chapter The Solar System 8 section 1 Planet Motion What You ll Learn how models describe the motion of planets how each planet orbits the Sun the location and characteristics of the planets Before You Read Have you ever looked at the night sky and seen stars overhead? Why do you think that some of these stars may really be some of the planets in our solar system? Study Coach Note-Taking Strategies As you read, underline or highlight key words or ideas that identify the important information. A Compare Scientific Models Make a three-tab Foldable like the one shown below. As you read this section, record information about each model on the inside of each tab. geocentric Both heliocentric Read to Learn Models of the Solar System It would be difficult to make a model of the solar system if you did not know that Earth rotates. All you can see from Earth is the movement of planets across the sky, and they all seem to move in a path around Earth. It s easy to understand why many ancient scientists thought Earth was the center around which everything they saw in the sky revolved. What is the geocentric model? Before scientists understood that Earth rotates on its axis, their models of the solar system placed Earth at the center. In those models, the Sun and all other planets orbit around Earth. The geocentric model of the solar system has Earth at its center and everything else revolving around it. Many early Greek scientists thought that the Sun, the planets, and the Moon were each a part of a separate sphere that rotated around Earth. They thought the stars were fixed in another sphere that rotated around Earth, too. The sphere with the stars moved in a regular, predictable way. The spheres with the planets seemed to move in irregular patterns. The planets seemed to wander across the sky. In fact, the word planet comes from the Greek word that means wanderer. In time, these ideas led to the model of the solar system that was developed by Ptolemy. 118 CHAPTER 8 The Solar System
Geocentric Modifications The Ptolomaic model of the solar system was accepted for centuries. However, there were many problems with it. For one thing, the planets seem to move backwards from time to time against the background of the stars. Many changes were made in the Ptolomaic model to explain these problems. In time, the model became very complex. What is the heliocentric model? The Polish astronomer Copernicus published a different model of the solar system in 1545. His model put the Sun at the center, as shown below. The heliocentric model shows that the planets orbit the Sun. Copernicus stated that the Moon revolved around Earth, which was a planet. Earth and other planets revolved around the Sun. He also said that the apparent motion of the planets, stars, and the Sun around Earth was due to Earth s rotation. Mars Venus Mercury Earth Picture This 1. Explain The heliocentric model of the solar system. Jupiter Saturn Uranus Neptune Pluto What did Galileo observe? The Italian astronomer Galileo used a telescope to look at the planets. Galileo saw that the ideas of Copernicus could explain why Venus went through phases like the Moon. The phases could be explained if Venus orbited the Sun and passed between the Sun and Earth on each orbit. Galileo also saw moons in orbit around Jupiter. Galileo s findings convinced him that Venus and Earth revolve around the Sun and that the Sun is the center of the solar system. Understanding the Solar System Copernicus model was better than earlier models. However, it showed the orbits of the planets as perfect circles. In the 1600s, German mathematician Johannes Kepler found that the planets do not orbit the Sun in circular paths. They move in ellipses. 2. Analyze Information What two planets did Galileo study that convinced him Earth moves around the Sun? Reading Essentials 119
Different Speeds Kepler found that the planets in our solar system travel at different speeds. The closer a planet is to the Sun, the faster it travels. Mercury, the planet closest to the Sun, takes just 88 days to travel once around the Sun. Pluto, the farthest planet, orbits the Sun once in 248 years. Measuring Distances Distances in the solar system are huge. Because they must work with these huge distances, astronomers have created some special units. The astronomical unit (AU) equals the average distance from Earth to the Sun, about 150 million km. Scientists use the AU to measure distances in the solar system. 3. Define What is the astronomical unit? What is this distance in kilometers? 