Larger image and description
Discovered byUrbain Le Verrier
John Couch Adams
Johann Galle
Discovered in1846
Orbital characteristics
Mean radius4,498,252,900 km
Revolution period164y 288d 13h
Synodic period367.5 days
Avg. Orbital Speed5.4778 km/s
Number of satellitess11
Physical characteristics
Equatorial diameter49572 km
Surface area7.65×109 km2
Mass1.024×1026 kg
Mean density1.64 g/cm3
Surface gravity11.0 m/s2
Rotation period16h 6.5m
Axial tilt29.58°
Escape Speed23.71 km/s
Surface temp
50K53KN/A K
Atmospheric characteristics
Atmospheric pressurekPa

Neptune is the eighth planet from the sun in our solar system. It is a gas giant, named after the Roman god of the sea.

After the discovery of Uranus, it was noted that the orbits of Uranus, Saturn, and Jupiter were not acting according to the laws of Kepler and Newton. Adams and Le Verrier independently calculated the location of another planet, Neptune, which was located by Galle, on September 23, 1846; within 1° of where Adams and Le Verrier had predicted it to be. Later, it was noted that Galileo had observed Neptune, in 1611, but he had thought it was a star.

Table of contents
1 Physical characteristics
2 The exploration of Neptune
3 The moons of Neptune

Physical characteristics

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.

The internal structure resembles that of Uranus - a rocky core covered by an icy crust, buried deep under its thick atmosphere. The inner 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.

The exploration of Neptune

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.

The moons of Neptune

Neptune has a faint planetary ring system of unknown composition. 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. Evidence that the rings are incomplete first arose in the mid-1980s, when stellar occultation experiments were found to occasionally show an extra "blink" just before or after the planet occulted the star. Images by Voyager 2 in 1989 settled the issue, when the ring system was found to contain several faint rings, the outermost of which, Adams, contains three prominent arcs now named Liberty, Equality and Fraternity. The existence of arcs is very difficult to understand because the laws of motion would predict that arcs spread out into a uniform ring over very short timescales. The gravitational effects of Galatea, a moon just inward from the ring, are now believed to confine the arcs. Several other rings were detected by the Voyager cameras. In addition to the narrow Adams Ring 63,000 km from the center of Neptune, the Leverrier Ring is at 53,000 km and the broader, fainter Galle Ring is at 42,000 km. A faint outward extension to the Leverrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57,000 km.

Neptune has eleven known moons.

Neptune's natural satellites
Name Diameter (km) Mass (kg) Mean orbital
radius (km)
Orbital period
Naiad 58 Unknown 48,200 0.294396 days
Thalassa 80 Unknown 50,000 0.311485 days
Despina 148 Unknown 52,600 0.334655 days
Galatea 158 Unknown 62,000 0.428745 days
Larissa 193 (208 × 178) Unknown 73,600 0.554654 days
Proteus 418 (436 × 416 × 402) Unknown 117,600 1.122315 days
Triton 2700 2.14×1022 354,760 -5.87685 days
Nereid 340 Unknown 5,513,400 360.1362 days
S/2002 N2* 17.5 Unknown 19,556,000 2405.21 days
S/2002 N3* 17.5 Unknown 20,857,000 2649.22 days
S/2002 N1* 17.5 Unknown 21,930,000 2856.30 days

* Awaiting confirmation and naming.