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Unformatted text preview: Microscopic creatures live in the roots of your eye- lashes. Don’t worry. Everyone has them, and they are harmless.* They hatch, fight for survival, mate, lay eggs, and die in the tiny spaces around the roots of our eyelashes without doing us any harm. Some live in renowned places—the eyelashes of a glamorous movie star—but the tiny beasts are not self-aware; they never stop to say. ”Where are we?” Humans are more intelli- gent; we have the ability and the responsibility to won- der where we are in the universe and how we came here. In this chapter, we begin exploring our solar sys— tern: the sun and its family of planets. We must avoid, however. the great pitfall of science. We must avoid memorizing facts instead of understanding nature. Iupiter is 317.33 times more massive than Earth. That is a fact, and it is boring. The excitement of science comes from understanding nature. We want to under- stand how the sun and planets formed, and that will tell us why Jupiter is so massive. Facts fit together to build understanding, so we begin our study of the solar system by developing a unifying hypothesis for the origin of the sun and plan- ets. As we learn facts about the solar system we can use them to test and improve our hypothesis, and that will increase our understanding of nature. A unifying hy- pothesis is like a basket into which we can put facts. The facts are easier to remember because they make sense, and the combination of hypothesis and support ing facts represents real knowledg%real understand— ing—and not just memory. We have no choice; we must understand our solar system out of self-preservation. We live on a planet with 6 billion other humans, and we are altering our world’s environment in unplanned and unknown ways. We could make Earth uninhabitable. The deadly desarts of Mars and sulfuric acid fogs of Venus may help us understand our own more comfortable world. We must also study planets as astronomers. It seems likely that roughly half of all stars have plane- tary systems, so there are more planets in the universe than stars. Also. the chemical reactions that give rise to life seem to need the moderate conditions of the sur- face of planets. To search for life in the universe, we must understand planets. Above all, we study the solar system because it is our home in the universe. Because we are an intelligent species, we have the right and the responsibility to wonder what we are. Our kind have inhabited this solar system for at least a million years, but only within the last hundred years have we begun to understand what a solar system is. Like sleeping passengers on a train, we waken, look out at the passing scenery, and mutter: “What place is this? Where are we now?” *Demodex follicclorum has been found in 97 percent of individuals and is a characteristic of healthy skin. 1 9-1 THE GREAT CHAIN OF ORIGINS We are linked through a great chain of beginnings that leads backward through time to the first instant when the universe began about 14 billion years ago. The gradual discovery of the links in that chain is one of the most exciting adventures of the human intellect. In earlier chapters, we discussed some of that story: the origin of the universe in the big hang. the formation of galaxies, the origin of stars, and the growth of the chemical elements. Here we will explore further to consider the formation of planets. EARLY HYPOTHESES The earliest theories for Earth’s origin are myths and folktales that go back beyond the beginning of recorded history. In addition, almost all religions contain an ac- count of the origin of the world. It was not until about the time of Galileo that philosophers began searching for rational explanations for natural phenomena. While people like Copernicus, Kepler, and Galileo tried to find logical explanations for the motions of Earth and the other planets, other philosophers began thinking about the origin of the planets. The first rational theory for Earth’s origin was pro- posed hy the French philosopher and mathematician Rene Descartes [15964650]. Because he lived and wrote before the time of Newton, Descartes did not have the concept of gravitation as the dominant force in the uni- verse. Rather, he believed that force was communicated by contact between bodies and that the entire universe was filled with vortices of whirling particles. In 1644, he proposed that the sun and planets formed when a large vortex contracted and condensed. Thus, his hy- pothesis explained the general properties of the solar system known at the time. A century later, in 1745, the French naturalist Georges— Louis de Buffon [1707—1788] propOsed an alternative hypothesis that the planets were formed when a pass ing star collided with or passed close to the sun and pulled matter out of the sun and the star. The matter condensed to form the planets, and they fell into orbit around the sun [I Figure 19-1a). This passing star hy- pothesis was popular off and on for two centuries, but it contains serious flaws. First, stars are very small compared to the distances between them. and thus they collide very infrequently. In the entire history of our galaxy, stars have probably collided only a few times. hasty-Astronomy” The AceAstronomy icon throughout the text indicates an opportunity tor you to test yourself on key concepts and to explore animations and interactions on the AceAstronomy Web site at http:Ilastronomyhrookscole .com/seedsBe CHAPTER 19 THE ORIGIN OF THE SOLAR SYSTEM 407 b FIGURE 19-1 [a] The passing star hypothesis was (:ataslrophic. [I proposed that the sun was hil by 01' had a very close encounler with a passing star and that matter torn from the sun and the star formed planets orbiting the sun. The theory is no longer accepted, “3] Laplace's nebular hypothesis was evolulionary. IL suggested that a conlracting disk ofmatter conserved angular momentum. spun faster. and shed rings of malter that torn-led plan— ets. The hypothesis could not explain the sun's low angular momentum and was never fully accepted. even though the modern theory shares some of its features. More important, the gas pulled from the sun and the star would be much too hot to condense to make plan- ets. Furthermore, even ifplenets formed, they would not go into stable