Exploring Our Solar System

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1 CHAPTER 21 Exploring Our Solar System LESSON 1 Our Solar System What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After 1. Our solar system has eight planets. 2. Earth s atmosphere is mostly oxygen. Key Concepts How do objects in the solar system move? How did distance from the affect the makeup of the objects in the solar system? What objects are in the solar system? Origin and Structure of Our Solar System A solar system is a group of objects that revolve around a star. You might be familiar with our solar system the eight planets and other objects that revolve around the. Our is not the only star in the universe that has a solar system. But our solar system is the only one scientists can study in detail. Formation of the Solar System Did you know there are clouds in space? Clouds in space are made mostly of hydrogen gas. Five billion years ago, our solar system formed from a spinning cloud of hydrogen gas and dust. When gravity caused the cloud to collapse, the cloud began to spin faster. The cloud also got hotter. When the center of the cloud became hot enough for nuclear reactions to occur, a star formed. That star was our. The stars you see in the night sky formed in much the same way as the formed. As the cloud containing the continued to spin, it flattened, with the at its center. Small pieces of ice and rock orbiting the clumped together and formed small, rocky or icy bodies called planetesimals (pla ne TE sih mulz). Gravity pulled some of the planetesimals together. The larger bodies that formed became planets, asteroids, and other objects. Ask Questions As you read, write questions you may have next to each paragraph. Read the lesson a second time and try to answer the questions. When you are done, ask your teacher any questions you still have. Reading Check 1. Summarize How did planetesimals become planets? Reading Essentials Exploring Our Solar System 377

2 Solar System Asteroid belt Earth Mars Uranus Venus 4 AU 30 AU 5000 AU Saturn Neptune Jupiter Mercury Oort cloud Inner solar system Planets of the outer solar system Outer solar system Visual Check 2. Locate Which planet is farthest from the? Reading Check 3. Recognize How far does the solar system extend? REVIEW VOCABULARY silicates compounds composed mostly of silicon and oxygen, with smaller amounts of other atoms Distances Within the Solar System The solar system is so large that scientists rarely use kilometers to measure distances within it. Instead, they use astronomical units. An astronomical unit (AU) is Earth s average distance from the, nearly 150 million km. Scientists often divide our solar system into regions, depending on distance from the. Refer to the figure above as you read about regions in the solar system. The inner solar system extends to about 4 AU from the. The inner solar system contains the planets closest to the Mercury, Venus, Earth, and Mars. It also contains the asteroid belt between Mars and Jupiter, 2 4 AU from the. The four planets farthest from the Jupiter, Saturn, Uranus, and Neptune are part of the outer solar system. The outer solar system extends from Jupiter s orbit to the Oort (ORT) cloud. The Oort cloud is a large, spherical shell of icy planetesimals. Evidence suggests the Oort cloud orbits the from about 5,000 AU to as far as 50,000 AU. Makeup of the Solar System You live on a planet with a solid, rocky surface. But the solar system is made mostly of hydrogen gas. Ices, rocks, and metals make up less than 2 percent of our solar system s mass. Hydrogen gas is the least-dense material in the solar system. The, Jupiter, and Saturn contain most of the hydrogen in the solar system. They are the only bodies with enough mass to have large amounts of hydrogen. Ices are denser than gases. They are made mostly of water, carbon dioxide, methane, or ammonia. Rocks are denser than ices and are made mostly of silicates. Metals, such as iron, are the solar system s densest materials. 378 Exploring Our Solar System Reading Essentials

3 Movement in the Solar System The contains 99 percent of the solar system s mass. Most other objects in the solar system revolve around the. The s enormous gravitational pull holds these objects in orbit. Revolution is the movement of one object around another object. The closer an object is to the, the faster it revolves. While most objects in the solar system revolve, they also rotate, or spin, on their axes. Both of these movements rotation and revolution are regular and predictable. Direction of Motion The motion of an object around the is like the motion of an object whirled on a string. The string pulls on the object just as the s gravity pulls on planets and objects that revolve around it. Our solar system formed from a spinning cloud of gas and dust. Objects that formed from this material spun in the same direction. Planets and most other solar system objects still revolve around the in the same direction. If you were far above Earth s North Pole and looked down on the solar system, you would see objects revolving in a counterclockwise direction. The and six of the eight planets, including Earth, also rotate in a counterclockwise direction. Venus and Uranus rotate clockwise. The View from Earth Earth rotates on its axis once every 24 h. But on Earth, it seems as though you are standing still, and the, the Moon, and stars move around you. While Earth rotates from west to east, objects in the sky appear to move from east to west. The same is true on a merry-go-round. As you move in one direction on a merry-go-round, objects around you seem to move in the opposite direction. Objects in Our Solar System Our solar system contains billions of objects. Scientists group these objects into categories based on their makeup, their size, their distance from the, and whether they orbit the or another object. From Gas and Dust to Solids Recall that the solar system formed from a cloud of gas and dust that was extremely hot at its center, where the formed. As regions beyond the cooled, some of the gases solidified into ices, rocks, and metals. Use four sheets of paper to make an eight-layer book. Use it to identify and describe objects in the solar system. Objects In The Solar System Inner and Outer Planets Dwarf Planets Asteroids Natural Satellites Kuiper Belt Objects Comets Meteoroids, Meteors, Meteorites Key Concept Check 4. Explain Why do most objects in our solar system move in the same direction? Reading Essentials Exploring Our Solar System 379

