Neptune

Introduction

Voyager 2 image of Neptune taken as it flew past on 2 August 1989. Image courtesy NASA/JPL-Caltech In the years following the discovery of Uranus in 1781, astronomers became aware that the motion of the planet was perturbed from the expected path along its orbit. Two mathematicians, Urbain le Verrier in France and John Couch Adams in England attributed the perturbations to a more distant planet and predicted its location in the sky.

Adams approached the Royal Observatory but the then Astronomer Royal, George Airy, did not consider his work particularly important and asked him to contact Challis who searched for the planet using the Northumberland telescope in Cambridge. Le Verrier's predictions were more accurate and taken seriously by astronomers at the Berlin observatory who used them to find Neptune in September 1846.

Neptune orbits at a mean distance of 30.1 AU or 4500 million km from the Sun. It takes 165 years to complete each revolution. With a diameter of 49,500 km it is slightly smaller than Uranus but has a larger mass. Neptune rotates in just over 16 hours.

Atmosphere and structure

A Voyager 2 image of the Great Dark Spot seen in 1989. Image courtesy NASA/JPL-Caltech Neptune is the most distant of the 'gas giants' from the Sun. Like the other large outer planets it has a deep atmosphere made up of hydrogen and helium with a little methane. The temperature at the cloud tops – the visible 'surface' – is only 53K. The interior of Neptune is probably a mixture of ices, dense hydrogen and helium surrounding a rocky core about as big as the Earth.

Large storms seen as bright and dark spots come and go in the upper atmosphere, where winds blow at up to 2000 km per hour, faster than any other planet in the solar system. Neptune's atmosphere changes on short timescales.

For example, the Great Dark Spot was a feature observed by Voyager 2 when it arrived at the planet in 1989. The spot had disappeared by the time the Hubble Space Telescope was pointed at Neptune in 1994.

Neptune's rings

A Voyager 2 image of Neptune's rings. Image courtesy NASA/JPL-Caltech Like Jupiter, Saturn and Uranus, Neptune has a ring system. The rings consist of dark material and completely encircle the planet. Thicker clumps of matter spread out along arcs in several of the rings.

Neptune's moons

Eleven moons make up Neptune's retinue of satellites. Prior to the Voyager 2 mission, only 2 were known – Triton and Nereid. Voyager detected 6 more and in 2003 a further 3 were found.

Triton is by far the largest moon with a diameter of 2700 km. It has a thin nitrogen-rich atmosphere above a surface as cold as Pluto, with a temperature of just 35K – the highly reflective surface prevents it from absorbing much heat.

The moon has a very unusual axis of rotation, with a tilt with respect to the Sun of 127 degrees. This gives it the same kind of seasons as are found on Uranus, albeit with the changes taking place over a few days rather than decades. A quarter of Triton is probably water ice with rock making up the remainder of its mass. The relatively large moon is covered with ridges and valleys. Triton's relatively few craters suggest that melting and re-freezing of the surface ices quickly erode older features.

A mosaic of images of Trition from Voyager 2. Image courtesy NASA/JPL-Caltech Perhaps the strangest features on Triton (and anywhere else in the solar system) are the ice volcanoes, possibly driven by seasonal changes. Plumes from these were seen in the Voyager images, one of which rose 8 km and extended 140 km downwind. The material in the plumes freezes out as dark streaks.

Neptune's other moons are all very small, with the next largest Proteus only 418 km, then Nereid just 390 km across. The remainder are all less than 200 km in diameter.

Observing Neptune

At magnitude 7.8, Neptune is invisible to the unaided eye. It can be found with a pair of binoculars and a good star chart and will appear to move from one night to the next. A good amateur telescope will show a tiny disk with a diameter of just 2 arcseconds.

Significant features are only visible with the largest professional Earth-based observatories and the Hubble Space Telescope.

Questions to think about

• What is the key difference in the appearance of the atmospheres of Uranus and Neptune?
• Triton is Neptune's largest moon and has a surface temperature of only 38K, making it the coldest in the solar system. However it still has active features on its surface. Describe their appearance and a possible explanation of their origin.