Neptune

Neptune

**Neptune** is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth and slightly larger than Neptune.

Neptune is not visible to the unaided eye and is the only planet in the
Solar System found by mathematical prediction rather than by empirical observation. Neptune is similar in composition to Uranus, and both have compositions that differ from those of the larger gas giants, Jupiter and Saturn. Like Jupiter and Saturn, Neptune's atmosphere is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, but contains a higher proportion of "ices" such as water, ammonia, and methane. However, its interior, like that of Uranus, is primarily composed of ices and rock, and hence Uranus and Neptune are normally considered "ice giants" to emphasise this distinction. Traces of methane in the outermost regions in part account for the planet's blue appearance.

In contrast to the hazy, relatively featureless atmosphere of Uranus,
Neptune's atmosphere has active and visible weather patterns. For example, at the time of the *Voyager 2* flyby in 1989, the planet's southern hemisphere had a Great Dark Spot comparable to the Great Red Spot
 on Jupiter. These weather patterns are driven by the strongest
sustained winds of any planet in the Solar System, with recorded wind
speeds as high as 2,100 kilometres per hour (580 m/s; 1,300 mph). Because of its great distance from the Sun, Neptune's outer atmosphere is one of the coldest places in the Solar System. Neptune has a faint and fragmented ring system (labelled "arcs").

## Physical Characteristics

Neptune's mass of 1.0243×1026 kg, is intermediate between Earth and the larger gas giants: it is 17 times that of Earth but just 1/19th that of Jupiter.

Neptune's weather is characterised by extremely dynamic storm systems,
with winds reaching speeds of almost 600 m/s (2,200 km/h;
1,300 mph)—nearly reaching supersonic flow. More typically, by tracking the motion of persistent clouds, wind speeds
 have been shown to vary from 20 m/s in the easterly direction to
325 m/s westward. Most of the winds on Neptune move in a direction opposite the planet's rotation.

In 1989, the Great Dark Spot, an anti-cyclonic storm system spanning 13000 × 6600 km, was discovered by NASA's *Voyager 2* spacecraft. The storm resembled the Great Red Spot of Jupiter. Some five years later, on 2 November 1994, the *Hubble Space Telescope*
 did not see the Great Dark Spot on the planet. Instead, a new storm
similar to the Great Dark Spot was found in Neptune's northern
hemisphere.

## Satellites

Neptune has 14 known moons. Triton is the largest Neptunian moon, comprising more than 99.5% of the mass in orbit around Neptune, and it is the only one massive enough to be spheroidal. Triton has a retrograde orbit, indicating that it was captured rather than forming in place; it was probably once a dwarf planet in the Kuiper belt. It is close enough to Neptune to be locked into a synchronous rotation, and it is slowly spiralling inward because of tidal acceleration. It will eventually be torn apart, in about 3.6 billion years, when it reaches the Roche limit.

### Rings of Uranus

Neptune has a planetary ring system, though one much less substantial than that of Saturn.
 The rings may consist of ice particles coated with silicates or
carbon-based material, which most likely gives them a reddish hue.