Neptune is the eighth planet from the sun. It was the first planet to get its existence predicted by mathematical calculations before it was actually seen through a telescope on Sept. 23, 1846. 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. Previously, astronomer Galileo Galilei sketched the planet, but he mistook it for a star due to its slow motion. 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.
Only one mission has flown by Neptune – Voyager 2 in 1989 – meaning that astronomers have done most studies using ground-based telescopes. Today, there are still many mysteries about the cool, blue planet, such as why its winds are so speedy and why its magnetic field is offset.
While Neptune is of interest because it is in our own solar system, astronomers are also interested in learning more about the planet to assist with exoplanet studies. Specifically, some astronomers are interested in learning about the habitability of worlds that are somewhat bigger than Earth.
Those that are closer to Earth's size are called "super-Earths", while those that are closer to Neptune's size are "mini-Neptunes." However, there is some debate about those terms given that today's telescope technology does not make it possible to view how much atmosphere is on those planet types, making it difficult to make a distinction.
Like Earth, Neptune has a rocky core, but it has a much thicker atmosphere that prohibits the existence of life as we know it. Astronomers are still trying to figure out at what point a planet is so large that it may pick up a lot of gas in the area, making it difficult or impossible for life to exist.
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, according to a NASA fact sheet. Neptune's rocky core alone is thought to be roughly equal to Earth's mass, NASA says.
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 mph (2,400 km/h), 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 mph (1,200 km/h). 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 other Great Dark Spots over the past decade. A new one was observed in 2016.
Neptune's magnetic poles are tipped over by roughly 47 degrees compared with the poles along which it spins. As such, the planet's magnetic field, which is about 27 times more powerful than Earth's, undergoes wild swings during each rotation.
By studying the cloud formations on the gas giant, scientists were able to calculate that a day on Neptune lasts just under 16 hours.
Neptune's elliptical, oval-shaped orbit keeps the planet 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 completed 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
Magnetic field: Roughly 27 times more powerful than Earth's
Composition: 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, according to Tristan Guillot, author of "Interiors of Giant Planets Inside and Outside the Solar System" in the journal Science.
Internal structure: Mantle of water, ammonia and methane ices; Core of iron and magnesium-silicate
Orbit & rotation
Average distance from the sun: 2,795,084,800 miles (4,498,252,900 km). By comparison: 30.069 times farther than Earth
Perihelion (closest approach to the sun): 2,771,087,000 miles (4,459,630,000 km). By comparison: 29.820 times that of Earth
Aphelion (farthest distance from the sun): 2,819,080,000 miles (4,536,870,000 km). By comparison: 30.326 times that of Earth
Neptune has 14 known moons, named after lesser sea gods and nymphs from Greek mythology. 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 the only spherical moon of Neptune — the planet's other 13 moons are irregularly shaped. It is also 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, according to NASA. 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 km), showing its interior appears warm. Scientists are investigating the possibility of a subsurface ocean on the icy moon. In 2010, seasons were discovered on Triton.
In 2013, scientists working with SETI caught sight of Neptune's "lost" moon of Naiad using data from the Hubble Space Telescope. The 62-mile-wide (100 km) moon had remained unseen since Voyager 2 discovered it in 1989.
Also in 2013, scientists using the Hubble Space Telescope found the 14th moon, dubbed S/2004 N 1. It is Neptune's smallest moon and is just 11 miles (18 km) wide. It got its temporary name because it is the first satellite (S) of Neptune (N) to be found from images taken in 2004, according to NASA.
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, according to an article in the journal Icarus.
Research & exploration
NASA's Voyager 2 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.
Additional reporting by Nola Taylor Redd and Elizabeth Howell, Space.com contributors.
Learn more about Neptune:
- How Big Is Neptune?
- How Far Away Is Neptune?
- Neptune's Atmosphere
- What Is Neptune Made Of?
- How Was Neptune Formed?
- What Is the Temperature of Neptune?
Learn more about the solar system: