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Neptune is the eighth planet from the sun. It is a gas giant.

After the discovery of Uranus, it was noted that the orbits of Uranus, Saturn, and Jupiter were apparently being perturbed by an additional unknown mass in the outer solar system. John Couch Adams and Urbain Jean Joseph Le Verrier used calculations to predict the region of the sky where the mysterious additional planet was likely to be located, and it was subsequently independantly discovered by the German astronomer Johann Gottfried Galle of Berlin Observatory on September 23 1846. Later it was found that Galileo had much earlier observed Neptune. Evidence of the fact was found in his notes, but he had thought it was a star. Had he observed Neptune only a matter of days earlier, its orbital motion would have been far more obvious. With an orbital period of 165 years, Neptune will first return to the point in its orbit where Galle discovered it in 2011. Due to Pluto's eccentric orbit, Neptune is sometimes the farthest known planet from the Sun.

Neptune is never visible with the naked eye. With the use of a telescope it appears as a blue-green disk, similar in appearance to Uranus; the blue-green colour comes from the methane in its atmosphere. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25 1989.

Orbiting so far from the sun Neptune receives very little heat. Its 'surface' temperature is -218 degrees Celsius (below zero). However, the planet seems to have an internal source of heat. It is thought that this may be leftover heat generated by infalling matter during the planet's birth, now slowly radiating away into space. Neptune's atmosphere has the highest wind speeds in the solar system, up to 2000 km/h, thought to be powered by this flow of internal heat.

Neptune's atmosphere contains :

The internal structure resembles that of Uranus - a rocky core covered by a icy crust, buried deep under its thick atmosphere. The first two thirds of Neptune is composed of a mixture of molten rock, water, liquid ammonia and methane. The outer third is a mixture of heated gases comprised of hydrogen, helium, water and methane. Like Uranus, and unlike the uniform composition of Jupiter and Saturn, Neptune's internal structure is thought to consist of distinct layers. Like Uranus, Neptune's magnetic field is strongly tilted relative to its rotational axis at 47° and offset at least 0.55 radii (about 13,500 kilometers) 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 orientation.

Neptune has a faint planetary ring system of unknown compostion. The rings have a peculiar "clumpy" structure, the cause of which is not currently known but which may involve gravitational interaction with small moons in orbit near them.

Data for Neptune:

  • Mass: 1.0247×1026 kg (17.2 earth masses)
  • Equatorial diameter: 49,532 km
  • Surface Area: 7,650,000,000 km2
  • Mean relative gravity: 1.66
  • Surface gravity: 1.23 (Earth = 1)
  • Mean orbital radius: 4,504,000,000 km (30.06 AU)
    • Aphelion: 4,539,800,000 km
    • Perihelion: 4,432,500,000 km
  • Orbital period (length of year): 165 years
  • Rotational period: 15.8 hours
  • Axial tilt: 29.56°

Neptune has eight known moons.

Neptune's natural satellites
NameOrbital radius (km)Diameter (km) Mass (kg)
Naiad 48,200 58 Unknown
Thalassa 50,000 80 Unknown
Despina 52,600 148 Unknown
Galatea 62,000 158 Unknown
Larissa 73,600 193 (208 x 178) Unknown
Proteus 117,600 418 (436 x 416 x 402) Unknown
Triton 354,760 2700 2.14×1022
Nereid 5,513,400 340 Unknown

Solar system:
Sun - Mercury - Venus - Earth - Mars - Asteroids - Jupiter - Saturn - Uranus - Neptune - Pluto - Comets