Neptune, the eighth most-distant planet in the solar system, is unique: its existence was first calculated mathematically, before it was ever located by astronomers. As the last of the gas giants, it is now the farthest planet in the solar system (after Pluto was reduced to “dwarf planet” status in 2006). Like the closer gas giant Uranus, no space probe has ever been sent to study Neptune; all of our close-in analysis, we got from the Voyager flybys during the 1980s.
ICE GIANT
All gas giants share several characteristics in common: a small, Earth-sized rocky core, surrounded by an enormous atmosphere of hydrogen and helium gas. Neptune is larger than Uranus, but is dwarfed by the other gas giants: it is 17 times as heavy as Earth, but only a few percent of the mass of Jupiter.
In the case of the more distant, colder planets, Uranus and Neptune, this cloud also consists of a large layer of liquid and ice, mostly water, methane and ammonia. In Neptune’s case, this atmosphere is tinged an attractive blue colour (probably Neptune’s best-known feature) as a result of high methane content. We will need to send a probe to learn much more about its weather systems, but, in general, Neptune is considerably warmer than Uranus, and therefore has stormier weather. Its winds whip about the planet at nearly supersonic speeds, and Voyager spotted two Dark Spots (similar to Jupiter’s Great Red Spot, though much smaller) when it sped by in the 1980s.
RINGS AND MOONS
All of the gas giants in our solar system have at least some form of ring system – although none are as beautiful or distinctive as the enormous shimmering disks around Saturn. Neptune’s, like Uranus’s, are red-tinged and filled with dark, mostly icy material. Only three sets of rings have been identified, and scientists currently believe that Neptune’s rings are much less long-lived and durable than those of the other gas giants. According to the New Scientist, some will even be gone within the next century.
Neptune also has 13 moons. For the most part, these have proved less exciting than the larger moons around Jupiter and Saturn, like Europa and Titan. (On the other hand, part of the lack of interest stems simply from ignorance: we don’t know as much about Neptune’s moon as we do about Saturn’s or Jupiter’s.) The largest moon, Triton, is exceptional; it’s one of the largest in the solar system, and is probably a Kuiper Belt dwarf planet captured by Neptune’s gravity. This would place it in the same category of objects as Pluto and Eris, except that those still orbit the Sun directly. Triton has a very thin atmosphere and its surface is pockmarked by geyser or volcano activity.
DISCOVERY AND EXPLORATION
Neptune is too far from the Earth to be seen with the naked eye. As a result, although it had been sighted several times and identified as a star by early astronomers, it was not until the 19th century that mathematical analyses of Uranus’s orbit showed it was too far away from the Sun. John Herschel and others suggested, and ultimately confirmed, that the reason for Uranus’s strange orbit was that it was being tugged outwards by the gravity of another planet: Neptune.
Until the present, most detailed study of the planet has been limited to Earth-based and orbital telescopes, especially the Hubble Space Telescope. The Voyager 2 space probe travelled within several thousand miles of Neptune and one of its moons in 1989. Since then, there have been no further spacecraft to visit Neptune. NASA continues to hold a concept for a Neptune space probe in reserve, but funding restrictions mean that other, more popular missions have been prioritized for the foreseeable future.