orbits. The hypotheses of Descartes and Button fall into two broad categories. Descartes's hypothesis is evolu- tionary in that it calls upon common, gradual events to produce the sun and planets. If it is correct, stars with planets are very common. Buffon‘s hypothesis, on the 408 PART :1 THE SOLAR 8Y8 I'EM other hand, is catastrophic. It calls upon unlikely, sud- den events to produce the solar system. and thus it im- plies that solar systems are very rare. While our imagi- nations may be tempted by oolliding stars. modern hypotheses for the origins of the planets are evolution- ary. with, as we will see, a few astonishing catastrophes thrown in [Window on Science 19-1). Our modern theory ofthe origin of the solar system had its true beginning with Pierre-Simon de Laplace Many theories in science can be classi— fied as either evolutionary. in that they involve gradual processes, or catas- trophic, in that they depend on specific. unlikely events. Scientists have generally preferred evolutionary theories. Never- theless. catastrophic events do occur. Something in us prefers catastrophic theories. perhaps because we like to see spectacular violence from a safe dis- tancer which may explain the success of movies that include lots of car crashes and explosions. Also. cataclysmic theo- ries resonate with Old Testament ac- counts ct catastrophic events and spa— theories. (1749—1327), the brilliant French astronomer and mathe- matician. In 17'96, he combined Descartes’s vortex with Newton’s gravity to produce a model of a rotating cloud of matter contracting under its own gravitation and flat- tening into a disk——the nebular hypothesis. As the disk grew smaller, it had to conserve angular momentum and spin faster and faster. Laplace reasoned that, when it was spinning as fast as it could. the disk would shed its outer edge to leave behind a ring of matter. Then the disk could contract further, speed up again, and leave another ring. Thus, the contracting disk would leave he- hind a series of rings, each to become a planet circling the newborn sun at the center of the disk [Figure 19-1b]. According to the nebular hypothesis, the sun should be spinning very rapidly, or, to put it another way, the sun should have most of the angular momen- tum of the solar system. [Recall from Chapter 5 that an- gular momentum is the tendency of a rotating object to continue rotating] As astronomers studied the planets and the sun, however, they found that the sun rotated slowly and that the planets moving in their orbits had most of the angular momentum in the solar system. In fact, the rotation of the sun contains only show 0.3 per- cent of the angular momentum of the solar system. Be- cause the nebular hypothesis could not explain this angular momentum problem, it was never fully suc- cessful, and astronomers toyed with various versions of the passing star hypothesis for over a century. In contrast to the astronomy of earlier centuries, 20th-century astronomy applied modern physics to the stars and galaxies and so illuminated our origins. We can now trace that great chain of origins that began with the big bang and led to the matter of which we are made. We will review the story of the origin of matter, and then we will add to it the story ofhow that matter formed our world. WINDOW ON SCIENCE 'I 9-1 Two Kinds of Theories: Evolution and Catastrophe cial acts of creation. Thus. we have an understandable interest in catastrophic Nevertheless, most scientific theories are evolutionary. Such theories do not depend on unlikely events or special acts of biblical creation, and thus they are open to scientific investigation. For example, geologists much prefer theo~ ries of mountain building that are evo— lutionary. with the mountains being pushed up slowly as centuries pass. All the evidence of erosion and the folding of rock layers show that the process is gradual. Because most such natural processes are evolutionary. scientists sometimes find it difficult to accept any theory that depends on catastrophic events. We will see in this and later chapters that catastrophes do occur. Earth. for example, is bombarded by debris irom space. and some of these impacts are very large. As you study astronomy or any other natural science. notice that most theories are evolutionary but that we must allow for the possibility of un- predictable catastrophic events. A REVIEW OF THE ORIGIN OF MATTER Look at your thumb. The matter in your thumb came into existence within minutes of the beginning of the uniVerse. Astronomers have strong evidence that the universe began in an event called the big bang [Chapter 18], and by the time the universe was three minutes old, the protons, neutrons. and electrons in your thumb had come into existence. You are made of very old matter. Although those particles formed quickly, they were not linked together as they are today. Most of the mat- ter was hydrogen, and about 25 percent was helium. Very few heavier atoms were made in the big bang. Al- though helium is very rare in our bodies, your thumb contains many of these ancient hydrogen atoms un» changed since the universe began. Within a few hundred million years after the big bang, matter began to collect to form galaxies contain- ing billions of stars. Astronomers understand that nu- clear reactions inside stars combine low-mass atoms such as hydrogen to make heavier atoms [Chapter 8]. Generation after generation of stars cooked the original particles, linking them together to build atoms such as carbon, nitrogen, and oxygen [Chapter 15}. Look at your thumb. Even the calcium in the bone and the iron in the blood Were assembled inside stars. The atoms heavier than iron were created by rapid nuclear reactions that can only occur when a massive star explodes [Chapter 13}. Gold and silver are rare in our bodies, but iodine is critical in your thyroid gland, and there are no doubt a few of those iodine atoms cir- culating through your thumb at this very moment, thanks to the violent deaths of massive stars. Our galaxy contains at least 100 billion stars, of which our sun is one. It formed from a cloud of gas and dust about 5 billion years ago, and the atoms in your thumb were part of that cloud. How the sun took shape, CHAPTER 19 THE ORiGIN OF THE SOLAR SYSTEM 409 ...
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