4 Key Concept Check 5. Evaluate How did distance from the affect the makeup of objects in the solar system? Visual Check 6. Differentiate How do the inner and outer planets differ? Ices, Rocks, and Metals Ices formed far from the, where temperatures were extremely cold. Closer to the, temperatures were too high for ices to form. Most gases there solidified into rocks and metals. The densest matter, the metals, sank to the centers of the largest objects throughout the solar system. The The is made mostly of hydrogen gas. It also contains helium and tiny amounts of other elements. The is the only star in our solar system, and it is the largest object in the solar system. The s is 10 times that of Jupiter and more than 100 times that of Earth. Planets A planet orbits the, is large enough to be nearly spherical in shape, and has no other large object in its orbital path. The figure below shows the eight planets in their order from the. The four inner planets Mercury, Venus, Earth, and Mars formed from rocks and metals. As shown in the figure, they are smaller than the outer planets. They have few or no moons and rotate slowly. The four outer planets Jupiter, Saturn, Uranus, and Neptune formed mostly from gas and ice. They are large and have many moons. They rotate quickly and have rings. The table on the next page describes the planets. Mercury Earth Venus Mars Jupiter The Planets Saturn Uranus Neptune Distances not to scale 380 Exploring Our Solar System Reading Essentials

5 The Planets of the Solar System Planet Distance from the and Diameter 0.39 AU from the 4,900 km Makeup and Atmosphere Mercury Mercury has a large metal core under its small rocky mantle. Its surface is covered with craters and looks much like the surface of Earth s moon. Mercury has no permanent atmosphere. Venus 0.72 AU from the 12,100 km Venus is similar to Earth in size and makeup. Its rocky mantle surrounds a molten or partially molten metal core. Its thick carbon dioxide atmosphere traps thermal energy, making Venus s surface the hottest of all the planets. Earth Mars 1 AU from the 12,800 km 1.5 AU from the 6,800 km Most of Earth is covered by a thin layer of liquid water. Earth has a rocky mantle, a molten outer metal core, and a solid inner metal core. Its atmosphere is 80 percent nitrogen and 20 percent oxygen. Mars has a rocky mantle and a partially molten metal core. Its thin atmosphere is mostly carbon dioxide. Iron in its surface rock gives the planet a reddish color. The surface of Mars has ice but no liquid water. Interpreting Tables 7. Name the largest and smallest planets, by. Jupiter Saturn Uranus Neptune 5.2 AU from the 143,000 km 9.6 AU from the 121,000 km 19 AU from the 51,100 km 30 AU from the 49,500 km Jupiter has more mass than all the other planets combined. Under its atmosphere of hydrogen gas is a layer of liquid hydrogen. Rock and metal have sunk to its core. A thin, barely visible ring system surrounds it. Saturn s makeup is similar to that of Jupiter, but its atmosphere is hazier. Saturn s rings, the most distinctive rings of all the outer planets, are made mostly of small particles of ice. Uranus has a hydrogen gas outer layer; a fluid inner layer made of water, methane, and ammonia; and a rocky core. Uranus has a blue-green color because of the small amount of methane in its cloud layers. It has thin rings. Neptune has more mass than Uranus but is slightly smaller. Neptune and Uranus have similar makeup. Neptune is a deeper blue because it has more methane in its atmosphere. Like the other outer planets, it has thin rings. Reading Essentials Exploring Our Solar System 381

