Summary This lesson will explore the process of how planets form. Planets can be large and made of gasses, or smaller and rocky, like our Earth. The formation process determines the chemical composition of our planet but also can result in the Goldilocks Conditions that enable life on Earth to exist. THINKING CONCEPTUALLY After watching the video, ask yourself if you think there could be Earth-like planets in other solar systems. Do you think that other solar systems are currently forming in different parts of the Universe? What evidence might there be for this? Threshold 4: Earth & the Solar System (2:49 minutes) https://www.oerproject.com/OER-Media/Videos/SBH/Unit-4/4-0-Earth-Formation-Solar-System/TH4-EarthSolar-System?PageId=&Id=8584&share=link Purpose The formation of our Solar System, including Earth, marks the fourth threshold in the course. Students must learn the ingredients and Goldilocks Conditions that made the formation of the Sun, Earth, and the rest of our Solar System possible in order to understand why the Earth is such a unique planet. Preview Large clouds of dust and gas remain in the aftermath of the death of stars. These chemically complex areas in space begin to pull together and the process of star formation happens all over again. In the aftermath of star formation, the leftover materials can coalesce to form planets. Understanding Content Use these questions and prompts at the appropriate stopping points to check in with the students and ensure they are getting the key concepts covered in the video. 1. 1:28 What enabled planets to form? answer: The complex cloud of chemicals formed by the death of stars enabled much greater complexity than hydrogen and helium alone. 2. 2:15 When did the Earth form? answer: About 4.5 billion years ago. HOW DID EARTH AND THE SOLAR SYSTEM FORM? Summary Planets form by a process called accretion in the aftermath of star formation. In our Solar System, the lighter elements were pushed far from the Sun, and this led to the formation of the gas planets, which lie far from the Sun. The heavier elements remained closer to the Sun and formed the rocky planets like our Earth. THINKING CONCEPTUALLY After watching the video, ask yourself if you think that the Earth is still forming today. Can you think of other examples of accretion elsewhere in the Solar System? How Did Earth and the Solar System Form? (12:05 minutes) https://www.oerproject.com/OER-Media/Videos/SBH/Unit-4/4-0-Earth-Formation-Solar-System/How-EarthFormed?PageId=&Id=8584&share=link Purpose The process of planet formation is a complex one, involving the forces of gravity and electromagnetism and the discipline of chemistry. To understand why the formation of planets and solar systems marks a new level of complexity in the Universe, students need to understand how planets form and what makes them different from stars. Preview As the stars die out and explode in supernovae, we start to see a collection of elements floating across the Universe. Out of these elements, new stars and planets will form. Understanding Content Use these questions and prompts at the appropriate stopping points to check in and ensure you are getting the key concepts covered in the video. Part I 1. 1:34 In the last unit, you learned that about 98 percent of the Universe is hydrogen and helium, and that about 2 percent was everything else. What percentage of the Earth is made up of these other elements? answer: About 90%. 2. 2:30 How did all these elements get concentrated in planets like the Earth? answer: A lot of this has to do with the formation of molecules, and the formation of molecules has a lot to do with electron arrangement in atoms. Some atoms, like hydrogen and oxygen, are very reactive. Others, like helium, are not. 3. 3:00 Can we predict the qualities of a molecule from the atoms that make it up? answer: No. Water, for example, is made up of two elements: hydrogen and oxygen. When separate, both of these elements are gases. When combined in molecules of water, they are liquids. 4. 3:47 Are the bonds between the atoms in molecules the same? answer: No. There is a lot of variety in the types of bonds that hold atoms together. Diamonds, for example, are composed of carbon in a form where the bonds are very strong and rigid. Graphite, on the other hand, is also composed of carbon, but its bonds are weaker and much less rigid. Part II 5. 4:50 Are there molecules in space? answer: Yes. Scientists can see molecules like water, carbon dioxide, ammonia, and acetic acid when they use spectroscopes to explore space. They can also see silicates, the basic building blocks for many of the rocks found on the Earth. 6. 6:25 What is a protoplanetary disk? answer: A protoplanetary disk is a rotating disk of dense gas and matter surrounding a newly formed star. 7. 7:43 Why are the planets closest to our Sun different from the planets further away? answer: After the Sun formed, its intense heat drove away lighter elements like hydrogen and helium. These elements would come to dominate the gas planets like Jupiter and Saturn, which orbit very far from the Sun. The inner planets were formed from heavier elements that were not driven away, so the planets closer to the Sun are rockier. 