|Size (diameter)||49,244 km, or about 4 Earths|
|Mass||1.0 × 1026 kg, or about 17 Earths|
|Length of a year (orbital period)||165 years|
|Number of moons||14|
|Average distance from the Sun||4.5 billion km|
|Temperature||Between -218 and -200 degrees Celsius|
The eighth planet from the Sun, Neptune is considered an ice giant because it is largely made up of water, ammonia, and methane in solid form. As our solar system took shape about 4.5 billion years ago, Neptune was likely formed in a massive, ancient cloud of gas, dust, and ice which collapsed into a spinning disc with our Sun at its centre. Scientists believe the differences between ice giants and gas giants (like Saturn and Jupiter) are due to their slightly different formation histories.
Rotation and orbit
Based on measurements taken by the Voyager 2 probe, different parts of Neptune may rotate at different speeds since the planet is not a solid body. Neptune's equator seems to rotate once every 18 hours while its polar regions spin once every 12 hours. This difference in rotational speed between the different regions is the largest of any planet and causes the strongest winds in the solar system, as fast as 2100 km/h!
Neptune takes 165 years to complete one full orbit around the Sun.
After about 75 years of being a planet, Pluto was reclassified as a dwarf planet by the International Astronomical Union in . Neptune therefore regained its status as the outermost planet in the solar system.
Atmosphere and surface
Like the other giant planets in our solar system, Neptune does not have a solid surface. Scientists believe that the interior of the planet is made up of a solid rocky core covered by a hot and dense liquid layer of water and ammonia. The atmosphere surrounding the interior of the planet is made up of mostly hydrogen and helium.
Neptune's blue colour is caused by the methane found in its atmosphere, which absorbs red light. Scientists are unsure why Uranus and Neptune are different shades of blue despite their very similar atmospheres. Like that of Jupiter, Neptune's atmosphere contains many storm systems including the Great Dark Spot, which is about as wide as Earth.
When astronomers in the 19th century first studied the orbit of Uranus, they observed that the planet's path around the Sun differed from their expectations in a way that suggested it was being acted upon by other forces. They suspected that the variation might be caused by the existence of an unknown planet beyond Uranus. Using complex calculations, two different astronomers independently predicted the location of this new planet, Neptune, in the sky – and in , it was found at almost that exact spot! Although it had been observed beforehand, Neptune is the only planet to have been discovered through mathematical calculation.
Exploration of Neptune
Launched in to explore the outer solar system, NASA's Voyager 2 was the only spacecraft to fly by Neptune. It observed Neptune from a distance of about 4400 km in after having studied Uranus. The probe gathered information on Neptune's magnetic field, atmosphere, and several of the planet's moons. Voyager 2 also discovered Neptune's Great Dark Spot.
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