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11.10
The outer planets
  
  Beyond the asteroid belt lie the outer planets, Jupiter, Saturn, Uranus, Neptune and Pluto- four giants and one midget.  
  The distance of the inner planets from the Sun seems a mere step by comparison with the vast and increasing distances between the outer planets.  
  These extend out into the col vastness of space, so far from the Sun that the most remote can hardly be seen from the Earth even with the best terrestrial telescopes. Even so, with the sole exception of Pluto, we have a remarkable amount of detailed knowledge of them all.  
11.10.1 Jupiter  
  148,000 km in diameter at its Equator and 778 million km from the Sun. 16 known moons, rotates once every ten hours, orbits the Sun once in 11.86 years. Jupiter is the largest planet in the solar system and the fifth out from the Sun, which orbits at a distance of 778.3 million km- over five times the distance of the Earth from the Sun. Its composition is very different from that of the Earth. It has an inner solid-rock core surrounded by a liquid zone and a massive atmosphere. Most of Jupiter consists of hydrogen and helium indicates that these elements were attracted to the massive core by gravitation in the course of the formation of the planet. Jupiter can be considered to represent a failed star. If it has been several times larger, its gravitational field would have produced core temperatures and pressure high enough to start off a nuclear fusion reaction, using the plentiful supply of hydrogen fuel.  
  Because so much of Jupiter consists of hydrogen and helium, its average density is very low- about 1.3 g per cubic cc. Jupiter spins on its axis at a tremendous speed for anything so large, and rotates completely in just under ten Earth hours. This speed causes it to deviate considerably from a spherical shape and its diameter at the equator- 142,800 km is over 9000 km greater than the distance between the North and South poles. Such a shape, a flattened sphere is called an oblate spheroid.  
  The Jovian atmosphere consists almost entirely of hydrogen and helium, with traces of water vapor, methane, ammonia, hydrogen sulphide and various other organic compounds. The atmosphere is believed to contain heavily layers of clouds of different colours, forming orange-brown coloured bands, readily visible by telescope from Earth and described as belts. Lightning storms and auroras are common and these were observed by the Voyager spacecraft in 1979. Strong east and west winds sweep round parallel to the equator.  
  The high internal temperature means that the electrons are stripped off hydrogen atoms making them electrically charged particles ( ions). The rotation currents of these ions around the core of the planet produces a powerful magnetic field 4,000 times stronger than that of the Earth. The axis of this field does not quite coincide with the axis of rotation of the planet so the field wobbles rapidly sending out a strong radio signals.  
  All the other giant outer planets radiate energy, but Jupiter radiates about twice as much as it receives from the Sun, implying that its has an internal energy source.  
  Jupiter has four large moons (over 3,600 km in diameter), which remarkably were detected by Galileo in 1610, using one of the first astronomical telescopes. These moons- Callisto, Ganymede, Europa and Io- move in a circular orbits around the planet. Ganymede is the largest planetary moon in the solar system. However, Europa is the most remarkable with oceans of water (partly frozen) and signs that the surface of the planet has been torn by glaciation (the same as Earth). However given the life conditions where Europa is located, any likely life on Europa will be at a low level of sophistication (e.g. bacteria, etc). Jupiter are also has numerous smaller bodies and dust particles and a tenuous ring system- much less conspicuous than that of Saturn- of microscopic particles of size close to the wavelength of light.  
  The Tail and front of Jupiter  
  Most importantly from the perspective of understanding a phenomena of planets orbiting the Sun, Jupiter has two fields of asteroids, separate to its ring system that travel just in front and behind the planet. These are called the Trojans and their size and number provide some indication that planets have the capacity to have tails of debris of significant size (some objects in the Trojan fields being as large as 2km across).  
  Why does Jupiter rotate so fast?  
  Jupiter is in the frustrating position of not being large enough yet to produce sufficient internal het to kick along the thermal fusion process to the point of self generating photon and then proton-proton conversion into Helium- the lowest and most powerful nuclear fusion/fission reaction.  
  Its rotation provides greater pressure and therefore higher temperatures deeper into its atmosphere.  
  This high rotation and temperatures is critically important in the fundamental role of Jupiter. Jupiter is the solar systems magnet, while the Sun is its light and het source.  
11.10.2 Saturn  
  Saturn is the sixth planet from the Sun, is the most remarkable in appearance on account of its conspicuous ring system. This can easily be seen with a small telescope. Saturn is somewhat smaller than Jupiter and is the second largest of the planets. Its diameter at the equator is 120,660 km and it rotates once in just over ten and a half Earth hours. Like Jupiter its is an oblate spheroid and is composed mainly of hydrogen and helium with a rocky inner core similar to that of Jupiter. Like the other three giant outer planets, Saturn has a dense gaseous atmosphere and it is this that is seen on optical observation. Its density is only about half that of Jupiter, indicating that the core is relatively smaller.  
