Of the four 'gas giants' that inhabit the outer Solar System, Neptune is both the smallest
and the furthest from the Sun - the last major gaseous outpost before Pluto and the Kuiper Belt. As such it has never been
observed in detail by a dedicated orbiter. Nontheless, in 1989 the Voyager 2 spacecraft supplied much information and many pictures about this
distant, mysterious world, before it raced past it into the outer reaches of the solar system. Neptune was in fact the first
planet to be 'predicted', and then subsequently observed. John Adams and Joseph Le Verrier independantly proposed an orbit
for the planet in 1845, based on peculiarities in Uranus' orbit, and observations a year later by German astronomer Johann Galle confirmed this prediction.
Named after the Roman God of the
Sea Neptune is certainly reminiscent of sealike colours, as it appears a shade of blue-green to us from space.
Underneath Neptune's atmosphere however, scientists believe there to be an ocean of liquid methane
and ice slush surrounding a rocky core. The reason for Neptune's blue colour however is essentially the same as for Uranus - because the Methane
in the planet's atmosphere absorbs red light, so the sunlight reflected back to us from the atmosphere is predominantly blue. Neptune's atmosphere extends very far down, eventually merging into water and other melted ices,
above its liquid outer core, which is approximately the same size as the planet Earth.
It is assumed that the composition of Neptune is similar to that of Uranus, so we're looking at a uniform distribution
of elements, rather than the internal layering which characterises Jupiter and to a lesser degree Saturn. Interestingly, Neptune's winds are up to nine times stronger
than those on Earth, and three times stronger than those on Jupiter, despite its low energy input and great distance from the Sun.
Neptune is quite similar to Jupiter in that it has
several oval hurricanes. It also has one 'Great Dark Spot' which, like Jupiter's famed Great Red Spot
is a massive hurricane/storm. Unlike the Red Spot though, it seems to have recently vanished from view, leaving Astronomers unsure as to whether it still exists.
Not much was known about Neptune before it was
visited by the Voyager space probe, which made many interesting discoveries, for instance it cleared up the
'Neptune arc' problem. When viewed from Earth, faint arcs were spotted around Neptune, but Voyager showed that there are
actually three complete rings around the planet, which vary in thickness.
Neptune's rotational axis is tilted at a large 30° to the plain of its solar orbit, and consequently the
planet experiences some rather extreme seasons, each lasting 41 years. During its 'summer' season therefore, each pole is in constant sunlight for
41 years! Due to the very elliptical nature of Pluto's orbit, on certain occasions Neptune can end up being further away from the Sun than
Pluto, as its orbit cuts across Neptune's. On average however, Neptune's orbit is a massive 4,500 million km away from the Sun - 30
times the distance between the Sun and the Earth.
Neptune's eight moons are: Naiad, Thalassa, Depoina, Galatea,
Larissa, Proteus, Triton and Nereid. The most interesting is Triton which has many
geyser-like eruptions, spewing invisible Nitrogen gas and dust particles many kilometres
into the atmosphere. It's an icy world of frozen methane just 2720 km wide - smaller than
Earth's moon! To find out more look at our Triton section.