4. Identify Is Mars an inferior or a superior planet? Why? How are planets classified? There are nine planets in our solar system. These planets have been classified in several ways. One system classifies planets according to size and other characteristics. The planets similar to Earth are called terrestrial planets. The giant planets, composed mainly of gas, are called jovian planets. Two other classifying systems are based on a planet s location in the solar system. One system is used more often and uses the categories of inner and outer planets. This most used system classifies inner planets (Mercury, Venus, Earth, and Mars) as ones that orbit between the Sun and the asteroid belt. Outer planets (Jupiter, Saturn, Uranus, Neptune, and Pluto) orbit beyond the asteroid belt. The other system bases the categories on the planets locations compared to Earth and the Sun. It classifies planets whose orbits are between Earth and the Sun (Mercury, Venus) as inferior planets, and those that orbit the Sun beyond Earth as superior planets. What was the origin of the solar system? It is not likely that the Sun formed as a single star. Evidence shows that it probably formed as part of a multiple star system, or star cluster. Scientists hypothesize that a huge cloud of gas, ice, and dust began forming our solar system about 4.6 billion years ago. This cloud, called a nebula, would have been a million billion kilometers across. A huge shock wave from nearby space caused the immense cloud to begin condensing. The shock wave also caused the nebula to split into small parts. In time, this fragmentation made thousands of cloud fragments. The Sun formed from a cloud fragment like the one shown on the next page. Eventually, the process of fragmentation stopped, but condensing due to gravitation continued. 120 CHAPTER 8 The Solar System
Picture This 5. Interpret a Diagram The Latin word nebula means mist, vapor, or cloud. How does the figure look like a nebula? The Sun s Size The cloud fragment from which the Sun formed was the size of our entire solar system. As it rotated slowly in space, the cloud continued to contract, and its matter became squeezed into an ever-smaller space. The cloud s density increased, and its increasing gravity pulled more gas, ice, and dust toward the center. The forces made the cloud fragments rotate faster, which in turn caused the cloud to flatten into a disk with a dense center. The Sun s Temperature Temperatures rose inside the dense disk. They finally reached about ten million degrees Celsius. At this temperature, hydrogen fuses into helium, and the process of fusion converts some mass into energy. The Sun was born. The leftover mass in the outer part of the cloud fragment condensed to form planets and other objects in the solar system. Other Solar Systems Until recently, ours was the only known solar system. Now we know that many other stars have planets around them. Upsilon Andromedae, a yellow star about 44 light-years away, has planets in orbit around it. Extrasolar planets are planets in orbit around other stars. How many extrasolar planets are there? The discovery of extrasolar planets makes it easy to imagine that there may be billions of planets in orbit around stars throughout our galaxy. So far, over 100 stars with planets have been found. Many more are likely to exist. NASA is making instruments that will look for planets similar to Earth in the planetary systems. One day, we may even find an Earth-like planet orbiting a star like the Sun. 6. Explain What is mass converted to in the fusion process? Reading Essentials 121
After You Read Mini Glossary astronomical unit: a measure that equals the average distance from Earth to the Sun, about 150 million km. extrasolar planet: a planet in orbit around other stars geocentric model: a model of the solar system that has Earth at its center heliocentric model: a model that shows that the planets in our solar system orbit Sun 1. Review the terms in the Mini-Glossary. Explain how the geocentric model of the solar system differs from the heliocentric model. 2. The planets of our solar system are classified based on their location relative to the Sun and to the asteroid belt. Label each of the planets in the figure according to this classification system. Sun Asteroid Belt 3. You highlighted key words and ideas. How did this help you understand the information in this section? Visit gpescience.com to access your textbook, interactive games, and projects to help you learn more about planet motion. End of Section 122 CHAPTER 8 The Solar System
