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NEPTUNE

The Outermost Planet

The last major planet in the solar system, Neptune is another world of superlatives. It’s the farthest planet from the Sun, at an average distance of 4.5 billion kilometers (2.8 billion miles). It takes 165 Earth years to circle the Sun once. At 24,764 kilometers (15,338 miles) across, it’s the smallest of the gas giants. It’s also the coldest world of all the planets, with temperatures dipping down to –221°C (–365.8°F) at the top of its atmosphere. Neptune also has the strongest winds. They blow up to 2,100 kilometers (1,304 miles) per hour. Those winds, in combination with the rotation of the atmosphere and planet, the almost Earth-like tilt of its axis, plus an unknown source of internal heat, give Neptune distinct seasons and drive some very fierce storms.

Neptune has a layered interior structure somewhat similar to the other gas giants, but with a few differences. There’s a small rocky core, covered with a mantle made of a mixture of water, methane, and ammonia ice. Above those are a lower atmosphere consisting of hydrogen, helium, and methane gases and an upper atmosphere with clouds that contain ice particles. The methane gases in the top of the atmosphere give Neptune its characteristic deep blue color. Since Neptune has a higher ice content than Jupiter or Saturn, it’s often referred to as an ice giant.

Like the other planets of the solar system, Neptune formed in the same cloud of gas and dust as the Sun some 4.5 billion years ago. It may have formed closer to the Sun, along with the other planets, and then migrated out to its present position.

Neptune Facts

Closest point to Sun: 4.4 billion kilometers (2.7 billion miles)
Most distant point from Sun: 4.5 billion kilometers (2.8 billion miles)
Length of year: 164.8 Earth years
Length of day: 16.1 hours (based on rotation of interior)
Tilt of axis: 28.32 degrees
Gravity: 1.12 Earth’s gravity

Neptune’s Moons and Rings

There are thirteen known moons orbiting Neptune. They probably didn’t form with Neptune, but were captured later on by the planet’s gravitational pull. The largest moon, Triton, was discovered in 1846 and was imaged in great detail by the Voyager 2 spacecraft in 1986. Triton orbits its planet in what’s called a retrograde orbit. That means it orbits against Neptune’s rotation. This alone is a good clue that Triton didn’t form with the planet but was captured later on. It’s 2,700 kilometers (nearly 1,700 miles) across, making it one of the largest moons in the solar system.

Like its sibling gas giants, Neptune has a ring system, although it’s not nearly as massive as Saturn’s. Very little is known about the particles that make up the rings, but scientists suspect that they may be ice particles coated with some kind of complex organic material. Oddly enough, the rings seem to be disappearing, and the reasons why are still being debated.

Although Triton had been observed from Earth for years, nobody expected to find what Voyager 2 found when it swept past in 1989. This frozen moon has a mottled surface made of nitrogen, water, and methane ices. A large part of the surface is covered with cantaloupe terrain, so called because it looks like the skin of that fruit. There’s also a rock-metal core hidden deep inside. Most fascinating of all is that some kind of internal activity drives nitrogen geysers up into Triton’s thin atmosphere. They show up as dark plumes that rise some 8 kilometers (about 5 miles) above the frosty surface.

Who Discovered Neptune?

The tale of Neptune’s discovery is a triumph of mathematical prediction by a man named Urbain Le Verrier (1811–1877). Actually, Neptune was inadvertently observed by such a luminary as Galileo Galilei, who thought he’d found a star. Others spotted this dim object (which can’t be seen with the naked eye), but failed to recognize it as a planet. Fast-forward to the 1840s, when observers were trying to determine what was perturbing the orbit of Uranus. Several predicted that the gravitational pull of another planet was tugging on Uranus. People set to work doing mathematical calculations, and in 1846, Le Verrier presented his work showing where an outer planet might be found.

Another astronomer, John Couch Adams, had been working on similar calculations and also predicted the position of a hidden planet, although not as accurately as Le Verrier had done. On September 24, 1846, astronomers in Berlin, using Le Verrier’s predictions, found Neptune. After a great deal of back-and-forth between France and England, both Adams and Le Verrier were given credit for the discovery. In recent years, new evidence has been found to support Le Verrier’s claim to Neptune’s discovery.

Exploring Neptune

Only one spacecraft has ever visited Neptune. On August 25, 1989, the Voyager 2 mission flew by the planet. It sent back a collection of high-resolution images of Neptune and Triton and probed the strength of the planet’s magnetic field. Oddly enough, that magnetic field is tilted away from the rotational axis of the planet. Voyager 2 discovered six moons and a ring that hadn’t been observed from Earth. While the spacecraft was in the system, it also captured images of several storms that astronomers nicknamed the Great Dark Spot, Scooter, and Dark Spot 2.

Most of what we know about Neptune these days comes from observations made using ground-based telescopes and the Hubble Space Telescope. There are no active plans for new missions to Neptune, although there is ongoing research into new generations of long-duration robotic probes that could be sent to the outer planets in the distant future.

One of the great finds of the Voyager 2 mission was the discovery of a giant anti-cyclonic storm raging through the planet’s upper cloud decks. It looked dark, so it was immediately dubbed “The Great Dark Spot.” It lasted for several years, disappearing before it could be imaged by the Hubble Space Telescope (which had not yet been launched when the Voyager 2 spacecraft got to Neptune). Other storms have come and gone, and in recent years a new “Great Dark Spot” has emerged in the northern hemisphere.