Neptune Facts

Neptune is now the most distant planet (sorry, Pluto) and is a cold and dark world nearly 3 billion miles from the Sun.

Neptune is the eighth planet from the Sun and the fourth largest (by diameter). Neptune is smaller in diameter but larger in mass than Uranus.

Planet Profile

orbit: 4,504,000,000 km (30.06 AU) from Sun
diameter: 49,532 km (equatorial)
mass: 1.0247e26 kg

History of Neptune

In Roman mythology Neptune (Greek: Poseidon) was the god of the Sea.

After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton’s laws. It was therefore predicted that another more distant planet must be perturbing Uranus’ orbit. Neptune was first observed by Galle and d’Arrest on 1846 Sept 23 very near to the locations independently predicted by Adams and Le Verrier from calculations based on the observed positions of Jupiter, Saturn and Uranus. An international dispute arose between the English and French (though not, apparently between Adams and Le Verrier personally) over priority and the right to name the new planet; they are now jointly credited with Neptune’s discovery. Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune’s actual orbit fairly quickly. Had the search for the planet taken place a few years earlier or later it would not have been found anywhere near the predicted location.

More than two centuries earlier, in 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune’s motion would have been obvious to him. But, alas, cloudy skies prevented observations on those few critical days.

Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Much of we know about Neptune comes from this single encounter. But fortunately, recent ground-based and HST observations have added a great deal, too.

Because Pluto’s orbit is so eccentric, it sometimes crosses the orbit of Neptune making Neptune the most distant planet from the Sun for a few years.

Neptune’s composition is probably similar to Uranus’: various “ices” and rock with about 15% hydrogen and a little helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but rather to be more or less uniform in composition. But there is most likely a small core (about the mass of the Earth) of rocky material. Its atmosphere is mostly hydrogen and helium with a small amount of methane.

Neptune’s blue color is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint.

Like a typical gas planet, Neptune has rapid winds confined to bands of latitude and large storms or vortices. Neptune’s winds are the fastest in the solar system, reaching 2000 km/hour.

Like Jupiter and Saturn, Neptune has an internal heat source — it radiates more than twice as much energy as it receives from the Sun.

At the time of the Voyager encounter, Neptune’s most prominent feature was the Great Dark Spot (left) in the southern hemisphere. It was about half the size as Jupiter’s Great Red Spot (about the same diameter as Earth). Neptune’s winds blew the Great Dark Spot westward at 300 meters/second (700 mph). Voyager 2 also saw a smaller dark spot in the southern hemisphere and a small irregular white cloud that zips around Neptune every 16 hours or so now known as “The Scooter” (right). It may be a plume rising from lower in the atmosphere but its true nature remains a mystery.

However, HST observations of Neptune (left) in 1994 show that the Great Dark Spot has disappeared! It has either simply dissipated or is currently being masked by other aspects of the atmosphere. A few months later HST discovered a new dark spot in Neptune’s northern hemisphere. This indicates that Neptune’s atmosphere changes rapidly, perhaps due to slight changes in the temperature differences between the tops and bottoms of the clouds.

Neptune also has rings. Earth-based observations showed only faint arcs instead of complete rings, but Voyager 2’s images showed them to be complete rings with bright clumps. One of the rings appears to have a curious twisted structure (right).

Like Uranus and Jupiter, Neptune’s rings are very dark but their composition is unknown.

Neptune’s rings have been given names: the outermost is Adams (which contains three prominent arcs now named Liberty, Equality and Fraternity), next is an unnamed ring co-orbital with Galatea, then Leverrier (whose outer extensions are called Lassell and Arago), and finally the faint but broad Galle.

Neptune’s magnetic field is, like Uranus’, oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers.

Neptune can be seen with binoculars (if you know exactly where to look) but a large telescope is needed to see anything other than a tiny disk. There are several Web sites that show the current position of Neptune (and the other planets) in the sky, but much more detailed charts will be required to actually find it. Such charts can be created with a planetarium program.

Neptune’s Satellites

 Neptune has 14 known moons; 7 small named ones and Triton plus four discovered in 2002 and one discovered in 2003 and S/2004 N1 in 2013.

