Astronomers find evidence for the existence of a ring system around Neptune.ġ989: Voyager 2 becomes the first and only spacecraft to visit Neptune, passing about 4,800 kilometers (2,983 miles) above the planet's north pole.Ģ002: Using improved observing techniques, astronomers discover four new moons orbiting Neptune: Laomedia, Neso, Sao and Halimede.Ģ003: Another moon, Psamathe, is discovered using ground-based telescopes.Ģ005: Scientists using the Keck Observatory take images of the outer rings and find that some of the ring arcs have deteriorated.Ģ011: Neptune completes its first 165-year orbit of the sun since its discovery in 1846.Ģ013: A scientist studying Neptune's ring arcs in archival Hubble Space Telescope images finds a previously unknown 14th moon of Neptune, provisionally designated S/2004 N 1.Ģ016: Scientists using the Hubble Space Telescope discover a new dark spot on Neptune, the first new atmospheric vortex seen in the 21st century.Neptune is the eighth planet from the Sun and the farthest IAU-recognized planet in the Solar System. Neptune's largest moon, Triton, is found the same year.ġ983: Pioneer 10 crosses the orbit of Neptune and becomes the first human-made object to travel beyond the orbits of the planets of our solar system. Scientists also use the Hubble Space Telescope and powerful ground-based telescopes to gather more information about this distant planet Significant Eventsġ612: Galileo incorrectly records Neptune as a fixed star during observations with his small telescope.ġ846: Using mathematical calculations, astronomers discover Neptune, increasing the number of known planets to eight.
Voyager returned a wealth of information about Neptune and its moons-and confirmed evidence the giant world had faint rings like the other gas planets.
More than 140 years later, in 1989, NASA's Voyager 2 became the first-and only-spacecraft to study Neptune up close. Seventeen days later, Neptune's largest moon Triton was discovered as well. Le Verrier sent his predictions to Johann Gottfried Galle at the Berlin Observatory, who found Neptune on his first night of searching in 1846. Because Uranus didn't travel exactly as astronomers expected it to, French mathematician Urbain Joseph Le Verrier proposed the position and mass of a then-unknown planet that could cause the observed changes to Uranus' orbit. More than 200 years later, the ice giant Neptune became the first planet located through mathematical predictions rather than through regular observations of the sky. Galileo recorded Neptune as a fixed star during observations with his small telescope in 16.
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