chapter The Solar System 8 section 2 The Inner Planets Before You Read Two of the inner planets are closer to the Sun than Earth is. How do you think this location affects what it s like on the surfaces of these planets? What You ll Learn how the inner planets compare with Earth the characteristics of the inner planets about the missions to Mars Read to Learn Planets Near the Sun The outermost fragments of the cloud that formed the Sun were not pulled into the star. The remaining gas, ice, and dust particles collided and stuck together. The gravitational pull of these objects attracted more particles to them. Close to the Sun, high temperatures vaporized lighter elements so they could not condense. This explains why planets close to the Sun have fewer light elements than planets farther away. As a result, planets near the Sun are small, rocky, and have iron cores. Mercury Mercury is the second-smallest planet, and the closest planet to the Sun. Much of what we know about Mercury came from the Mariner 10 mission that was sent to photograph the planet during 1974 and 1975. This space probe photographed 45 percent of Mercury s surface. The photographs show a surface much like that of Earth s Moon. Mercury s surface is covered in impact craters. The space probe found that Mercury has a magnetic field and a larger-than-expected iron core. Mercury s thick mantle is missing some lighter materials that scientists expected to find. One theory is that Mercury collided with another body soon after it formed. Their cores merged to form one large iron core. Some of the lighter materials vaporized into space. Study Coach Two-Column Notes As you read about the planets, write the name of each planet in one column and details about it in another. B Organize Information Make a Foldable like the one shown below. Label the left side Inner Planets and the right side Outer Planets. As you read this section, add information about the inner planets. Inner Planets Outer Planets Reading Essentials 123
1. Predict If Mercury had an atmosphere like Earth s, predict, in general, how the high and low temperatures might change each day. Picture This 2. Identify Trace the features of the Venusian surface that are similar to volcanoes on Earth. What is Mercury s surface like? Mercury s large core shrank much more rapidly than its thin outer layers. As the outer layers adjusted, they wrinkled, forming dramatic cliffs as high as three kilometers. This is similar to what happens when an apple dries and shrivels up. Is there an atmosphere around Mercury? Mercury is small, has a low gravitational pull, and undergoes extreme temperatures on its surface. These conditions show that Mercury has no atmosphere. Gases that have been detected on Mercury, such as helium and hydrogen, turned out to be from the solar wind. They don t stay long and blow away into space. The lack of an atmosphere and its closeness to the Sun give Mercury a surface temperature of about 427 C during the day and 170 C at night. Venus Venus, the second planet from the Sun, resembles Earth in some ways. Its size and mass are almost identical to Earth s. Yet the atmosphere of Venus is different. Venus has a dense atmosphere that has 92 times the surface pressure of Earth s at sea level. Most of the atmosphere of Venus is carbon dioxide, and its clouds contain droplets of sulfuric acid. The atmosphere is so dense, only about two percent of sunlight reaches the surface. Carbon dioxide traps the solar energy that does reach the surface, causing what is called a greenhouse effect. Due to this intense greenhouse effect, temperatures on the surface of Venus are between 450 C and 475 C. In the 1970s, the former Soviet Union sent the Venera lander probes to Venus. One photographed and mapped the surface before landing. An hour after landing, the probe was destroyed by intense heat. In 1995, the U.S. Magellan probe sent back radar images of Venus that revealed the surface features shown in the figure below. 124 CHAPTER 8 The Solar System
Earth Earth is the third planet from the Sun. Earth is unlike other planets in the solar system. Its surface temperature allows water to exist in its three forms: gas, liquid, and solid. Earth has an oxygen-rich atmosphere that supports life and burns up most incoming meteors. The ozone layer in Earth s stratosphere blocks harmful ultraviolet rays from the Sun and protects organisms from this intense radiation. Life exists all over Earth. Life has been found at extreme termperatures and pressures on Earth. These findings encourage scientists to think that life may be possible on other planets in the solar system. Mars Mars is the fourth planet from the Sun. It is called the red planet because iron oxide in some of its surface rocks gives it a reddish color. From Earth you can see Mars s red color and polar ice caps. The polar caps are made mainly of frozen carbon dioxide and frozen water. The tilt of Mars s axis and its seasons are similar to Earth s tilt and seasons. During the Martian winter, the polar ice caps get bigger, then shrink in summer. The color of Mars s surface changes according to its seasons. At one time, people thought this indicated the growth and dying