Leicester University Physics Department

Neptune

picture : Neptune taken from Los Alamos National Laboratory, US

Solar System Index

The Sun

Mercury

Venus

Earth

The Moon

Mars

Jupiter

Saturn

Uranus

Neptune

Pluto

The Kuiper Belt

Introduction

Neptune's rings Neptune is the eighth planet in the solar system, though for 20 Earth years in every 248 its orbit is outside Pluto's, the ninth. It is a gas giant, having several moons and a massive, fluid atmosphere. Its axial tilt is about 28 degrees from perpendicular, and it has three distinct, dark rings and one faint ring.

The Atmosphere

Neptune's Great Dark Spot Neptune's blue colour is due to methane in the atmosphere, which absorbs red light. Similar swirling weather patterns to those of Jupiter cover the entire planet, and Neptune too seems to have a semi-permanent atmospheric feature: the Great Dark Spot. This is similar in many ways to Jupiter's red one, and is roughly half the size, but it appears to disappear and reappear elsewhere every so often. There is also a small, white cloud that zips around the planet every sixteen hours or so, known as the Scooter. No-one really knows what causes it, but many scientists believe that it is a plume rising from the lower atmosphere.

The Moons of Neptune

Neptune, like the rest of the gas giants, has many moons, most of which were discovered by the Voyager 2 probe. Only Nereid and Triton had been discovered previously. Nereid's only remarkable feature is the eccentricity (or tilt) of it's orbit, being higher than any natural satellite in the solar system. This is possibly a result of the unusual orbit of its fellow-satellite, Triton.

Triton

Triton Neptune's largest moon, Triton, has a retrograde orbit. This means that it orbits in the opposite direction to all the others. The only other retrograde moons orbit Saturn and Jupiter, and all are far smaller. No-one has been able to account for this, but it suggests that Triton came from a different place to the rest of Neptune's moons, and was possibly captured by the planet. Because of this, the radius of Triton's orbit is slowly decreasing, and it will eventually crash into the gas giant. When this happens, its fragments may form a bright ring that would be easily visible from Earth. One of the more interesting side-effects of this retrograde orbit is that the immense forces acting on Triton cause it to be volcanically active. The erupted materials are probably nitrogen, dust or methane, and are often ejected several miles above its surface like geysers on Earth, only much larger.