Neptune
Neptune is the only planet in the Solar System that no one ever stumbled upon by looking up. It was found on paper first. Strange wobbles in the orbit of Uranus told astronomers that something massive, something unseen, had to be tugging at it from farther out. In 1821, Alexis Bouvard published tables of Uranus's orbit, then watched the real planet drift away from his predictions. He guessed an unknown body was pulling the strings. Two men, working separately, turned that guess into coordinates. On the evening of the 23rd of September 1846, a telescope at the Berlin Observatory swung to the predicted spot, and there it was, within a degree of the calculation. How do you find a world by mathematics alone? Why does this distant blue giant blow the fastest winds in the Solar System? And what happens to a planet whose existence was first proven by the trouble it caused?
Galileo's drawings from the 28th of December 1612 and the 27th of January 1613 contain plotted points that sit exactly where Neptune was on those nights. He mistook it for a fixed star when it passed close to Jupiter, so history does not credit him with the discovery. At his first sighting, Neptune had just turned retrograde that very day, so its motion was far too slight for his small telescope to catch. A 2009 study suggested Galileo at least noticed his "star" had shifted against the others.
In 1843, John Couch Adams began working the orbit of Uranus, asking Sir George Airy, the Astronomer Royal, for extra data, which arrived in February 1844. Across 1845 and 1846, Adams produced several estimates for a planet beyond Uranus. Urbain Le Verrier reached his own calculations in the same years, independently, but stirred no enthusiasm among his countrymen.
In June 1846, Airy noticed how closely Le Verrier's figure matched Adams's and urged James Challis to hunt for the planet. Challis searched the sky uselessly through August and September. He had actually observed Neptune on 4 and the 12th of August 1845, a year before its true discoverer, but his out-of-date star maps and rough methods meant he never recognized what he had seen. Challis blamed his neglect on his charts and his distracting work on comets.
Meanwhile Le Verrier wrote to Johann Gottfried Galle at the Berlin Observatory. A student named Heinrich d'Arrest suggested comparing a freshly drawn sky chart against the real sky to spot anything that had moved. On the very evening Galle received the letter, he found Neptune just northeast of Iota Aquarii, one degree from Le Verrier's predicted spot and about twelve degrees from Adams's. A nationalistic rivalry erupted between the French and the British over credit. The eventual consensus gave Le Verrier and Adams joint honors, though since 1966 Dennis Rawlins has questioned Adams's claim to co-discovery.
Galle's first instinct was to call the new world Janus. In England, Challis suggested Oceanus. Le Verrier, claiming the right to name his discovery, proposed Neptune, while falsely stating that the French Bureau des Longitudes had already approved it. By October he had changed his mind and wanted the planet named Le Verrier, after himself, backed loyally by observatory director François Arago.
Resistance to naming a planet after a living astronomer was fierce outside France. French almanacs retaliated by reviving Herschel for Uranus, after that planet's discoverer Sir William Herschel, and Leverrier for the new one. Struve settled the matter on the 29th of December 1846 before the Saint Petersburg Academy of Sciences, favoring Neptune after the planet's color seen through a telescope. The name stuck, fitting the pattern of planets named for Greek and Roman deities. Neptune was the Roman god of the sea, identified with the Greek Poseidon.
Many languages carried the meaning rather than the sound. In Chinese, Vietnamese, Japanese, and Korean, the planet became "sea king star." In Mongolian it is Dalain van, in modern Greek it is Poseidon, and in Māori it is Tangaroa, each name a sea god in its own tradition. In Hebrew, the Academy of the Hebrew Language ran a vote in 2009 and chose Rahab, after a sea monster from the Book of Psalms, though the Latinate Neptun stays in common use.
Neptune holds 17 times the mass of Earth, yet it is just one nineteenth the mass of Jupiter. Its equatorial radius of 24,764 kilometres is nearly four times that of Earth, and its gravity at one bar is 1.15 times Earth's surface gravity, beaten only by Jupiter. With no well-defined solid surface, it is built mostly of gases and liquids.