back of vegetation. Today we know that these color changes result from seasonal surface winds. When wind blows dust off one area, the area may appear darker. In another season, the wind may weaken and the area may again be covered with light-colored dust. Does Mars have an atmosphere? Mars has an atmosphere. It is much thinner than Earth s and is made mostly of carbon dioxide. Its thin atmosphere does not filter out harmful radiation from the Sun. Surface temperatures range from 37 C to 123 C. This large temperature difference leads to strong winds that can cause dust storms. What are the moons of Mars? Mars has two small moons with many craters. They are called Phobos and Deimos. Phobos is moving slowly toward Mars and is predicted to hit the planet s surface in 50 million years. Deimos, the smaller moon, orbits the planet about 23,500 km above the surface. 3. Explain What are the states in which water exists on Earth s surface? 4. Compare What causes Earth and Mars to have seasons? Reading Essentials 125
Was Mars once wet? Today, Mars appears to be a dry planet. It may not always have been this way. Space probes sent to Mars indicate that liquid water once occurred on the planet and may still be there in solid form. So-called blueberries (named for their round shape) were photographed by the spacecraft Opportunity. The blueberries are deposits most likely composed of hematite. They form only in the presence of water. Later probes revealed surface features, such as sediment layers and gullies, which appear to have been made by flowing water. Channels at the edges of craters strongly suggest that Mars once may have had liquid water. Picture This 5. Identify Highlight each of the features in the figure that might have been made by flowing water. 6. Explain What is Olympus Mons, and why is its size significant? NASA NASA NASA on Mars Mars has been a major focus of NASA planetary exploration for many years. The Mariner 9 space probe (1971 1972) was one of the earliest Mars missions. It revealed long channels on the planet that might have been cut by flowing water. This Mariner mission discovered a long canyon called Valles Marineris. If this canyon were on Earth, it would stretch from New York to San Francisco. Mariner 9 also found some extinct volcanoes on Mars. One, Olympus Mons, is the largest volcano discovered in the solar system. Until 2003, NASA s Mariner 9, Viking probes, Mars Global Surveyor, Mars Pathfinder, and Odyssey were the sources of most of the information about Mars. The most recent data about Mars come from the Mars Exploration Rover Mission and its two landers, Spirit and Opportunity. 126 CHAPTER 8 The Solar System
What were the Viking probes? The Viking 1 and Viking 2 probes landed on Mars in 1976. Each spacecraft had an orbiter and a lander. The orbiters photographed the entire surface of Mars. The landers touched down on the planet s surface to do experiments. The first results of the biological tests seemed to show the presence of life. However, scientists soon realized that the same results might have come from chemical reactions. The biological experiments found no definite evidence of life on Mars. What have other missions discovered? Cameras on the Global Surveyor revealed that the walls of a huge Martian canyon, the Valles Marineris, have layers like those in the Grand Canyon. The cameras also showed a vast flat region. Similar to a mud flat, it covers a large area of Mars northern hemisphere. Water once may have covered this region but then froze to form the poles or soaked into the ground. The Mars Pathfinder gathered data showing that the iron on Mars surface may have leached out of groundwater. A recent probe, Mars Odyssey, showed evidence that water in the form of frost may occur in a thin layer beneath the surface in the regions around the poles. What is the Mars Exploration Rover mission? The rover Opportunity discovered the blueberries you read about earlier. In addition, it found evidence that they were made of hematite. This rover also found that the blueberries might have been deposited in a standing body of water. Another rover, the Spirit, found other deposits of hematite in Gusev Crater. The Mars Exploration Rover mission placed two landers on Mars. This mission provided a lot of evidence that water existed on Mars in the past. What are Martian meteorites? Space probes are not the only way we have learned about Mars. Meteorites have been found on Earth that may have been blasted into space from Mars. In 1984, a Martian meteorite was found in Antarctica. Scientists who studied it found long, egg-shaped microscopic structures in it. They thought these structures might be fossilized evidence of life on Mars. Scientists published a report in 1996 that started a debate. Some scientists suggested other possible causes of the egg-shaped structures. No firm conclusion can be made from this evidence. Research will continue about whether life exists on Mars or did exist on Mars in the past. 7. Explain Other than in polar ice caps, where is frozen water likely to occur on Mars? 8. Evaluate Do you think it is important to search for signs of life on other planets, such as Mars? Why or why not? Reading Essentials 127
