Neptune seen by Voyager 2 in 1989 at a range of 7.1 million
km (4.4 million miles). At center is the Great Dark Spot
Neptune is normally the eighth planet from the Sun, although the dwarf planet Pluto,
because of its elongated orbit, occasionally comes closer for periods of
20 years around its perihelion. Neptune
is the most distant of the gas giants.
It is also marginally the smallest of the gas giants in the solar system,
after Uranus, although its volume is still
60 times greater than that of the Earth and it is slightly more massive
than Uranus. Neptune and Uranus are more appropriately referred to as ice
giants because their composition is substantially different from that
of Jupiter and Saturn.
Discovery and naming
Neptune was discovered in 1846 by Johann Galle and Louis d'Arrest following predictions
by Urbain Leverrier. In fact, Galileo Galilei, as his notes reveal, had seen Neptune much earlier, in 1612 and
again in 1613, but thought it was a star. The name "Neptune,"
after the Roman god of the sea, was first suggested by Leverrier and adopted
internationally soon after.
Atmosphere and weather
Neptune's deep atmosphere extends 10–20% of the way to the center
of the planet. At high altitudes it consists mostly of hydrogen (80%) and helium (19%). Increasing concentrations
of methane, other hydrocarbons (including ethane, acetylene, and diacetylene), ammonia,
and water vapor occur in the dark, warmer, lower regions of the atmosphere.
Finally the atmosphere blends into the superheated liquid interior.
|The Great Dark Spot imaged by Voyager at a distance
of 2.8 million km (1.7 million miles). The smallest detail visible
is about 50 km (31 miles) across
Neptune's blue color is mainly due to methane in the atmosphere, which absorbs
red light. There is a noticeable difference, however, between the vivid
blue of Neptune and the blue-green of Uranus, whose coloration is also the
result of atmospheric methane. Some additional component must be responsible
for Neptune's distinctive hue but the identity of this extra component remains
Despite its great distance from the Sun and lower energy input, Neptune
has the strongest winds recorded on any planet – three times stronger
than Jupiter's and nine times stronger than Earth's. In 1989, Voyager
2 tracked a large, dark, oval storm in Neptune's southern hemisphere.
This hurricane-like "Great Dark Spot"
was big enough to contain the entire Earth, spun counterclockwise, and moved
westward at almost 1,200 km/hr (750 mph). Winds blowing near the Great Dark
Spot were clocked at up to 2,400 km/h (1,500 mph). (Subsequent images from
the Hubble Space Telescope showed no sign of the Great Dark Spot. A comparable
spot appeared in 1994 in Neptune's northern hemisphere but had disappeared
by 1997.) Voyager 2 also photographed clouds casting shadows on a lower
cloud deck, enabling scientists to visually measure the altitude differences
between the upper and lower cloud decks. Another feature revealed by Voyager
was a small, irregularly shaped, eastward-moving cloud, which scudded around
Neptune every 16 hours or so. Nicknamed "Scooter" it is thought that it
might have been a plume rising above a deeper cloud deck.
Below the atmosphere is a liquid hydrogen layer including helium and methane.
The lower layer is liquid hydrogen compounds, oxygen, and nitrogen. It is
believed that the planet's core comprises rock and ice. Average density,
as well as the greatest proportion of core per planet size, is the greatest
among the gaseous planets.
|Credit: Lunar and Planetary Institute
It probably has a rocky core covered by a icy crust, buried deep under a
thick atmosphere. The first two thirds of Neptune is thought to be composed
of a mixture of molten rock, water, liquid ammonia and methane; the outer
third is a mixture of heated gases.
As in the case of Uranus, Neptune's magnetic field is strongly tilted relative
to its rotational axis, by 47°, and offset at least 0.55 radii (about 13,500
km) from the planet's physical center. Comparing the magnetic fields of
the two planets, scientists think the extreme orientation may be characteristic
of flows in the interior of the planet and not the result of Uranus' sideways
Moons and rings
Neptune has 4 rings (see Neptune, rings)
and 13 known moons (see Neptune, moons),
the largest of which is Triton. See also Neptune Trojan.
Exploration by spacecraft
Neptune has been visited to date by only one spacecraft, Voyager 2, which
flew by on August 25, 1989. Various other Neptune
missions have been discussed but nothing is currently planned or being
||4,498 million km (2,796 million miles, 30.1 AU)
||49,528 km (30,782 miles), 3.88 × Earth
||16.11 hours (16 h 6 min 36 s)
|mass (Earth = 1)
|gravity at cloud tops (Earth =
||23.5 km/s (84,600 km/h, 52,600 mph)
||80% hydrogen, 18.5% helium, 1.5% methane
|mean temperature (cloud tops)
||-220 ºC (-364 ºF)
|number of moons