Satellite(000 km)(km)(kg)DiscovererDate
Naiad4829?Voyager 21989
Thalassa5040?Voyager 21989
Despina5374?Voyager 21989
Galatea6279?Voyager 21989
Larissa7496?Voyager 21989
S/2004 N11059?Showalter2013
Proteus118209?Voyager 21989

Neptune’s Rings

Diffuse41900151989N3R, Galle
Inner53200151989N2R, LeVerrier
Plateau5320058001989N4R, Lassell, Arago
Main629301989N1R, Adams

(distance is from Neptune’s center to the ring’s inner edge)

Interesting Facts about Neptune

  • Neptune was named after the Roman god of the Sea. This is because of its blue ocean like colour.
  • Neptune was first discovered in 1846. The planet was found by Jean Joseph Le Verrier. It was discovered later than all the other planets because it is not visible to the naked eye and so was not known to the ancients.
  • At first Neptune was called Le Verrier, named after the man who discovered it. This name did not stick and it was quickly renamed Neptune to fall in line with all the other planets named after Roman gods.
  • Neptune was discovered by watching the orbit of Uranus. It was seen that Uranus was being pulled from its orbit by a gravitational force at the same point each time it went around the Sun. Further investigation led scientists to find Neptune.
  • It takes 164.8 Earth years for Neptune to orbit the Sun. That is a massive 60,190 earth days! This makes it the planet that is slowest to move around the Sun. In fact in 2011 it completed its first full journey around the Sun since Neptune was first discovered in 1846.
  • Neptune is one of the Gas Giants. It is made of layers of 29% helium, 80% hydrogen and traces of methane gas. It does not have a solid surface but is thought to have a solid core, similar to the size of planet Earth.
  • Like Uranus it is sometimes called an Ice Giant because it has an inner layer of water, ammonia and methane ice.
  • Neptune has a Great Dark Spot storm. This is similar to the Great Red Spot on Jupiter. The storm itself is about the same size as planet Earth.
  • Although Neptune is slow to move around the Sun, it spins quickly on its axis making a day on Neptune 18 hours long. This is six hours less than Earth takes to rotate.
  • Neptune is 2793 million miles (4495 million kilometres) from the Sun. As far as we know it is the planet furthest away from the Sun in our Solar System.
  • Neptune is 2700 million miles (4345 kilometres) away from Earth.
  • The Voyager 2 is the only spacecraft to have reached Neptune. It flew past in 1989 and sent the first close up pictures of the planet back to Earth. Because of how far away Neptune is, it was four hours and 6 minutes before the signal reached Earth.
  • Neptune has a ring system. It is nothing quite as striking as Saturn’s ring system but it does have faint rings surrounds the planet. These are most likely made up of grains of dust and ice particles and are reddish in colour.
  • Neptune has 14 moons. The biggest of these is called Triton and was discovered a number of months after the discovery of Neptune. Triton is a very special moon as it orbits Neptunes but turns the opposite direction on its axis. This means they are spinning in opposite directions. Imagine walking around an object like a car (the Sun) whilst spinning around clockwise (Neptune) with a friend walking around you in an anti-clockwise circle (Triton).
  • Neptune’s moons are all named after Greek Water Gods. This is because Neptune is named after the main God of the Sea. The names include Neired, Proteus, Despina and Thalassa.
  • Neptune has a diameter of 29,297 miles (47,150 kilometres). This makes it the third biggest planet in the solar system.
  • Neptune is extremely cold. The planet has an average temperature of -214 degrees Celcius (-353 degrees fahrenheit) The temperature stays low most of the time because it is so far away from the Sun.
  • Neptune has a unique strong magnetic field. It is around 27 times stronger than the one on Earth. It is special compared with other planets because it is not in line with the planet’s axis and so is tipped on its side at 47 degree angle.
  • Currently there are no further missions set to explore Neptune further.

More about Neptune and its satellites

Open Issues

  • Neptune’s magnetic field is off center and at a large angle to its rotation axis. What processes in the interior generate this oddly shaped field?
  • What accounts for the relative lack of hydrogen and helium in Neptune (and Uranus)?
  • Why are Neptune’s winds so strong in spite of the fact that it is so far from the Sun and has a relatively weak internal heat source?
  • What happened to the Great Dark Spot?
  • Can we design a useful Neptune orbiter mission cheap enough to be funded?