The Solar System Dana Desonie, Ph.D. Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required) AUTHOR Dana Desonie, Ph.D. To access a customizable version of this book, as well as other interactive content, visit www.ck12.org CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based collaborative model termed the FlexBook®, CK-12 intends to pioneer the generation and distribution of high-quality educational content that will serve both as core text as well as provide an adaptive environment for learning, powered through the FlexBook Platform®. Copyright © 2014 CK-12 Foundation, www.ck12.org The names “CK-12” and “CK12” and associated logos and the terms “FlexBook®” and “FlexBook Platform®” (collectively “CK-12 Marks”) are trademarks and service marks of CK-12 Foundation and are protected by federal, state, and international laws. Any form of reproduction of this book in any format or medium, in whole or in sections must include the referral attribution link http://www.ck12.org/saythanks (placed in a visible location) in addition to the following terms. Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made available to Users in accordance with the Creative Commons Attribution-Non-Commercial 3.0 Unported (CC BY-NC 3.0) License (http://creativecommons.org/ licenses/by-nc/3.0/), as amended and updated by Creative Com- mons from time to time (the “CC License”), which is incorporated herein by this reference. Complete terms can be found at http://www.ck12.org/terms. Printed: October 29, 2014 www.ck12.org Chapter 1. The Solar System CHAPTER 1 The Solar System CHAPTER OUTLINE 1.1 Sun 1.2 Interior of the Sun 1.3 Outer Layers of the Sun 1.4 Surface Features of the Sun 1.5 Planets of the Solar System 1.6 Inner versus Outer Planets 1.7 Mercury 1.8 Venus 1.9 Moon 1.10 Mars 1.11 Jupiter 1.12 Saturn 1.13 Uranus 1.14 Neptune 1.15 Exoplanets 1.16 Asteroids 1.17 Comets 1.18 Meteors 1.19 Dwarf Planets 1.20 References 1 www.ck12.org Introduction How well do humans know the solar system? Humans have been exploring the solar system for centuries. Mostly we have used telescopes or simply our eyes. But the Sun, all of the planets, and many dwarf planets, asteroids and comets have been studied more closely. Spaceships have taken rovers, probes or other technologies to study these bodies. Sometimes a spacecraft just flies by to take a look. So while it’s very difficult to get humans very far from Earth, our spacecraft have become our eyes. We know an incredible amount about the solar system compared with even a few decades ago. This is because of the tremendous spacecraft and the instruments they carry. In this concept, we will explore a small fraction of what scientists have learned about our solar system. 2 www.ck12.org Chapter 1. The Solar System 1.1 Sun • Describe the basic properties of the Sun. How can we learn about something as hot as the Sun? Humans have sent spacecraft up to study our star. The SOHO spacecraft has been in operation since 1996. The craft orbits the Sun in step with Earth but closer to it. SOHO has sent back amazing images. Onboard instruments have also sent back mountains of data. The data is mostly about the Sun’s outer layers. The Sun Our Sun is a star, a sphere of plasma held together by gravity. It is an ordinary star that is extraordinarily important. The Sun provides light and heat to our planet. This star supports almost all life on Earth. The Sun is the center of the solar system, which includes all of the planets and other bodies that orbit it. It is by far the largest part of the solar system ( Figure 1.1). Added together, all of the planets make up just 0.2 percent of the solar system’s mass. The Sun makes up the remaining 99.8 percent of all the mass in the solar system! 3 1.1. Sun www.ck12.org FIGURE 1.1 The sizes of the planets relative to the Sun, if the Sun were the size of a bas- ketball. Vocabulary • solar system: The Sun and all of the bodies that orbit it. • star: A massive sphere of plasma held together by gravity Summary • The Sun is an ordinary star. It supports nearly all life on Earth. • The Sun is at the center of the solar system. • The Sun makes up almost all of the mass of the solar system. Explore More Use the resource below to answer the questions that follow. • Introduction to the Sun and Solar System at http://www.youtube.com/watch?v=VXRczjxteFc (3:00) MEDIA Click image to the left for use the URL below. URL: http://gamma.ck12.org/flx/render/embeddedobject/118819 1. Why is it that you can’t see the star closest to Earth in the night sky? 2. Where is the Sun located? 3. Why is our Sun special? Is it a special type of star? 4. What is the structure of the solar system from the center to the farthest reaches? 5. Which type of solar system bodies come from the farthest part of the solar system? 4 www.ck12.org Chapter 1. The Solar System Explore More Answers 1. The Sun is the star closest to the Earth. It is only visible in the daytime because it’s light makes the day. 2. The Sun is located at the center of the solar system. 3. The Sun is an ordinary star. Without it, however, Earth would be cold, dark, and uninhabitable, so it is special to life on our planet. 4. The Sun is at the center of the solar system. Surrounding it first are the inner planets: Mercury, Venus, Earth, and Mars. They are followed by the asteroid belt and then the outer planets, also called the gas giants: Jupiter, Saturn, Uranus, and Neptune. Next are the dwarf planets, including Pluto, and finally, there is the Oort cloud. 5. comets Review 1. What are the major properties of the Sun? 2. Where is the Sun located? 3. Why is the Sun important to life on Earth? Review Answers 1. The Sun is plasma held together by gravity; it emits light and heat. 2. The Sun is located at the center of the solar system. 3. The Sun is the source of all the planet’s light and nearly all of its heat. Without the Sun, there would be no life on Earth. 5 1.2. Interior of the Sun www.ck12.org 1.2 Interior of the Sun • Describe the internal layers of the Sun. What does the Sun look like? Different instruments pick up different features of the Sun. None can show us the Sun’s interior. But studies from the SOHO spacecraft and other craft allow us to study the interior. We know a surprising amount about a body that is so hot and so far away. Layers of the Sun The Sun is made almost entirely of the elements hydrogen and helium. The Sun has no solid material. Most atoms in the Sun exist as plasma. Plasma is superheated gas with an electrical charge. Because the Sun is made of gases, it does not have a defined outer boundary. Like Earth, the Sun has an internal structure. The inner three layers make up what we would actually call “the Sun.” Internal Structure Because the Sun is not solid, it does not have a defined outer boundary. It does, however, have a definite internal structure with identifiable layers ( Figure 1.2). Since the layers are not solid, the boundaries are fuzzy and indistinct. From inward to outward, the layers are: the core, the radiative zone, and the convection zone. 6 www.ck12.org Chapter 1. The Solar System FIGURE 1.2 The layers of the Sun. The Core The core is the Sun’s innermost layer. The core is plasma. It has a temperature of around 15 million degrees Celsius (°C). Nuclear fusion reactions create the immense temperature. In these reactions, hydrogen atoms fuse to form helium. This releases vast amounts of energy. The energy moves toward the outer layers of the Sun. Energy from the Sun’s core powers most of the solar system. Radiative Zone The radiative zone is the next layer out. It has a temperature of about 4 million °C. Energy from the core travels through the radiative zone. The energy travels at an extremely slow rate. Light particles are called photons. In the radiative zone, photons can only travel a few millimeters before they hit another particle. The particles are absorbed and then released again. It may take 50 million years for a photon to travel all the way through the radiative zone. The Convection Zone The convection zone surrounds the radiative zone. In the convection zone, hot material from near the Sun’s center rises. This material cools at the surface and then plunges back downward. The material then receives more heat from the radiative zone. The first video describes the basics of our Sun, including how it is powered by nuclear reactions: http://www.youtube.com/watch?v=JHf3dG0Bx7I (8:34). 7 1.2. Interior of the Sun www.ck12.org MEDIA Click image to the left for use the URL below.