The label "ice giant" sets Neptune and Uranus apart from Jupiter and Saturn, because they are smaller and carry higher concentrations of volatiles. The atmosphere makes up only about 5 to 10 percent of Neptune's mass. Beneath it lies a mantle worth 10 to 15 Earth masses, rich in water, ammonia and methane, called icy even though it is a hot, dense supercritical fluid. Scientists sometimes name it a water-ammonia ocean.
At a depth of 7,000 kilometres, conditions may force methane to break into diamond crystals that rain downward like hailstones. Experiments at Lawrence Livermore National Laboratory hint that the top of the mantle could be an ocean of liquid carbon with solid diamonds floating in it. At the very centre sits a core likely made of iron, nickel and silicates, with a mass about 1.2 times that of Earth. The pressure there reaches roughly 700 gigapascals, about twice the pressure at Earth's centre, and the temperature may climb to 5400 kelvin.
Winds on Neptune reach almost 600 metres per second, faster than the speed of sound. These are the strongest sustained winds of any planet in the Solar System, and most of them blow against the planet's own rotation. At the cloud tops, the prevailing winds run from 400 metres per second along the equator down to 250 metres per second at the poles. At 70 degrees south, a high-speed jet tears along at 300 metres per second.
Neptune radiates about 2.61 times as much energy as it receives from the Sun, a far larger gap than Uranus's 1.1 times. This internal heat fuels the weather, even though Neptune sits over 50 percent farther from the Sun than Uranus and catches only about 40 percent of Uranus's sunlight. The source of the heat remains unknown.
Neptune's axial tilt of 28.32 degrees, close to Earth's, gives it seasons, but its long orbit stretches each season to forty Earth years. In 2007, the upper troposphere of its south pole was found to be about 10 kelvin warmer than the rest of the atmosphere, warm enough to let frozen methane escape into the stratosphere. That hot spot exists because the south pole has faced the Sun for the last quarter of Neptune's year. As the planet moves on, the south pole will darken, the north pole will brighten, and the methane release will shift to the north.
In 1989, NASA's Voyager 2 found the Great Dark Spot, an anticyclonic storm in Neptune's southern hemisphere resembling Jupiter's Great Red Spot. Five years later, on the 2nd of November 1994, the Hubble Space Telescope looked and the Great Dark Spot was gone, replaced by a similar new storm in the northern hemisphere. In 2018 a fresh main dark spot and a smaller one were studied, and in 2023 came the first ground-based observation of a dark spot on Neptune.
Dark spots sit lower in the troposphere than the bright clouds, so they appear as holes punched in the upper cloud decks. Stable for months at a time, they are thought to be vortices, often shadowed by bright methane clouds forming near the tropopause. When a spot drifts too close to the equator, it may dissipate, though the exact mechanism is not fully understood.
Voyager 2's radio astronomy experiment caught around 60 lightning flashes during the 1989 encounter. A plasma wave system recorded 16 electromagnetic wave events, possibly triggered by lightning in the ammonia clouds of the magnetosphere. Neptune is predicted to flash lightning at one nineteenth the rate of Jupiter, concentrated at high latitudes, yet its lightning seems to resemble Earth's more than Jupiter's.
The outermost ring of Neptune, called Adams and sitting 63,000 kilometres from the centre, holds five bright arcs named Courage, Liberté, Egalité 1, Egalité 2 and Fraternité. The laws of motion say arcs should spread into a smooth ring, so their survival was hard to explain. Astronomers now credit the moon Galatea, just inside the ring, with herding them into shape. Two inner rings, the Le Verrier Ring at 53,000 kilometres and the broader, fainter Galle Ring at 42,000 kilometres, complete the three main bands.