6 Visual Check 8. Identify Which is the largest dwarf planet? Reading Check 9. Locate Where do most asteroids exist? 10. Analyze Why are the moons around the outermost planets mostly ice rather than rock? Dwarf Planets Dwarf planets orbit the and are nearly spherical in shape, but they share their orbital paths with other objects of similar size. Eris is the largest known dwarf planet. All known dwarf planets, including Eris and Pluto, are smaller than Earth s moon, as shown in the figure on the right. Our solar system has at least five dwarf planets. However, scientists hypothesize that the solar system might contain hundreds of dwarf planets, most of them orbiting the beyond Neptune. At least one dwarf planet, Ceres (SIHR eez), shown in the figure, orbits the between the orbits of Mars and Jupiter, in the asteroid belt. Earth s Moon Dwarf Planets Asteroids Asteroids are small, rocky or metallic objects that are left over from the solar system s formation. There are hundreds of thousands of asteroids in the asteroid belt, but they are so small that their total mass is less than the mass of Earth s moon. Though most asteroids exist in the asteroid belt, some exist elsewhere in the solar system. Most asteroids are irregularly shaped and have craters on their surfaces. Natural Satellites A satellite is an object that orbits a larger object other than a star. Natural satellites are also known as moons. The solar system has over 170 moons. Most of them orbit planets or dwarf planets. However, some moons orbit smaller objects, such as asteroids. For example, the asteroid Ida is 50 km long. It is 20 times larger than Dactyl (DAK tul), its moon. Just as planets have different makeups, satellites have different makeups depending on their location. The satellites of the inner planets are mostly rock. Jupiter s moons are a mixture of rock and ice. The moons around the three outermost planets are mostly ice. Eris Pluto Ceres 382 Exploring Our Solar System Reading Essentials

7 Kuiper Belt Objects You read that the asteroid belt lies between the orbits of Mars and Jupiter. The Kuiper (KI pur) belt, shown in the figure below, is a similar but much larger belt of objects between 30 and 50 AU from the. Like the asteroid belt, the Kuiper belt contains objects left over from the solar system s formation. While asteroids are mostly rock and metal, Kuiper belt objects are mostly ice. Pluto is the bestknown object in the Kuiper belt. Kuiper Belt and Comets Reading Check 11. Contrast How do Kuiper belt objects differ from objects in the asteroid belt? Comet Hyakutake Kuiper belt Jupiter Saturn Uranus Halley s Comet Visual Check 12. Observe Which direction does a comet s tail point? Neptune Comets Comets are small objects made mostly of ice. Most comets formed in the Kuiper belt or the Oort cloud. Comets revolve around the in long, stretched-out orbits, as shown in the figure above. As a comet s orbit nears the, some of the comet s ice becomes gas and forms the comet s tail. The tails of some comets extend millions of kilometers into space. A comet loses mass with each orbit. After a certain number of orbits, the comet breaks up. Meteoroids, Meteors, and Meteorites A meteoroid is a solar system object that is smaller than an asteroid or a comet. Some meteoroids are the result of collisions between asteroids. Some are debris from comets. A meteor is the streak of light created when a meteoroid enters Earth s atmosphere. Most meteoroids are no bigger than a grain of rice. They are visible only when they get hot and glow as they pass through Earth s atmosphere. A few large meteoroids become meteorites. A meteorite is a meteoroid that strikes Earth. Key Concept Check 13. Name What objects are in our solar system? Reading Essentials Exploring Our Solar System 383

8 Mini Glossary astronomical unit (AU): Earth s average distance from the, nearly 150 million km dwarf planet: an object that orbits the and is nearly spherical in shape, but shares its orbital path with other objects of similar size meteor: the streak of light created when a meteoroid enters Earth s atmosphere meteoroid: a solar system object that is smaller than an asteroid or a comet planet: an object that orbits the, is large enough to be nearly spherical in shape, and has no other large object in its orbital path satellite: an object that orbits a larger object other than a star meteorite: a meteoroid that strikes Earth 1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explains some differences between a planet and a dwarf planet. 2. Use the descriptions given in the graphic organizer to identify objects in our solar system. Object Description Object Description What do you think the only star in our solar system small object made mostly of ice a meteoroid that strikes Earth nearly spherical object that orbits the but shares orbital path with other similarly sized objects 3. Does the rise in the east or west on Venus? Explain your answer. Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. result of collisions between asteroids or debris from comets orbits a larger object other than a star small, rocky or metallic object with an irregular shape and craters streak of light created when a meteoroid enters Earth s atmosphere END OF LESSON 384 Exploring Our Solar System Reading Essentials