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Humans live on a small planet in a tiny part of a vast universe. This part of the universe is called the solar system, and is dominated by a single brilliant star-the sun. The solar system is the earths neighbourhood and the planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are the Earths neighbours. They all have the same stars in the sky and orbit the same sun. Scientists believe the solar system began about 5 billion years ago, perhaps when a nearby star exploded and caused a large cloud of dust and gas to collapse in on itself. The hot, central part of the cloud became the sun, while some smaller pieces formed around it and became the planets.
Other fragments became asteroids and comets, which also orbit the sun. The early solar system was a turbulent mix of hot gas and rocky debris. In the solar system everything is affected by the suns gravity. The planets and a variety of other objects, including comets, move the way they do because of the suns gravitational attraction. Our planet, Earth, is the third out from the sun. The planets are all different.
Their differences are largely the result of their different distances from the sun. We call the planets that are closer to the sun, including the Earth, the inner planets. They are small rocky worlds. The outer planets are much larger and are made from much lighter materials.
All but two planets, Mercury and Venus, have moons in orbit around them. The Moon The explorers found a dead world. There is no air and no water. Without an atmosphere, the sky is black at midday and the temperatures swing by hundreds of degrees from day to night. No rivers or oceans have eroded the surface; no volcanoes are rebuilding the boring landscape. The surface has survived unchanged for billions of years.
Smooth plains and lava flows that froze on the Moon long before life arose on the Earth. Elsewhere, the Moon still bears the scars of every rock that has hit it from space, both large and small. The footprints of the visiting astronauts will survive for millions of years. Crash and splash Astronomers now believe the Moon is the remains of a giant cosmic traffic accident.
In its very early days, the Earth was hit by a runaway planet the size of Mars. White-hot molten rock were splashed into space, and solidified into a ring of rocks around the Earth. These then came together to make up the moon. Ice on the moon Circling over the moons poles in 1998, the American Lunar Prospector spacecraft discovered patches of ice within some deep craters, where it is always shaded from the Sun, so the ice never melts or evaporates. Future manned expeditions could melt the ice into water, for drinking, washing and turning into rocket-fuel: there is enough ice to make a lake 10 km across and 10 m deep. Earth If you approached the Solar System from space, one planet would stand out as very odd.
The third world from the sun is brightly coloured, in shades of mainly blue, with patches of red and green, and constantly shifting patterns of white cloud. And it has an unusually large moon; which-by contrast- is dull and uniformly brown. The perfect planet Come closer, and you find blue is liquid water. This is the only rocky planet with water. Test the atmosphere, and again this planet is unique: the air contains a lot of reactive gas oxygen. And finally, take a closer look at the green areas.
Here there is ample vegetation and animal life. This is the only planet in the solar system obviously inhabited life forms. The earth is special mainly because it is located at the perfect distance from the sun. The temperature here is just right for water to exist as a liquid: get in as close Venus and the water just boils away as steam, while as far out as the orbit of Mars water is frozen to ice. Soon after the birth of earth, it was a very different place from the one we know today. Its air was made of the unbreathable and poisonous gases we still find on other planets, such as carbon dioxide and hydrogen cyanide.
Comets fell from the sky as giant fireballs, and lethal ultraviolet light rays from the Sun irradiated the surface. In all this mayhem, simple chemicals from the air and the installing comets dissolved in the oceans. They gradually joined together in a primordial soup to make the first living cells Life on Earth Life became an important force in shaping the environment. The first plant cells in the sea took in carbon dioxide from the air, and gave out oxygen by the process of photosynthesis.
Animals emerged which could breathe the oxygen. And some of the oxygen turned into an ozone layer high in the atmosphere that shielded the surface from the harmful ultraviolet rays of the sun. Lifeforms emerged on to dry land, evolving into the familiar variety of plants and animals we know today. Mercury Mercury is the planet nearest to the sun.
It is about half as far from the Sun as our Earth is. So when Mercury comes at its closest to the sun, it will appear twice as large in the sky as it appears to us on Earth. There are extreme temperature differences on Mercury. The temperature in the intense sunlight is very, very hot at 420 C. The temperature at night is 180 C. There is no protective atmosphere on Mercury to provide air or to keep the planet at an even temperature.
It would be impossible to live there. Mercury takes just 88 Earth days to go once around the sun, a Mercury year. It rotates very slowly, just once every 58 Earth days. A Mercury day lasts 176 Earth days. Nobody knew what Mercury looked like until a space probe called Mariner 10 flew past in 1973 and took close-up pictures of Mercury's surface. These showed that it is covered with craters and mountains.
Geography and craters Mercury looks much like our own moon, since the rocky surfaces of both bodies have been extensively cratered by impacts from meteorites. Mercury is thought to have a large iron core that occupies three-quarters of its diameter. As on the moon, the youngest craters on Mercury are bright and surrounded by rays of material ejected by the force of the meteoric impact. Between Mercury's main craters are older, smoother areas called inter-crater plains, dotted with small craters.
Secondary craters, caused by debris from the main impact, lie closer to the main craters than on the moon because of Mercury's stronger gravity. Neptune A bizarre volcanic moon accompanies the windiest planet. The blue planet Neptune is seen here from above the largest of its eight moons, Triton. Until Voyager 2 flew past in 1989, astronomers knew hardly anything about these two worlds. They are so from the Sun you need a telescope to see them at all. Neptune was tracked down by the power of the human mind.
Astronomers in England and France realised that Uranus was being pulled by the gravity of a planet further out and calculated where that planet must lie. In 1846 astronomers in Berlin discovered it. Twins and spins Neptune is the twin of Uranus in size, and is also made mainly of water. But Voyager 2 found that Neptune is much more interesting to look like at: it has white clouds and large dark storms.
The spacecraft found six new moons orbiting Neptune, in addition to the two already discovered from Earth. Even more exciting than Neptune itself is its largest moon, Triton. Even before Voyager, astronomers were intrigued that it orbits Neptunes own spin. It was probably once a separate planet, like Pluto, which was captured by Neptune in a traumatic event that melted its surface. Pluto The most distant planet from the sun is also the weirdest.
In fact it may not even be a planet at all, but an ice dwarf. No one suspected Pluto existed until 1930, when the American astronomer Clyde Tombaugh discovered a faint speck of light out beyond Neptune. Then the surprises began to appear. First, Pluto follows a very elongated path rather than a neat circle around the Sun. Its orbit crosses over Neptunes more circular route, so that sometimes Pluto is actually closer to the Sun than Neptune is. Its orbit it also tilted up more than any other planet.
Double planet With better telescopes, astronomers found that Pluto is far smaller than any other planet-only half the size of Mercury is. And its not alone. In 1978, a moon was discovered orbiting very close to Pluto itself: only a powerful telescope can show them apart. This moon, Charon, is half Pluto's own size, so that the two look more like a double planet and its moon. Charon lies so close to Pluto that it looms large in the sky. Pluto and Charon always turn the same face to each other as they orbit: from one side of Pluto Charon is always visible, while an imaginary inhabitant on the other side would never see Charon at all.
In the 1990 s, astronomers started finding many smaller worlds orbiting the Sun in this region, rather like the asteroid belt further in. Because they are made mainly of frozen water and gases, they are called ice dwarfs.
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