After You Read Mini Glossary Earth: the third planet from the Sun; water exists in three forms; supports life Mars: the fourth planet from the Sun; polar caps of frozen carbon dioxide and frozen water Mercury: the planet closest to the Sun; small, rocky, with extreme temperatures Venus: the second planet from the Sun; has a dense, carbon dioxide-rich atmosphere with an intense greenhouse effect 1. Review the terms in the Mini-Glossary. Explain one way in which Earth differs from at least two of the other three inner planets. 2. Complete the table. Mercury Venus Planet Position Relative Atmosphere Surface to the Sun Earth Mars 3. You made two-column notes as you read this section. How did this help you understand the information in this section? Visit gpescience.com to access your textbook, interactive games, and projects to help you learn more about the inner planets. End of Section 128 CHAPTER 8 The Solar System
chapter 8 The Solar System section 3 The Outer Planets Before You Read On Earth, the land is solid and familiar. Write a sentence about how life on a planet with no solid surface might differ from life on Earth. What You ll Learn how the outer planets are alike and different the characteristics of the outer planets about NASA missions to outer planets Read to Learn Why are the outer planets so different? Recall how the small, rocky inner planets formed. High temperatures prevented the lighter elements from condensing. The outer planets formed differently. In the outer parts of the cloud that formed the planets, the lighter elements condensed, collided, and stuck together. The gravitational pull of the growing masses attracted more particles. This process produced the giant planets found in the outer part of the solar system. Only Pluto does not fit this description. Jupiter Jupiter, the fifth planet from the Sun, is the largest planet in the solar system. It is made mainly of hydrogen and helium, with some ammonia, methane, and water vapor. Scientists theorize that there is an ocean of liquid metallic hydrogen in the middle of the planet. Below this liquid may lie a solid, rocky core that is larger than Earth. The core has extreme pressure (50 million Earth atmospheres) and temperature (40,000 C). These conditions make Jupiter s core far different from any rock on Earth. Jupiter s colorful clouds are bands of white, red, tan, and brown. Lightning has been observed within these clouds. The planet has constant storms of swirling, high-pressure gas. The Great Red Spot is the most spectacular of these storms. Study Coach Sticky-Note Discussion Use sticky-notes to mark those pages or passages you have questions or comments about or that you find most interesting. Discuss your questions and interesting facts after you read the section. B Organize Information Continue to use the Foldable you made for the last section. As you read this section, add information about the outer planets. Inner Planets Outer Planets Reading Essentials 129
1. Explain How did scientists get information about Jupiter s moons? 2. Identity What large moon of Jupiter probably does not have an ocean under its crust? What space probes have gone to Jupiter? In 1979, Voyager 1 and Voyager 2 flew past Jupiter. The Galileo space probe reached Jupiter in 1995. These probes provided data about the makeup and motion of its atmosphere. The space probes gathered data about Jupiter s moons and discovered new ones. What are Jupiter s moons like? Jupiter has more than 60 moons. Many are very small and may be captured asteroids. Four are large enough to be considered small planets. Galileo was the first to discover these moons when he looked through his telescope. Jupiter s large moons are called Io, Europa, Ganymede, and Callisto. Ganymede is the largest moon in our solar system and is larger than the planet Mercury. Io is caught in a constant tug of war between the gravitational pulls of Jupiter and Europa. This tug of war heats up Io s interior and makes it the most volcanically active body in the solar system. The volcanoes on Io were first seen in photographs from the Voyager probes. The most recent findings of the Galileo space probe indicate that all of Jupiter s large moons, except Io, likely have an ocean of water under an ice-rock crust. Data from the space probe and the models of the