8. 8:36 What is accretion? answer: Material floating in space gathers into larger bodies through gravity and collisions until they form planets. 9. 9:03 What’s the likely way the Moon was formed? answer: A large object crashed into the Earth, sending a huge chunk of debris into orbit. Eventually, this material accreted into our Moon. 10. 10:15 What is an exoplanet? answer: An exoplanet is a planet orbiting around a star other than our Sun. Part III 11. 11:32 What makes planets more complex than stars? answer: Planets are more complex than stars because they have a more complex structure, and they are composed of a greater diversity of elements. HOW OUR SOLAR SYSTEM FORMED A CLOSE LOOK AT THE PLANETS ORBITING OUR SUN By Cynthia Stokes Brown, adapted by Newsela Planets are born from the clouds of gas and dust orbiting new stars. Billions of years ago, circumstances were just right for the planets in our Solar System to form. The Solar System that we live in consists of a mediumsize star (the Sun) with eight planets orbiting it. The planets are of two different types. The four inner planets, those closest to the Sun, are Mercury, Venus, Earth, and Mars. They are smaller and composed mainly of metals and rocks. The four outer planets — Jupiter, Saturn, Uranus, and Neptune — are larger and composed mostly of gases. What are planets? Where did they come from? Why would some be rocky and some gaseous? What is our planet like? This essay will try to answer these questions. The Birth of the Sun Let’s quickly review how our star came into being. Five billion years ago, a giant cloud floated in one of the spiral arms of the Milky Way galaxy. This cloud, called a nebula by astronomers, was made up of dust and gas, mostly hydrogen and helium. It had just a small percentage of heavier atoms. These heavier atoms had been formed earlier in the history of the Universe when other stars aged and died. This cloud/nebula began to contract, collapsing in on itself. The atoms, once separated, began to bump against each other, generating heat. In the rising heat, the atoms collided more frequently and more violently. Eventually, they reached a temperature at which the protons at the centers of the atoms began to fuse, in a process called nuclear fusion. As they did, a tiny bit of matter transformed into a whole lot of energy, and a star was born. In this way, our Sun came into being. The Birth of the planets The material in the nebula that didn’t absorb into the Sun swirled around it into a flat disk of dust and gas. The Sun’s gravity held this “accretion disk” in orbit. Material in the disk accumulated by further accretion — by sticking together. Each planet began as microscopic grains of dust in the accretion disk. The atoms and molecules began to stick together, or accrete, into larger particles. By gentle collisions, some grains built up into balls. As they grew larger, they formed into objects a mile in diameter, called planetesimals. These objects were big enough to attract others by gravity rather than by chance. If the collisions of planetesimals occurred at high speeds, they could shatter the objects. But when impacts were gentle enough, the objects combined and grew. For some 10 to 100 million years these protoplanets orbited the Sun. Some revolved around it in egg-shaped circuits that resulted in more frequent collisions. Between the inner and outer planets lies an area filled with millions of asteroids — small rocky, icy, and metallic bodies left over from the formation of the Solar System. No planet formed in this area. Astronomers theorize that Jupiter’s gravity influenced this region so much that no large planet could take shape. Jupiter is 11 times the size (in diameter) of Earth and more than twice as big as all the other planets combined. It is almost large enough to have become a star. Of the four rocky planets, Mercury is the smallest, about two-fifths the size of Earth. Earth and Venus are almost the same size, while Mars is about half their size. Astronomers speculate that a smaller object must have hit Mercury, vaporizing its crust and leaving only the larger-than-usual iron core. Worlds collided, combined, and evolved for a dramatic period of time. When it was over, there remained eight stable planets that had swept their orbits clean. To be called a planet, it must orbit the Sun. It must also be massive enough for its own gravity to form it into a sphere, and has cleaned its neighborhood of smaller objects. In 2007, researchers at the University of California–Davis determined that our Solar System was fully formed 4.568 billion years ago. Scientists did this by determining the age of rocky materials from the asteroid belt. The Sun sent out energy and particles in a steady stream, called stellar winds. These winds proved so strong that they blew off the gases of the four planets closest to the Sun. The