(Editor's Note: Scientists working with SETI have caught sight of Neptune's "lost" moon of Naiad using data from the Hubble Space Telescope. Read more here: Neptune's 'Lost' Moon Spotted for First Time in 20 Years.)
Neptune’s History & Naming
The planet Neptune was discovered on Sept. 23, 1846. Neptune was the first planet to get its existence predicted by mathematical calculations before it was actually seen by a telescope. Irregularities in the orbit of Uranus led French astronomer Alexis Bouvard to suggest that the gravitational pull from another celestial body might be responsible. German astronomer Johann Galle then relied on subsequent calculations to help spot Neptune via telescope. In accordance with all the other planets seen in the sky, this new world was given a name from Greek and Roman mythology — Neptune, the Roman god of the sea.
Physical Characteristics of the Planet Neptune
Neptune’s cloud cover has an especially vivid blue tint that is partly due to an as-yet-unidentified compound and the result of the absorption of red light by methane in the planets mostly hydrogen-helium atmosphere. Photos of Neptune reveal a blue planet, and it is often dubbed an ice giant, since it possesses a thick, slushy fluid mix of water, ammonia and methane ices under its atmosphere and is roughly 17 times Earth's mass and nearly 58 times its volume. Neptune's rocky core alone is thought to be roughly equal to Earth's mass.
Despite its great distance from the sun, which means it gets little sunlight to help warm and drive its atmosphere, Neptune's winds can reach up to 1,500 miles per hour (2,400 kilometers per hour), the fastest detected yet in the solar system. These winds were linked with a large dark storm that Voyager 2 tracked in Neptune's southern hemisphere in 1989. This oval-shaped, counterclockwise-spinning "Great Dark Spot" was large enough to contain the entire Earth, and moved westward at nearly 750 miles per hour (1,200 kilometers per hour). This storm seemed to have vanished when the Hubble Space Telescope later searched for it. Hubble has also revealed the appearance and then fading of two other Great Dark Spots over the last decade.
Neptune's magnetic poles are tipped over by roughly 47 degrees compared with the poles along which it spins. As such, the planets magnetic field, which is about 27 times more powerful than Earth's, undergoes wild swings during each rotation.
Neptune’s Orbital Characteristics
Neptune is the eighth and most distant planet from the sun. Its elliptical, oval-shaped orbit makes it keep an average distance from the sun of almost 2.8 billion miles (4.5 billion kilometers), or roughly 30 times as far away as Earth, making it invisible to the naked eye. Neptune goes around the sun once roughly every 165 Earth years, and will complete its first orbit, since being discovered, in 2011.
Every 248 years, Pluto moves inside Neptune's orbit for 20 years or so, during which time it is closer to the sun than Neptune. Nevertheless, Neptune remains the farthest planet from the sun, since Pluto was reclassified as a dwarf planet in 2006.
Composition & Structure
- Atmospheric composition (by volume)
Hydrogen - 80 percent; Helium - 19.0 percent; Methane - 1.5 percent
Roughly 27 times more powerful than Earth's.
The overall composition of Neptune is, by mass, thought to be about 25 percent rock, 60 to 70 percent ice, and 5 to 15 percent hydrogen and helium. (Tristan Guillot, "Interiors of Giant Planets Inside and Outside the Solar System." Science Vol. 286 (5437), p. 72-77, October 1, 1999.)
- Internal structure
Mantle of water, ammonia and methane ices
Core of iron and magnesium-silicate
(Tristan Guillot, "Interiors of Giant Planets Inside and Outside the Solar System." Science Vol. 286 (5437), p. 72-77, October 1, 1999.)
Orbit & Rotation
Average Distance from the Sun
Metric: 4,498,252,900 km
English: 2,795,084,800 miles
By Comparison: 30.069 x Earth
Metric: 4,459,630,000 km
English: 2,771,087,000 miles
By Comparison: 29.820 x Earth
Metric: 4,536,870,000 km
English: 2,819,080,000 miles
By Comparison: 30.326 x Earth
Neptune has 13 known moons, named after lesser sea gods and nymphs from Greek mythology, just as Neptune itself was named after the Roman god of the sea. The largest by far is Triton, whose discovery on Oct. 10, 1846 was in a sense enabled by beer — amateur astronomer William Lassell used the fortune he made as a brewer to finance his telescopes.
Triton is unique in being the only large moon in the solar system to circle its planet in a direction opposite to its planet's rotation — this "retrograde orbit" suggests that Triton may once have been a dwarf planet that Neptune captured rather than forming in place. Neptune's gravity is dragging Triton closer to the planet, meaning that millions of years from now, Triton will come close enough for gravitational forces to rip it apart.
Triton is extremely cold, with temperatures on its surface reaching about minus 391 degrees F (minus 235 degrees C), making it one of the coldest places in the solar system. Nevertheless, Voyager 2 detected geysers spewing icy matter upward more than 5 miles (8 kilometers), showing its interior appears warm. Recently, seasons have been discovered on Triton.
Triton is the only spherical moon of Neptune — the planet’s other twelve moons are irregularly shaped.
The Rings of Neptune
Neptune's unusual rings are not uniform, but possess bright thick clumps of dust called arcs. The rings are thought to be relatively young and short-lived. Earth-based observations announced in 2005 found that Neptune's rings are apparently far more unstable than previously thought, with some dwindling away rapidly. (Imke de Pater et al., "The dynamic neptunian ring arcs: evidence for a gradual disappearance of Liberté and resonant jump of courage." Icarus, Volume 174, Issue 1, March 2005, Pages 263-272.)
Research & Exploration
NASA's Voyager 2 space satellite was the first and as yet only spacecraft to visit Neptune on Aug. 25, 1989. The satellite discovered Neptune's rings and six of the planet's moons — Despina, Galatea, Larissa, Naiad, Proteus, and Thalassa. An international team of astronomers relying on ground telescopes announced the discovery of five new moons orbiting Neptune in 2003.
Formation of Neptune
Neptune is generally thought to have formed with the initial buildup of a solid core followed by the capture of surrounding hydrogen and helium gas in the nebula surrounding the early sun. In this model, proto-Neptune formed over the course of 1 to 10 million years.
RELATED: See our Solar System Planets overview, or our broader Solar System Facts overview, or learn more about each of the other planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and the demoted dwarf planet Pluto.