moons made afterward show that the three large moons probably contain water. Some scientists speculate that Europa s large ocean might contain life. Saturn Saturn is the sixth planet from the Sun and is known for its colorful rings, as shown on the following page. It is not the only ringed planet, but it does have the largest and most complex ring system. In 2004, NASA s Cassini-Huygens spacecraft reached Saturn and began sending data. Data about Saturn, including data about its rings and moons, were collected during the spacefraft s approach. The data indicated that Saturn could have at least 34 moons. Low Density Although it is the second-largest planet in the solar system, Saturn has the lowest density. Saturn s cloud layers are not squeezed as tightly together as Jupiter s because Saturn has a weaker gravitational pull than Jupiter. 130 CHAPTER 8 The Solar System
What is Saturn s atmosphere like? Like Jupiter, Saturn is a large planet with a thick outer atmosphere composed mostly of hydrogen and helium, with some ammonia, methane, and water vapor. Deeper into Saturn s atmosphere, the gases begin to change into liquid hydrogen and helium. Below the atmosphere is Saturn s liquid ocean. The planet may have a small rocky core, but no rocky surface like Earth s. The pressures and temperatures at the core are great. What are Saturn s rings? Saturn s rings are composed of countless ice and rock particles, ranging in size from a speck of dust to many meters across. The rings occur in separate, broad bands, each composed of thousands of thin ringlets. The Cassini-Huygens spacecraft took close-up photographs of Saturn s rings. The pictures show a density wave and areas where the ring material bends up and down. Other photos indicate that the gravity of Saturn s moons affects its rings and the presence of a new ring associated with the moon, Atlas. More evidence may that the moon Atlas may have a ring system of its own. What happened to Cassini-Huygens at Saturn? Cassini-Huygens is actually two spacecraft in one. Cassini will continue to orbit Saturn for more than four years. When it passed Saturn s moon Titan in 2005, it released the probe Huygens, which went through Titan s thick atmosphere and landed on its surface. 3. Describe the gases in Saturn s atmosphere. Picture This 4. Locate and label in the figure Saturn s density waves and bends in the rings. Reading Essentials 131
5. Explain Why does methane in the atmosphere cause Uranus to look blue-green? 6. Describe How does Neptune s gravitational force affect Uranus? 132 CHAPTER 8 The Solar System Uranus Uranus is the seventh planet from the Sun. Uranus, discovered in 1781, is a large planet with 27 moons. Voyager 2 revealed numerous thin rings around the planet. The probe also found that the planet s magnetic field is tilted 55 degrees from its rotational poles. Uranus s atmosphere contains hydrogen, helium, and about two percent methane. Methane in the atmosphere gives the planet its blue-green color. Because methane absorbs red and yellow light, the clouds reflect only blue and green light. Uranus has no cloud bands and few storm systems because of its cold upper atmosphere. The lack of bands and storms may also be related to the planet s relative lack of internal heat. Some evidence suggests that beneath its atmosphere, Uranus has other layers. Uranus may have a mantle of liquid water, methane, and ammonia surrounding a rocky core. Neptune Neptune, the eighth planet from the Sun, is similar in size to Uranus. Neptune was discovered in 1846 when studies showed that the gravity of an unseen planet was pulling Uranus. Below its atmosphere, Neptune might have liquid water, methane, and ammonia. It probably also has a rocky core. The Voyager 2 probe also discovered that Neptune has rings, which are thin in some places and thick in others. Neptune s atmosphere is similar to Uranus s, but is has a little more methane, about three percent. Unlike Uranus, Neptune s atmosphere has some dark spots similar to Jupiter s Great Red Spot. Neptune s atmosphere is dynamic, probably because its internal heat is greater than Uranus s. Neptune has at least 13 moons. Triton is the largest with a 2,700-km diameter. Triton has a thin atmosphere composed mostly of nitrogen, with some methane. Voyager 2 detected geysers spewing methane on Triton. Pluto Pluto, the ninth planet from the Sun, is the smallest and least known of the planets. Pluto is considered the ninth planet because its orbit takes it farther from the Sun than Neptune. However, during part of its orbit, Pluto does move closer to the Sun than Neptune does. The two planets never collide because Pluto s orbit is inclined farther from the eliptical plane creating an angle that will not allow the orbits to intersect.
