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Mark Thompson
Sep 5, 2011, 7:00 UTC

Sen— The eighth and final major planet from the Sun, Neptune is unique amongst its planetary neighbours as its eventual discovery was the result of mathematical calculation. Discovery aside, it has a very similar composition to Uranus and at just under 4 times the size of the Earth deserves to be considered amongst the realm of the giant planets.

Despite Galileo having observed Neptune back in the 17th Century, he failed to identify its true nature so it wasn’t discovered properly until careful observations of the movement of Uranus highlighted strange discrepancies in its orbit. These were mathematically calculated to be the result of another giant planet orbiting further out from the Sun and in September of 1846, Neptune was first identified. There is some controversy around who actually ‘discovered’ it since two mathematicians working completely independently predicted its position. International agreement holds that it was discovered by Urbain Le Verrier and John Couch Adams.

Since its discovery we have been able to accurately determine that it takes 164.8 years to complete one orbit around the Sun at a distance of 4.5 billion kilometres (2.8 billion miles). In 2011 Neptune completed its first orbit since its discovery in 1846. Like most of the planets, the orbit of Neptune is elliptical, the difference between its closest (perigee) and most distant point (apogee) from the Sun is approximately 100 million kilometres.

In composition, Neptune is very similar to Uranus and, considered to be an ‘ice giant’ it differs slightly from the composition of Jupiter and Saturn. Jupiter and Saturn are composed almost entirely of gas whereas Uranus and Neptune are composed of an unusual material which is a hot dense liquid that has properties similar to ice. Neptune’s atmosphere actually only makes up about 20% of its diameter, the equivalent of its mantle is made up of water, ammonia and methane and its core is made of iron and nickel silicates. It’s the presence of methane in the atmosphere of Neptune which absorbs red light, giving the planet its distinctive blue colour.

The weather system on Neptune seems to be driven by several extreme storms, of note is the ‘Great Dark Spot’ analogous to the storm system on Jupiter called the ‘Great Red Spot’. Wind speeds in excess of 600 metres per second have been recorded. The storm we see today is different to the one discovered originally by the Voyager spacecraft suggesting the common nature of them. They are thought to occur in the troposphere, at lower altitudes than the lighter cirrus clouds often seen around them.

A system of rings surrounds Neptune, although they are by no means as spectacular as the rings around Saturn. Three rings have been identified to date and they are composed of countless particles of ice. The ring particles are accompanied in orbit by fourteen moons, the largest of which is Triton. Triton is thought to have been captured by Neptune’s gravity as it wandered too close rather than forming with Neptune about 4.5 billions years ago.