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.
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 (Source: NASA)
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
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 [in over a billion] 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.
Triton is the largest of Neptune's moons. Discovered in 1846 by British astronomer William Lassell — just weeks after Neptune itself was found — the moon showed some strange characteristics as astronomers learned more about it.
To NASA's knowledge, Triton is the only moon in the solar system that orbits in a direction opposite to the rotation of its planet. Additionally, its surface is a study of contrasts, with smooth plains appearing to be right next to cratered surfaces.
No spacecraft has gone to Neptune since the 1980s, but telescopes are capturing new data on Triton from the ground. A notable recent find was discovering seasons on the moon, using the European Southern Observatory's Very Large Telescope in Chile.
Renewed interest in Triton began shortly after the New Horizons mission flew past Pluto in 2015. NASA released the best high-resolution map of Triton to date in 2014 in anticipation of the flyby, because it is believed that Triton and Pluto could share a similar history. They are of similar size, have nitrogen in their atmospheres and also icy surfaces.
Discovery and flyby
Triton was found on Oct. 10, 1846, just 17 days after French astronomer Alexis Bouvard discovered Neptune. According to NASA, Triton was known simply as "the satellite of Neptune" until 1949, when a second moon (Nereid) was found. All of Neptune's moons, according to International Astronomical Union guidelines, are named after Roman or Greek mythological characters associated with Neptune, Poseidon or oceans.
Astronomers had to wait well over a century to see Triton as more than a dot, however. In 1977, NASA sent two Voyager probes on a one-way trip through the outer solar system, taking advantage of a rare planetary alignment that allowed them to move from one location to the next without using a lot of fuel. Voyager 2 flew by Neptune and its system on Aug. 25, 1989.
Among the spacecraft's most stunning finds: icy volcanism is likely taking place on Triton's surface. Voyager 2 took pictures of "several geyser-like volcanic vents that were apparently spewing nitrogen gas laced with extremely fine, dark particles," according to NASA. The agency estimates the particles go as high as 5 miles (8 kilometers) before flowing downwind and striking the surface.
It also appears that a large portion of the surface has melted. NASA said the melting was probably due to tidal heating that began when Neptune captured Triton, which could have left the satellite liquid for at least 1 billion years.
Discoveries after Voyager 2
Although no spacecraft will go by Neptune's system in the near future, computer models and higher-resolution telescopes on Earth are providing new information about Triton's history and environment. (NASA has also released new pictures of Triton from Voyager 2 at least as late as 2009.)
In 2010, long-range infrared observations with the European Southern Observatory's Very Large Telescope revealed that the thin atmosphere of Triton changes with the seasons. At the time the VLT looked at the planet, it was summer in the southern hemisphere, where the sun's warmth thickened the atmosphere.
New models of Neptune could also provide insights on Triton, such as one released in 2015 that looks at the gas giant's magnetosphere. The magnetic field appears to be particularly pronounced on one side, according to Voyager 2 data and the model, and scientists next want to figure out how this field could interact with Triton.
Other information about Triton's history could come from studying small bodies in the solar system, such as Comet 67P/Churyumov–Gerasimenko. The Rosetta spacecraft gathered data that showed molecular nitrogen on the comet's surface in 2015. From that information, scientists are trying to figure out where the nitrogen came from and how it compares to other bodies with nitrogen in the solar system, such as Triton and Pluto.
Astrobiologists are also considering that Triton could have water under its icy surface. "I think it is extremely likely that a subsurface ammonia-rich ocean exists in Triton," said the University of Maryland's Saswata Hier-Majumder in a 2012 Astrobiology Magazine article republished on Space.com. "[But] there are a number of uncertainties in our knowledge of Triton's interior and past, which makes it difficult to predict with absolute certainty."
One example: because no one is quite sure of the size of Triton's rocky core, that creates difficulty with calculating the amount of heating produced by the decay of radioactive isotopes, also known as radiogenic heating. More heating would increase the size of the ocean.
Flinging aside moons
In 2006, a model published in Nature suggested Triton was originally a member of a binary system that orbited the sun. During a close encounter with the planet, Neptune pulled Triton away from its companion. Researchers in 2017 also postulated that Triton was likely a double-object system, similar to dwarf planet Pluto and its large moon, Charon. In the Astronomical Journal, the authors also noted that when Neptune's gravity captured Triton, the secondary body escaped. Triton survived only because Neptune's system was bereft of a large moon. If there had been a moon moving in a normal (forward) orbit around Neptune, its gravitational influence meant Triton instead would have fallen into Neptune.
The authors also said their research showed that Triton played an important role in the formation of Neptune's system. Some models suggest that Neptune used to have other moons orbiting it. Then, when Triton was captured long ago, some of these moons were propelled into the planet and others were thrown out of orbit.
There are some other indications of chaos in Neptune's system, including the outlying moon Nereid. The moon has an extremely stretched-out orbit around Neptune and takes 360 Earth-days to make a single orbit. In addition, some of these smaller moons around Neptune may have been absorbed into Triton itself. "If we want to look for the primordial Neptunian satellites, my best guess would be to look inside Triton," lead author Raluca Rufu, a doctoral student at the Weizmann Institute of Science in Israel, told Space.com by email in December 2017.