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Transcript
The Solar System The Sun is the star at the center of the Solar System. It is about five billion years old. There are nine planets in the Solar System. These planets circle around the Sun. This is called orbits. A lot of astronomy people think that the Solar System divided into two parts. The Inner Solar System has Mercury, Venus, Earth and Mars. These are closest to the sun and they are called the terrestrial planets simply because they have very solid rocky surfaces. The Outer Solar System has Jupiter, Saturn, Uranus, and Neptune. They are sometimes called the gas giants. These two systems are separated by the asteroid belt. The Solar System also contains comets, moons, dust, gas and some minor planets. Mercury Mercury is the first planet from the Sun. It is named after Mercury, the messenger of the gods. The planet speeds around the Sun once every 88 days. Mercury is a small, rocky world. Mercury is very like the Moon. Its surface is covered with craters. It has no atmosphere and no water. The noon temperature at the equator can be about 450 °C but the nights are extremely cold, below -180 °C. Venus Venus is the second planet from the Sun. It appears as a brilliant morning or evening “star” – the brightest object in the night sky apart from the Moon. In some ways, Venus is Earth’s twin. It is about the same size and made of the same rocky materials. It also comes closer to Earth than any other planet. The planet’s temperature is 465 °C. This means that Venus is even hotter than Mercury. Without special protection, a visitor to Venus would die instantly – crushed by the immense air pressure, suffocated by the atmosphere, burned to a crisp by the scorching heat, and dissolved by the acid. The surface of Venus has thousands of volcanoes and impact craters. Mars Mars is often called the 'Red Planet' because it appears in the sky as an orange-red star. Because of color the ancient Greeks and Romans named it after their god of war. Mars is the fourth planet from the Sun. It orbits the Sun at an average distance of 228 million km, so human visitors would find it very cold. The average temperature is 63 degrees Celsius below zero – similar to winters in Antarctica. The nights are also bitterly cold. Violent storms can whip up clouds of dust. Sometimes these spread rapidly around the entire planet, hiding the surface from view. Jupiter Beyond the asteroid belt is Jupiter, the fifth planet from the Sun. Everything about Jupiter is large. It is so big that it could easily swallow all of the other planets (or more than 1,300 Earths). It also weighs more than twice as much as all the other planets. Jupiter is five times as far from the Sun as the Earth, so its surface temperature is low, around –145 °C. Jupiter is a giant ball of gas, with no solid surface. It is mainly made of the very light gases, hydrogen and helium. Telescopes show a cloudy atmosphere with colorful belts and spots. Jupiter has a faint ring of dust, over 100,000 km wide. It is also orbited by 63satellites. Four of these, discovered by the Italian scientist Galileo in 1610, are very big. They are Io, Europa, Ganymede and Calisto. Saturn the gas giant Saturn is the sixth planet from the Sun. It was the furthest of the planets until the telescope was invented. Saturn is made mainly of the light gases hydrogen and helium. 764 Earths would fit inside Saturn, but the gas giant weighs only 95 times as much as our rocky world. In a telescope, Saturn appears a pale yellow color. It has no solid surface, so what we are seeing are clouds that appear as light and dark bands. These clouds are blown along by very strong winds. Uranus Uranus is the seventh planet from the Sun. It was discovered by William Herschel in 1781. Uranus lies more than 2,800 million km from the Sun. The temperature of its cloud tops is -214 degrees C. It moves quite slowly and has a long way to travel, so each orbit lasts 84 years. Uranus is a giant world, the third largest planet in our Solar System. 64 Earths would fit inside it. Despite its size, it spins rapidly. A day on Uranus lasts only 17 hours 14 minutes. Uranus has 27 moons. None of these are very big. The largest satellites are Oberon and Titania, both more than 1,500 km in diameter. Uranus also has at least a dozen dark, dusty rings. At least one ring is created by meteorites crashing into a small satellite. Neptune Neptune was discovered in 1846 by Johann Galle, an astronomer at the Berlin Observatory. Neptune turned out to be almost an identical twin of Uranus. It is 57 times bigger than the Earth, but spins quite rapidly – one day lasts only 16 hours 7 minutes. Its average distance from the Sun is about 4,500 million km, and one year on Neptune lasts for almost 165 Earth years. It has an atmosphere of hydrogen, helium and methane. Its interior is made of ices, with a possible rocky core. Although the atmosphere is very cold (-220 degrees C), the blue planet has some very strong winds and violent storms. Neptune has at least five dark, narrow rings (named after Galle, Le Verrier, Adams and others who worked to discover the planet). It has 13 known moons. By far the largest is Triton, an icy world that is bigger than Pluto. Triton is very cold, so its thin atmosphere has frozen onto the surface. However, it does have many active ice volcanoes that spurt plumes of gas and dust. Triton is also unusual because it travels “the wrong way” (east to west) around Neptune. Pluto Pluto was discovered in 1930 by 24 year-old Clyde Tombaugh, who was using a special machine to compare photos of the sky. It turned out to be a tiny world, even smaller than our Moon. Pluto follows an elliptical (egg-shaped) orbit that varies between 7,381 million km (49 Sun-Earth distances) and 4,446 million km (30 Sun-Earth distances). Since one orbit lasts 248 years, no one born on Pluto would ever experience a single birthday! Very little is known about Pluto. Its surface is extremely cold (-230 degrees C) and seems to be covered with frozen ices. In recent years it has been fairly close to the Sun and enjoying a brief summer. Surface ices have vaporized, producing a thin atmosphere. However, it is now retreating into the cold depths of the Solar System and this atmosphere will soon freeze again. Pluto spins “backwards” (east to west) once every 6 days 9 hours. Its largest moon, Charon, takes the same length of time to orbit Pluto. This means that anyone living on one side of Pluto would never see Charon. Two smaller moons have recently been discovered with the Hubble Space Telescope. For many years, Pluto was accepted as the ninth planet from the Sun (even though it sometimes comes closer than Neptune). Today, it is regarded as a “ dwarf planet”. It is also one of the largest members of the Kuiper Belt, a family of icy worlds that occupy the space beyond Neptune.