loss of their gasses left the planets smaller. Only their rocks and metals remained intact. That’s why they are called rocky, or terrestrial, planets. The four outer planets were so far from the Sun that its winds could not blow away their ice and gases. They stayed in a gas form, with only a small rocky core. These four were made of more gas (namely hydrogen and helium) than the others to begin with. Heavier materi- als had already pulled closer by the Sun’s gravity in the original solar disk. Conditions on Earth When the rocky planets first formed, they were largely melted (molten) rock. Over hundreds of millions of years, they slowly cooled. Eventually Mercury and Mars, because they are small, solidified and became rigid all the way to their centers. Only on Earth, and possibly on Venus, have conditions remained in an in-between state. Earth has stayed partially molten. Its crust is solid rock, and its mantle is rigid in short-term time. But over geologic time the mantle flows slowly. And the center of Earth consists of a solid iron core rotating in hot liquid called magma. Some scientists and Big Historians use the term “Goldilocks Conditions” to describe conditions on Earth. In “Goldilocks and the Three Bears,” Goldi- locks wanders into the home of three bears, who are away. She tries out their porridge, their chairs, and their beds, finding some too hot or too cold, too hard or too soft, too large or too small, but one of each just right. Like- wise, Earth is not too hot or too cold, not too big or too little, not too near the Sun or too far away, but just right for life to flourish. Earth’s Moon The rocky object nearest to us is the Moon. Where did it come from? Good question. The Moon orbits Earth, not the Sun, so it is not a planet. The Moon is about one-fourth the size of Earth. The origin of the Moon remains mysterious, but since astronauts walked on the Moon in 1969 and brought back rock and soil samples, we know more about it now than before. The standard argument today holds that a small planet, about one-tenth the size of Earth, must have collided with Earth about 4.45 billion years ago. Earth was still red-hot beneath a thin new crust. Some of the material from the impact was absorbed into the liquefied Earth. However, some material ricocheted into space, where it settled into orbit and condensed as the Moon. At first, the Moon orbited much closer to Earth. It is still moving away at a rate of almost two inches (four centimeters) per year. The Moon significantly affects conditions on Earth. The impact that produced the Moon tilted Earth on its axis. This causes Earth’s seasonal variations in temperature, since the side tilted toward the Sun for one-half the year’s journey around the Sun receives more direct sunlight. Also, the Moon’s gravity causes the oceans’ tides. Additionally, it reduces the Earth’s wobble (which helps stabilize climate), and slows the spin of the Earth. The Earth used to complete a rotation on its axis in 12 hours, but now it takes 24. Pluto and beyond Before 2006, students learned that our Solar System had nine planets, not eight. The one counted as the ninth, Pluto, orbits out beyond Neptune. How- ever, in 2006, the International Astronomical Union declared that Pluto does not count as a planet. It is smaller than Earth’s Moon. It orbits way out in a belt of asteroids beyond Neptune and does not have enough gravity to clear the neighborhood around its path. Therefore, it was downgraded to a “dwarf planet,” or a planetesimal. Astronomers feel confident that our Solar System formed by accretion. They’re sure in their belief because a similar process is occurring in part of the Orion Nebula. This planet-forming area is on the near side of a giant cloud complex that embraces much of the constellation Orion, 1,500 light- years from Earth. Since 1993, astronomers have discovered several hundred stars there in the process of formation. Most of them are surrounded by rings of dust in accretion disks, just like the one they believe produced the solar planets. These clouds of dust and gas around new stars in the Orion Nebula may develop into planetary systems similar to our own. In 1995, astronomers in Switzerland found, for the first time, a planet beyond our Solar System orbiting an ordinary star. Such a planet is called an extra- solar planet, or an exoplanet. As of June 2012, more than 700 exoplanets had been discovered and confirmed. Most of them are giants, closer in size to Jupiter, as larger planets have proved easier to detect hundreds of light- years away. Most are not detected by direct imaging. They’re typically spotted indirectly by measuring the effect of their gravity on their parent star or by observing how the light of the parent star dims as the planet passes in front of it. In summary, planets are bodies orbiting a star. Planets form from particles in a disk of gas and dust, colliding and sticking together as they orbit the star. The planets nearest to the star tend to be rockier because the star’s wind blows away their gases