The Planets

Uranus

Uranus is the next cold giant gas planet. It is green in colour, but is largely featureless, although detailed observations have revealed some faint bands of cloud, but nothing like those seen on Saturn and Jupiter. It is the third largest planet. Although it is only half as big as Saturn, 63 earths could fit into one Uranus.

Uranus was discovered in 1781 by a British astronomer, William Herschel. It had actually been seen many times before this, but was ignored as simply another star in the night sky.

Its day lasts for 17 Earth hours, and its year is 84 Earth years. The average surface temperature is –216C. Uranus' atmosphere consists of hydrogen and helium, with some methane further down.

It has been visited by Voyager 2, but currently no more missions are planned to Uranus and Neptune.
(Image copyright Calvin J. Hamilton. National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation)

Most planets spin round like a top (on an axis nearly perpendicular to the plane of their orbit) but Uranus spins round tipped on its side (it rolls along like a giant bowling ball). It is thought that its tilt was due to a massive collision with a planet-sized body early in the history of the Solar System. Like Saturn it has rings, but these are also tipped round on its side.

Uranus' rings were not detectable from telescopes on earth, as they were so faint as to be lost in the planet's glare, and the chunks of material in the rings are dark. They were finally discovered in 1977, when the Voyager spacecraft flew by the planet.
The outermost ring is composed mostly of ice boulders several feet across.

You can also see some of the inner moons in this Hubble Space Telescope picture. (Copyright public domain)

The picture on the right shows a montage of the larger moons of Uranus. The five large moons show complicated surfaces. There are also a lot of small moons, the total number being at least 22. They tend to have craters, fault systems and canyons suggesting lots of moonquakes and meteor strikes over their history.

Titania has huge fault systems and canyons, craters and valleys. Ariel has only small craters, suggesting that activity there is more recent. It also has valleys, which are similar to, and larger than, those on Titania. It may have been hot long ago, but is cold now. Umbriel is ancient and dark, large and heavily cratered. Miranda is the innermost moon, and very strange. It has craters mixed with weird grooves, valleys and cliffs, and huge fault canyons 12 miles deep. It is made up of half water ice and half rocky material. The structures observed are caused by the upwelling of partially melted ices.
(Image copyright Calvin J. Hamilton. National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation)

The final cold giant gas planet, is Neptune. Neptune is blue. It has a day which is 16 Earth hours, and a year which is 165 Earth years. Neptune is the smallest of the gas giants, about half as big as Saturn. You could fit about 57 earths inside Neptune.

It was discovered in 1846. When people plotted out how Uranus moved across the sky, it wasn’t always where it was expected to be, if it was just moving around under the influence of the sun and other planets’ gravity. So two mathematicians (one French, and one a student here in Cambridge) worked out that this was because Uranus was being tugged by the gravity of another planet further out that had not yet been discovered. They predicted where it would be, and asked some people to look out for it with their telescopes. One such person searching for it was using the telescope here in Cambridge to search for it but missed it: he did put it in his notebook, but didn’t realise what he’d seen till afterwards. So Neptune has not yet gone once round the sun since it was first discovered! As Pluto's orbit is so non-circular , it sometimes crosses the orbit of Neptune. From 1979-1999, Neptune was the most distant planet in the solar system.

Like Saturn and Uranus, Neptune also has faint rings. It also has a Great Dark Spot, which was observed by Voyager scientists, and can be seen toward the center of the image above. The bright clouds to the south and east of the Great Dark Spot constantly change their appearances in periods as short as four hours.
(Image copyright Calvin J. Hamilton. National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation)

This Great Dark Spot is a huge tropical storm, big enough to swallow the entire planet Earth, which spins round every 16 days. It has wispy clouds like those we see on Earth. These appear to form above most of the methane atmosphere and thus are not blue.
(NASA copyright free policy)

On the left is a close-up of the bright cirrus-like clouds of Neptune. The white streaky clouds are from 48 to 160 kilometers (30 to 100 miles) wide and extend for thousands of miles. These can be seen some tens of miles above the methane atmosphere. The clouds change rapidly, often forming and dissipating over periods of several to tens of hours.
(Image copyright Calvin J. Hamilton. National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation)

Neptune has eight known moons. Most are small, but there is also one large moon called Triton, which is almost size of our Moon. There is a lot of geological activity on Triton still. Eruptions occur on the surface, which are called ‘ice volcanoes’ - they erupt with gases rather than with lava but gases. Triton also has polar caps and wind streaks on the surface (like Mars). It has very few craters, ridges, and so on, because of its freezing/thawing cycles.

Triton is the only large moon in the Solar System whose orbit is decreasing. At some very distant future time it will either break up (into a ring?) or crash into Neptune.
(Image copyright A. Tayfun Oner/Calvin J. Hamilton. National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation)

And finally … we come to Pluto, which is by far the smallest planet in the solar system. Pluto is five times smaller than earth, and is also smaller than seven of the Solar Systems moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton).
(Copyright: public domain)

It is a small rocky planet, furthest out from the Sun (usually) so we don’t really know much about it. Is the only planet that has not been visited by a spacecraft, and even the Hubble Space Telescope can only resolve the largest features on its surface. It is too far away to be likely to be visited in the near future, certainly in the next few decades, which means we don’t have a very good picture of it! This is the best so far, showing Pluto and its moon Charon.

Pluto was discovered in 1930, and in the same way as Neptune was found by looking at 'wobbles' in Uranus' orbit, so Pluto was predicted from 'wobbles' in the orbit of Neptune. Its day is 6.4 Earth days long, and its year is 248 Earth years. The average surface temperature is –223C, which is so cold that air of the kind we know would freeze.

Charon, its moon, wasn't detected until 1978, because the moon is so close to Pluto that the two worlds are typically blurred together. Pluto and Charon are much closer to being a double planet; as Charon is half the width of Pluto (our Moon is only a quarter the diameter of the Earth). Pluto's diameter is 1440 miles (2320 kilometers) and Charon's diameter is 790 miles (1270 kilometers).