The sky — astronomy

Neptune: The Planet Found by Mathematics

Neptune is the eighth and farthest planet from the Sun, and the first to be found through a mathematical prediction. In 1846 Urbain Le Verrier calculated where it should be, and Johann Galle spotted it in the sky. Voyager 2 in 1989 remains the only spacecraft to have visited it.

A prediction before the observation

Neptune is a rare case in the history of astronomy: the planet was located on paper before it was seen in a telescope. After Uranus was discovered in 1781, astronomers tracked its orbit and noticed that it did not move quite as Newton’s law of gravitation predicted. Perhaps another, unknown planet was pulling on it.

The French mathematician Urbain Le Verrier and the Briton John Couch Adams worked on the problem independently. Each calculated where such a planet would have to be to explain the discrepancies. Le Verrier sent his prediction to the Berlin Observatory. On the night of 23 September 1846, Johann Galle, assisted by Heinrich d’Arrest, pointed a telescope at the indicated spot and found an object that was not on the star charts. It lay about one degree from the predicted position. Historians often call this a triumph of celestial mechanics, the branch of physics that calculates how bodies move under gravity.

A dispute over priority followed between France and Britain. Today both calculators are credited.

What kind of planet it is

Neptune is an ice giant, like Uranus. It orbits about 30 astronomical units from the Sun (one astronomical unit is the average Earth–Sun distance), roughly 4.5 billion kilometres. One trip around the Sun takes about 165 Earth years, and in 2011 Neptune completed its first full orbit since its discovery.

Neptune’s diameter is about four times Earth’s, and its mass is about 17 times greater. A day there lasts about 16 hours. The planet is invisible to the naked eye; you need binoculars or a telescope, and even then it looks like a tiny bluish dot. For tips on getting started, see how to observe the sky.

Atmosphere and winds

Neptune’s atmosphere is mostly hydrogen and helium with a little methane. Methane absorbs red light, which is why the planet looks blue. Temperatures at the cloud tops are around −200 °C.

Neptune has the fastest winds measured in the Solar System: according to NASA they exceed 2,000 kilometres per hour. This is surprising, because the planet receives very little sunlight. Scientists are still studying what powers these storms.

In 1989 Voyager 2 photographed a huge dark storm called the Great Dark Spot, an enormous anticyclone. When the Hubble Space Telescope looked in 1994, that spot had vanished, showing how quickly Neptune’s atmosphere can change.

The planet also has a system of thin, faint rings. The five main ones are named Galle, Le Verrier, Lassell, Arago and Adams.

Triton

Neptune has 16 known moons. The largest is Triton, discovered by the British astronomer William Lassell only weeks after the planet itself. It is about 2,700 kilometres across.

Triton orbits Neptune in the direction opposite to the planet’s rotation, a retrograde orbit. The leading scientific explanation is that Triton was captured by Neptune’s gravity after originating in the Kuiper belt, the region of icy bodies beyond Neptune’s orbit. If so, Triton is likely a relative of Pluto in origin.

Triton’s surface is extremely cold, about −235 °C, and covered mostly with nitrogen ice. Voyager 2 saw dark streaks that are traces of plumes (geysers) of nitrogen gas and dust. It is one of the very few places in the Solar System where such activity is known on a body this cold.

The Voyager 2 visit

Voyager 2 was launched in 1977 and is still the only spacecraft to have flown past Neptune. Its closest approach came on 25 August 1989, a few thousand kilometres above the cloud tops near the north pole. It sent back images of storms, rings and Triton, and discovered several small new moons. Much of what we know about Neptune comes from those few days.

Since then the planet has been studied from Earth with powerful telescopes such as Hubble and large ground-based observatories. No dedicated new mission to Neptune has yet flown.

Neptune in culture

This article treats Neptune as an astronomical object. For how it is interpreted in the symbolic tradition of astrology, see Neptune in astrology. That is a cultural tradition, not scientific fact.

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