and because they are made of heavier materials attracted by the star’s gravity. In the Sun’s system, Earth is one of four rocky planets, but a unique one, with rigid and molten layers. What Was the Young Earth Like? Summary The layers of the Earth were—and still are—constantly moving, and it was this movement that resulted in the creation of separate continents. Life on each of the continents evolved independently until the continents were reunited much, much later in our story. THINKING CONCEPTUALLY Ask yourself if you think that the Earth is still forming today. Can you think of other examples of accretion elsewhere in the Solar System? What Was the Young Earth Like? (11:09 minutes) https://www.oerproject.com/OER-Media/Videos/SBH/Unit-4/4-1-What-Was-Young-Earth-Like/What-wasYoung-Earth?PageId=&Id=8584&share=link Purpose This video explains what life was like on the early Earth and asks us to think about what it would have been like to live on the Earth back then. It wasn’t pleasant, and it highlights the challenges emerging life forms had to overcome to survive. This video also previews the idea of plate tectonics, which will be the focus of the next two lessons in this unit. Understanding both the history of the Earth and plate tectonics are critical to understanding how the Earth became a place that could support life. Preview In its early years, the Earth didn’t look anything like it does today. Now that we’ve covered the process of accretion and how planets form, it’s helpful to think about what the Earth might have looked like during this time. Is this a place any of us would have wanted to live? Key Ideas—Factual This video is packed with information on both the formation of the Earth and plate tectonics. Both topics are important for understanding Unit 4 and how the Earth became a suitable place for life. Use these questions and prompts at the appropriate stopping points to check in with the students and ensure they are getting the key concepts covered in the video. Part I 1. 1:08 What challenges would humans have faced if they lived on the early Earth? answer: The surface of the planet was molten lava, there was no oxygen, the planet was being constantly bombarded by meteors and asteroids, and there were extreme levels of radiation. 2. 2:23 Why was the early Earth so hot? answer: There are three key reasons why the Earth was hot: First, the Earth was exposed to radiation from the elements created when a supernova exploded nearby, and those radioactive materials contributed to extreme heat. Second, the impact of meteors and other debris during the process of accretion generated a great deal of heat. Finally, the pressure generated by the process of differentiation on the early Earth created a lot of heat. 3. 3:57 Why is it important that the inner core of the Earth is metal? answer: The inner core is made up primarily of iron and nickel. Because they are metals, they are able to generate magnetic fields that help to shield the Earth from the radiation of the Sun, making it possible for living things to exist on Earth. 4. 4:18 How is the mantle different from the core of the Earth? answer: The mantle is made of rock, but it’s very hot and has the consistency of sludge. This sludge circulates. This circulation is called convection, and it will be important in the process of plate tectonics. 5. 4:40 How is the crust different from the mantle? answer: The crust is very light and thin and is solid. David Christian compares it to an eggshell. The crust can be moved by convection in the mantle. 6. 4:47 How did the Earth’s atmosphere form? answer: Gases from the Earth bubbled up and evaporated. Some were held close to the Earth by gravity while others escaped into space. Those that were held close to the Earth became the atmosphere. Part II 7. 6:18 What important hypothesis did Alfred Wegener make about the Earth, and what was his evidence? answer: Alfred Wegener was a meteorologist who hypothesized that the continents at one time fit together forming one giant continent. Two examples of evidence are: the shapes of the continents allow them to be put together like a jigsaw puzzle; and Wegener found similar geological formations in very different parts of the Earth (for example, similar mountains in Brazil and West Africa). 8. 6:33 What was this supercontinent called? answer: Pangaea. 9. 7:04 Why were many scientists unwilling to accept Wegener’s hypothesis? answer: Although Wegener had lots of evidence that suggested that the continents had at one time been joined, he couldn’t explain how continents could move. 10. 7:59 When and how did an explanation for the movement of the Earth’s plates come together? answer: Sonar was developed for military purposes during World War II. After the war, it was used to explore the ocean floor. Geologists discovered chains of volcanoes and realized that the lava rising from these volcanoes created new crust that pushed existing sections of crust apart. 11. 