Pluto s Features Pluto is unlike the other outer planets. It has a thin atmosphere and a solid, ice-rock surface. Charon, Pluto s only moon, orbits quite close to the planet. Comets and Other Objects Scientists know that there are many objects beyond Pluto, including comets. A comet is composed of dust and rock particles mixed in with frozen water, methane, and ammonia. As a comet approaches the Sun, it develops a distinctive tail. When the Sun s heat begins to vaporize the comet, dust and gases are released. These materials form a bright cloud, called a coma, around the nucleus, or solid part, of the comet. The solar wind pushes on the vaporized coma particles, forming a tail. The tail always points away from the Sun, as shown in the figure below. Origin Places Most comets come from two places. A huge disk of icy comets, called the Kuiper Belt, exists near Neptune s orbit. Farther out in space, the Oort Cloud completely surrounds the solar system. Gravity from the Sun or passing stars may disturb these regions and set comets on long journeys toward the Sun. Once in orbit around the Sun, comets reappear at regular times. The famous Halley s comet returns every 76 years. 7. List What substances make up comets? Picture This 8. Interpret a Diagram Describe the location of the tail of the comet in relation to the position of the Sun. Reading Essentials 133
9. Identify What are asteroids and where are they found? 10. Classify When does a meteoroid become a meteorite? What are asteroids? Asteroids are rocky objects formed from material similar to that of the planets. Most asteroids are found in an area between the orbits of Mars and Jupiter called the asteroid belt. Asteroids range in size from tiny particles to objects 940 km in diameter. What are meteoroids? Meteoroids are other rocky objects orbiting within the solar system. Meteoroids may have formed when asteroids collided or when comets broke up as they passed close to the Sun, leaving a trail of debris. When Earth passes through these trails, meteoroids may enter the atmosphere. Most objects burn up completely in Earth s atmosphere. We see them briefly as meteors or shooting stars. Some do not burn up completely. These land on Earth and are called meteorites. What is Sedna? Sedna is the unofficial name for another object in the solar system. It has puzzled scientists for some time. Sedna has been labeled a distant planetoid. With a diameter of 1,200 to 1,700 km, it is smaller than Pluto but larger than comets. Sedna also has a highly elliptical orbit, coming as close as 76 AU from the Sun, then traveling to a distance of 950 AU from the Sun. Some scientists suggest that Sedna s orbit was greatly affected by a passing star less than 100 million years after the Sun formed. This supports the idea that our Sun formed as part of a star cluster. A close encounter with another star so soon after the Sun s formation and so close to it (800 AU) could be explained if the Sun formed within a cluster. Another puzzle is Sedna s apparent rate of rotation of 40 days. The best way to explain this puzzle is if there is another object in orbit around Sedna. Even if Sedna has no companion, there may be similar objects, some even larger than Pluto, out there. At present, objects that are this far away and that have orbits this elliptical can be detected by astronomers only when they are closest to the Sun. 134 CHAPTER 8 The Solar System
After You Read Mini Glossary asteroids: rocky objects formed from material similar to that of the planets; occur in the asteroid belt comet: a body composed of dusk and rock particles mixed in with frozen water, methane, and ammonia Jupiter: the fifth planet from the Sun; the largest planet in the solar system meteoroid: a rocky object orbiting within the solar system Neptune: the eighth planet from the Sun; has rings and at least 13 moons Pluto: the ninth planet from the Sun; has a rocky surface and is the smallest and least known of the planets Saturn: the sixth planet from the Sun; known for its colorful rings Sedna: a distant planetoid with a highly elliptical orbit Uranus: the seventh planet from the Sun; a large, gaseous planet with 27 moons 1. Review the terms in the Mini-Glossary. Explain how asteroids, meteoroids, and comets are similar. 2. Fill in the Venn diagram to show how Pluto and the other outer planets are alike and different. In the diagram, list the special characteristics of each group of outer planets. Then, list their shared characteristics. Pluto Shared Jupiter, Uranus, Saturn, Neptune 3. You used sticky-notes to mark any questions or comments you had on the text. How did discussing these questions and comments help you understand the information in this section? Visit gpescience.com to access your textbook, interactive games, and projects to help you learn more about the outer planets. End of Section Reading Essentials 135