The first ring was detected in 1968 by a team led by Edward Guinan. A stellar occultation in 1984 hinted the rings had gaps, when they blocked a star going in but not coming out, and Voyager 2 confirmed the faint, fragmented system in 1989. Images from the W. M. Keck Observatory in 2002 and 2003 showed the rings decaying, with the Liberté arc possibly vanishing within a single century.
Triton, discovered by William Lassell just 17 days after Neptune itself, holds more than 99.5 percent of the mass orbiting the planet. Its retrograde orbit marks it as a captured world, probably once a dwarf planet from the Kuiper belt. It is spiralling inward and will be torn apart in about 28 billion years when it reaches the Roche limit. In 1989 Triton was the coldest object then measured in the Solar System, at an estimated 38 kelvin, kept frigid by its very high albedo. Because Neptune was god of the sea, its 16 known moons carry the names of lesser sea gods, from Nereid with one of the most eccentric satellite orbits in the Solar System to Hippocamp, found in 2013 by combining multiple Hubble images.
Neptune's gravity rules the Kuiper belt, the ring of small icy worlds stretching from its orbit at 30 AU out to about 55 AU. Over the age of the Solar System, certain zones were destabilised into gaps, such as the region between 40 and 42 AU. Yet stable orbits survive where an object's period locks into a simple ratio with Neptune's.
The most crowded of these is the 2:3 resonance, holding over 200 known objects that complete two orbits for every three of Neptune. They are called plutinos because Pluto is among them. Pluto crosses Neptune's orbit regularly, but the resonance guarantees the two can never collide.
Voyager 2 made its closest approach on the 25th of August 1989, the last major planet it could reach. Signals took 246 minutes to travel back to Earth, so the encounter ran mostly on commands planned in advance. The flyby gave the first accurate measurement of Neptune's mass, 0.5 percent less than calculated, a figure that disproved the idea of an undiscovered Planet X tugging at Neptune and Uranus. Future visits remain on the drawing board, including China's IHP-2 probe, which would fly past Neptune in January 2038, passing just 1,000 kilometres above the cloud tops.
Common questions
When was Neptune discovered and who found it?
Neptune was directly observed through a telescope on the 23rd of September 1846 by Johann Gottfried Galle at the Berlin Observatory. He found it within a degree of the position predicted mathematically by Urbain Le Verrier, who shares discovery credit with John Couch Adams.
Why is Neptune called an ice giant?
Neptune is called an ice giant because it is smaller than Jupiter and Saturn and carries higher concentrations of volatiles such as water, ammonia and methane. Its mantle, worth 10 to 15 Earth masses, is rich in these ices, a label it shares with Uranus.
How fast are the winds on Neptune?
Neptune has the strongest sustained winds of any planet in the Solar System, reaching almost 600 metres per second, faster than the speed of sound. These winds are fueled by internal heat, as Neptune radiates about 2.61 times as much energy as it receives from the Sun.
How was Neptune named?
Neptune was named after the Roman god of the sea, identified with the Greek Poseidon. Urbain Le Verrier proposed the name, and Struve endorsed it on the 29th of December 1846 before the Saint Petersburg Academy of Sciences, after the planet's color seen through a telescope.
What is Triton and how was it discovered?
Triton is Neptune's largest moon, holding more than 99.5 percent of the mass orbiting the planet. William Lassell discovered it just 17 days after Neptune itself, and its retrograde orbit indicates it was captured, probably once a dwarf planet from the Kuiper belt.
Has any spacecraft visited Neptune?
Voyager 2 is the only spacecraft to visit Neptune, making its closest approach on the 25th of August 1989. Signals took 246 minutes to reach Earth, and the flyby measured Neptune's mass, discovered six new moons, and revealed more than one ring.
How long does Neptune take to orbit the Sun?
Neptune orbits the Sun once every 164.8 years at an orbital distance of 30.1 AU. On the 11th of July 2011, it completed its first full barycentric orbit since its discovery in 1846.