8:57 How does continental crust differ from oceanic crust? answer: Continental crust is lighter than oceanic crust. When the two crusts collide, the heavier oceanic crust usually slides under the lighter continental crust. 12. 9:37 How were the Andes and Himalayas formed? answer: In the case of the Andes, the heavier oceanic crust ran into the lighter continental crust. The continental crust was pushed up by the oceanic crust. The Himalayas formed when continental crust collided with continental crust, and parts of both sections of continental crust were pushed up. 13. 10:59 What is plate tectonics? answer: The concept that the Earth's crust consists of separate plates in constant motion that crash into one another, creating mountains, volcanoes, and earthquakes. The Early Atmosphere Summary The atmosphere of the Earth keeps changing and adapting to the conditions around it. There is a never-ending cycle of warming, cooling, and recovery from traumatic events. THINKING CONCEPTUALLY It may come as a surprise to you that oxygen could have had such a devastating impact of some early forms of life. Can you think of other examples of elements or substances that are deadly to some species and not others? The Early Atmosphere (5:33) https://www.oerproject.com/OER-Media/Videos/SBH/Unit-4/4-1-What-Was-Young-Earth-Like/EarlyAtmosphere?PageId=&Id=8584&share=link Purpose The atmosphere, like the Earth, has evolved over time. These changes in the atmosphere can be traced to changes on Earth. Many factors can influence the climate, but geological forces such as increased volcanic activity, and biological changes such as the development of photosynthesis, are two examples. It’s important to understand that the atmosphere is part of a larger system, and changes in that system can bring about changes in the atmosphere. Preview After the Earth formed, the atmosphere began to develop and stabilize. Driven by a series of traumatic events on the Earth’s surface, the atmosphere formed and re-formed before achieving its current state. Key Ideas—Factual Use these questions and prompts at the appropriate stopping points to check in with the students and ensure they are getting the key concepts covered in the video. 1. 0:37 How did the first atmosphere of the Earth form? answer: The high temperatures of the early Earth caused water vapor and other gases to be released from the Earth’s surface, forming a blanket of steam around the Earth. This steam, supplemented by other gases, thickened over time and became the first atmosphere. 2. 0:48 What is the “greenhouse effect,” and how did the conditions on the early Earth allow for its development? answer: The buildup of denser gases in the young atmosphere kept some gases from escaping into space, which further thickened the atmosphere. This buildup allowed for the further insulating, heating, and melting of the surface of the Earth. 3. 1:41 How did the formation of the Moon impact the Earth? answer: The collision that led to the formation of the Moon generated tremendous energy and raised the Earth’s temperature. The oceans that had formed evaporated into steam and the Earth’s surface melted again. 4. 2:11 How did the Earth’s atmosphere change during the Hadean eon? answer: Once the Earth cooled in the aftermath of the Moon’s formation, the oceans developed again. Also, volcanoes began to release heavier gases like CO2 and methane into the atmosphere. 5. 3:07 How did the Earth’s atmosphere change during the Archaean eon? answer: Living things appeared and they began to impact the atmosphere. Some forms of life consumed hydrogen, others released methane. Most important, though, some organisms developed the ability to do photosynthesis, which produced oxygen, although it took some time for oxygen to begin to accumulate in the atmosphere. 6. 3:55 What was the Great Oxidation Event and why did it happen? answer: The Great Oxidation Event refers to what happened when Earth’s atmosphere went from containing no oxygen to having a huge buildup, the result of photosynthesis-capable life. This buildup of oxygen caused the dying off of organisms to which oxygen was toxic; however, there were also many organisms that took advantage of the oxygenated environment. The buildup of oxygen had another important consequence for the Earth: it generated a layer of ozone that further helped to shield the Earth from the harmful radiation of the Sun. 7. 4:50 How did the buildup of oxygen in the atmosphere impact that Earth’s climate? answer: The buildup of oxygen in the atmosphere crowded out some gases, like CO 2 and methane. This reduced the greenhouse capability of the atmosphere, lowering the Earth’s temperature. Ice formed, and the ice reflected the Sun’s rays, which did not allow the crust to absorb this energy. This led to a further drop in the Earth’s temperature. The Earth was covered with ice at these times, so scientists call it Snowball Earth. Volcanic activity continued, though, and eventually enough CO 2 and methane were pumped into the atmosphere to allow the greenhouse effect to make a comeback. Eventually the Earth’s temperature rose, melting the ice.