  Saturn orbits the Sun at an average distance of 1.427 billion km, taking nearly thirty Earth years to do so. The orbit is slightly elliptical and is inclined at a small angle to the Earths orbital plane (the ecliptic plane). The Saturnian atmosphere, largely of hydrogen and helium, shows shifting dark belts and light zones similar to those on Jupiter, but about every 30 years or so, a great white spot appears, thought to consist of frozen ammonia. Like Jupiter, Saturn has a powerful magnetic field.  
  Investigations, mainly by the Voyager spacecraft, showed that most of the ring particles are of ice, of size probably varying from centimetres to metres in diameter and that the structure of the system is far more complex than had been imagined. It actually consists of an elaborate of hundreds of separate ringlets, some of which are elliptical. In addition to the rings, Saturn has the largest satellite moon of all the planets. Over 20 orbiting bodies have been identified and six can easily be seen with a telescope. The largest moon is Titan, which is second in size only to the Jovian moon Ganymede and is the only satellite in the solar system known to have a significant atmosphere. It is largely composed of nitrogen. The atmosphere is determined of the moon of -180 degrees C at which other gases are either liquid or solid form. Some of the other moons are Phoebe, Hyperon, Iapetus, Mimas, Enceldaus, Tethys, Dione, Rhea, Epimetheus and Janus.  
11.10.3 Uranus  
  Uranus is the seventh planet and the fourth largest. It is not normally visible to the naked eye. It was first identified as a planet by the British Astronomer William Herschel (1738-1822) in 1781 who at first thought it was a comet.  
  It had been observed previously, but moved so slowly that it was originally thought to be a star. It is nearly 20 times as far from the Sun as is the Earth- some 2.870 billion km and takes over 84 Earth years for each revolution.  
  Remarkably the axis of Uranus, instead of being perpendicular to the plane of the orbit, lies almost in the plane. This means that in the course of a Uranium year, each pole has 42 years of day followed by 42 years of night.  
  Uranus has a diameter of 51,800 km and a density of 1.1.5 h per cc. Relatively more of the heavier elements, such as carbon, oxygen and nitrogen are present than in Jupiter and Saturn. Its dense atmosphere contains a considerable amount of methane, giving it a bluish colour. Most of this methane is contained in layers of clouds thought to be over 10,000 km thick and this dense cloud covering makes observation difficult. The chief atmospheric gas is hydrogen and, in addition to methane, helium and ammonia are present.  
  On 24 January 1986, eight and a half years after its launch, Voyager 2 sped past Uranus at a distance of only 81,500 km and at a speed of 72,400 kph. because Uranus is so far from the Sun, the ambient light is only about 1/400 that on Earth, so long photographic exposures were needed. This together with the speed of the craft made clear photography very difficult. In the event, little was revealed visually, the planet being seen to be blanketed with a pale blue haze.  
  Observations were, however made of the rate of rotation of the planet and a value of just over 17 Earth hours was obtained. Voyager 2 also confirmed the existence of a strong magnetic field, at least as strong as that of Saturn.  
  Uranus has five major moons, Oberon, Titania, Umbriel, Ariel and Muranda. Voyager 2 obtained remarkable photographs of these moons, especially of Ariel, with detail down to 2.4 km. The spacecraft also discovered 10 new moons ranging in size from about 16 km to 160 km in diameter. The planet also has an inconspicuous system of at least 11 dark rings.  
11.10.4 Neptune  
  The eighth planet from the Sun, Neptune is the most remote and least known of the giants. Its position was predicted before it was seen. It is so far from the Sun 4.497 billion km that it reflects too little light to be seen with the naked eye. One revolution of this enormous orbit takes no less time than 164.793 Earth years and the planet rotates on its own axis probably in about 18 hours. Neptune is 49,500 km in diameter and its spectrum shows that it is composed of hydrogen, methane and probably ethane. It is a very cold planet with a temperature well below -200 degrees C and its has two moons. Triton and Nereid. The former 2,720 km in diameter, is probably the coldest body in the solar system. It has ice caps of frozen methane, liquid nitrogen and water. In addition, there are at least six other smaller satellites ranging in size from 50 to 200 km in diameter.  
  Voyager 2 passed Neptune barely 4,850 km above the surface on 24 August 1989 and sent back thousands of remarkably clear images by radio. These showed a turbulent world beset by massive storms sweeping around its equator at 1,000 km per hour. The planet is covered with banks of silvery cirrus, thin, wispy clouds extending out some 50 km from the watery surface. The atmosphere consists largely of hydrogen, helium and methane and its core is of molten rock. Neptune has four remarkable incomplete, thin dust rings. The images of Triton were of astonishing detail, showing great canyons, craters and peaks.  
11.10.5 Pluto  
  Pluto is the ninth planet, the smallest most remote and the least known. Even the largest of telescopes show it as no more than a featureless blob. Its distance from the Sun averages 5.970 billion km, but its orbit is very eccentric and brings it inside the orbit of Neptune at its closest approach.  
  Fortunately their paths do not cross. Pluto takes around 246 Earth years to move once round the Sun. It has a diameter of about 4,000 km and probably consists of a rocky, silicate core surrounded by a blanket of ice of various gases and liquids including methane frost.  
  When furthest from the Sun, the planet is probably covered with methane snow. And there is methane in the very thin atmosphere.  
  Pluto has one moon, Charon which was discovered in 1978 and which is about half Pluto's diameter- so large compared with Pluto that they really should be considered a two planetary system.  
     
     
     
 
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