Neptune was discovered by Johann Galle and Heinrich d'Arrest at the Berlin Observatory on 23 September 1846, following a request for a search by French mathematician Urbain Le Verrier. Le Verrier had calculated the position of a hypothesized new planet based on discrepancies in Uranus's orbit. British astronomer George Airy challenged Le Verrier's prediction priority, claiming that John Couch Adams had predicted the planet's position before Le Verrier, which led to an international controversy between the astronomical communities in France and the United Kingdom. Both claims were given credence, as the British Royal Society awarded Le Verrier the prestigious Copley Medal for 1846 and gave Adams the same award two years later. In the mid-1990s, evidence from some of Airy's rediscovered papers showed that Adams kept modifying his predictions, which complicated the search for the new planet. When he produced the first calculated orbit for Neptune just a few months after its discovery, Adams was surprised to find that its mean distance from the Sun was just 30 AU, rather than the 36 AU assumed by Le Verrier, or the 37 AU assumed by himself.
Less than a month after Neptune's discovery, William Lassell discovered a moon, Triton, close to Neptune. Further work showed its orbit to be circular, inclined at about 30º to the ecliptic, and retrograde. Astronomers believed that as Triton was relatively large and close to Neptune, they both must have been formed at the same time, and so Neptune's spin was probably also retrograde. In 1949 Gerard Peter Kuiper discovered Nereid in a prograde, but elliptical orbit. The discovery of Triton allowed astronomers to estimate Neptune's mass as almost the same as Uranus. Although Neptune was about 50 percent farther from the Sun than Uranus, far infrared measurements in the early 1970s showed that Neptune was warmer than Uranus, indicating that it had an internal heat source.
Angelo Secchi obtained the first spectra of Neptune and Uranus in 1869, showing that both had broad absorption bands, with a particularly broad one in the yellow/orange region, explaining both planets’ blue appearance. Rupert Wildt showed in the early 1930s that these bands, and others found since, were caused by methane in the planets’ atmospheres. Based on spectra provided by Kuiper, Gerhard Herzberg announced in 1952 the discovery of molecular hydrogen in the atmosphere of Neptune. Before the Voyager 2 encounter with Neptune in 1989, ammonia and ethane had been detected in trace amounts. Astronomers estimated Neptune's rotation period as about 17.6–17.8 hours prograde, based on observed cloud motions.
The discovery of rings at Uranus (1977) and at Jupiter (1979) meant that Neptune was apparently the only gas giant planet without rings. A flurry of stellar occultation observations in the early 1980s provided no firm evidence for rings, although some of the observations yielded positive detections, but only on one side of the planet. In July 1984 two observatories obtained evidence of a partial ring or ring arc, though its true nature did not become clear until the Voyager 2 encounter five years later.
In 1978 Dale Cruikshank and Peter Silvaggio detected methane on Triton. In 1983 Cruikshank, Roger Clark, and Hamilton Brown detected nitrogen. In the following year, Lawrence Trafton predicted that Triton would have seasonal caps, probably of methane ice, condensing out of a tenuous methane atmosphere.
Voyager 2 arrived at Neptune in August 1989. It discovered six new moons, all interior to the orbit of Triton. It also found four narrow rings and one or more broad, dusty rings. The outermost (Adams) ring contained five ring arcs, which were strung together over about 10 percent of the ring circumference and were likely the source of most Earth-based positive detections. Voyager 2 provided vastly improved values for the mass of both Neptune and Triton. The helium mass fraction is 0.32 ± 0.05, identical to that of the Sun, within the given uncertainties. The planet's equatorial radius of 24,764 km is about 3 percent smaller than Uranus, but its higher density (1.638 g cm-3 ) yields a mass 19 percent higher than Uranus. The ratio of the total radiated heat to that absorbed from the Sun is 2.69, by far the highest of the planets. The magnetic dipole field tilt is large (47°; Uranus has a 59° tilt), and it is offset 55 percent of the way from the planet center to its cloud tops. Voyager 2 measured the rotation period from radio emissions to be 16.1 hours, although various cloud features rotated around the planet in periods ranging from 15.8–18.3 hours. Detailed images of the atmosphere, the rings, and Triton were obtained. Methane cloud altitudes, relative to the main cloud deck, were found to be about 50 km from shadow measurements. Several storm systems were imaged, including a feature named the Great Dark Spot. Triton's reflectivity is high, and its surface temperature is 30 K, making it the coldest surface in the solar system. Despite its frigidity, Voyager 2 found active plumes spewing nitrogen gas and trapped surface material about 8 km into a thin nitrogen atmosphere from beneath a nitrogen (not methane) ice cap. There are also strange grooves and a cantaloupe-like terrain on the non-ice-covered surface.
In Earth-based observations since the Voyager 2 encounter, five additional moons have been discovered, bringing the total to thirteen as of early 2007. Recent images of Neptune from the Hubble Space Telescope and from the Hawaii Infrared Telescope Facility show that the Great Dark Spot has disappeared and other storms have appeared, so the atmospheric storms appear relatively short-lived.
See also: Voyager
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