CHAPTER 7 The Birth and Evolution of Planetary Systems hn hk io il sy SY hn hk io il sy SY hn hk io il sy SY Ideas about the origins of the Sun, the Moon, and Earth objects in the Solar System. It is important for the stu- hn hk io il sy SY are older than written history. Greek and Roman mythol- dents to realize that you can determine many of the prop- ogy, as well as creation myths of the Bible, represent some erties of a planet by knowing its mass, size, and distance hn hk io il sy SY of humanity’s earliest attempts to explain how the heav- from the Sun. hn hk io il sy SY ens and Earth were created. Thousands of years and Although planetary scientists are confident they have hn hk io il sy SY the scientific revolution have ultimately debunked many the big picture of Solar System formation correct, the ancient creation stories, but our new theories of solar sys- details are still in question. Questions remain concerning hn hk io il sy SY tem formation are still relatively in their infancy. Jupiter’s formation. It might not have been able to form hn hk io il sy SY The nebular model of solar system formation was first via accretion but may have formed similarly to the Sun. proposed by Immanuel Kant in 1755. It has undergone To complicate matters further, Uranus and Neptune significant revision in the last 250 years, but the details of appear to be too large to have formed at their current posi- the pro cess remain elusive. In broad strokes, our current tions in the Solar System. Additionally, more than one understanding suggests that the Solar System formed hundred large extrasolar planets have been discovered from a collapsing cloud of interstellar gas and dust. Most in odd orbits about other stars, hinting that perhaps of the infalling material fell to the center of the cloud, our Solar System is not representative of solar systems where it became the proto- Sun, but a significant fraction throughout the galaxy. As you teach this chapter, be sure of the cloud’s mass was flattened into a disk- shaped neb- to remind your students that the story being told is still ula surrounding the proto- Sun. Because of the higher incomplete and probably even partially incorrect. It is, temperatures in the inner nebula, only relatively rare however, a great demonstration of science in action, illus- refractory materials (rock and metal) condensed from the trating how our understanding of the universe changes nebula. However, in the outer nebula, abundant volatile and grows as technology provides us with better observa- materials also condensed as ices. Rock and metal particles tions and more data. It also is very exciting to be able to in the inner nebula accreted into small protoplanetary teach your students about other worlds and the fact that seeds that gave birth to terrestrial planets. In the outer even more are being found as you read this and speak to nebula, ices dominated the larger protoplanetary seeds them. that became the Jovian planets. All of the protoplanets captured hydrogen and helium atmospheres from the DISCUssION POINTS nebula; but when nuclear fusion ignited the Sun, powerful solar winds cleared the nebula of loose material and • Based on the amount of material near the Sun, how stripped the small terrestrial planets of their atmospheres. likely is it that the Sun has siblings that formed from Meanwhile, because they were larger and farther from the the same parental molecular cloud? Discuss with stu- Sun, the Jovian planets retained their atmospheres despite dents the possibility of stellar siblings of any kind. buffeting from the solar winds. After the Sun cleared the • Have students use the equation for spin angular nebula, protoplanets on crossing orbits swept up any momentum to calculate the values for the Sun, Jupiter, remaining material and collided with each other, ulti- Saturn, Neptune, Earth, Pluto, and a typical molecular mately growing to the full- sized planets we see today. cloud. Discuss these in the context of conservation of Eventually the volcanically active terrestrial planets angular momentum. (aided by comet impacts that delivered volatile material • Ask students, if they had to choose just one technique from the outer Solar System) outgassed secondary atmo- among the leading choices for finding extrasolar plan- spheres. As implied from the above statements, the envi- ets that might be as habitable as Earth, which one they ronment created by the Sun dictated a great deal of the would pick. What makes that method advantageous properties for each of the planets, as well as the other over the others? —-1 —0 —+1 1 577-53231_ch01_4P.indd 1 3/9/13 8:16 AM 2 ◆ Chapter 7 The Birth and Evolution of Planetary Systems • Discuss with students the significance of finding as many extrasolar planets as we have so far. Consider the DEMONSTRATIONS AND ACTIVITIes discovery of Earth- like planets in habitable zones. Demonstration: Ranking Task for Formation What cultural impact does that have? What questions of Planetary Systems does it provoke? If there were life, would we be able to communicate with it? Note to Instructors hn hk io il sy SY Ranking tasks are excellent means of helping students hn hk io il sy SY EXPLORATION think about the progression of events, whether it is in hn hk io il sy SY space, as in sizes of objects, or in time, as in which came Exploration 1: Using the Transit Method hn hk io il sy SY first, second, and so on. This ranking task asks students to Detect Exoplanets to think about the pro cess of the formation of a star and hn hk io il sy SY Note to Instructors its planetary system. Rather than memorizing the steps, students should visualize the birth of a star and planets hn hk io il sy SY This alternate Exploration is about searching for extra- in steps that begin from a giant, cool, rotating molecu- solar planets (exoplanets) using transits. It pairs nicely hn hk io il sy SY lar cloud and end with fully formed planets orbiting a with the Exploration in the textbook since Doppler shift hn hk io il sy SY genuine star. This is listed as a demonstration rather is the way astronomers have found most of the known than an activity because students should be able to dis- hn hk io il sy SY extrasolar planets to date, but transits— specifically cuss, explain, and explore what they think is the correct with the Kepler mission— are how astronomers are cur- order. rently searching for new extrasolar planets. Transits hold the best potential for finding Earth- like extrasolar Learning Goals planets. • Chapter learning goal(s) addressed: Summarize the Learning Goals role that gravity, energy, and angular momentum play in the formation of stars and planets. Describe the • Explain thoroughly how transits can be used to dis- modern theory of planetary system formation. cover extrasolar planets. • State what mea sur able pa ram e ters transit observations Required Materials can yield. • Cards or strips of paper containing the steps Required Materials • Same steps attached to strips of magnetic sheeting • Computer with Internet access Instructions Pre- Post- Assessment Question Make enough copies of the list of the stages in star /planet formation (included with the worksheets for this chap- Assuming the mass of the parent star is known, what ter) to hand out to teams of 3– 5 students. Cut the sheet information about an extrasolar planet can be inferred into strips, shuffle the strips, and place sets of strips into from a mea sured light curve? envelopes. Students should work in teams to put the stages a. Orbital period into the correct order, according to the current theory b. Planet radius of planet formation. If events occur nearly simultane- c. Orbital radius ously, then those stages should be put into the same pile. d. All of the above can be inferred. After students have had enough time to discuss the order Answer: d of the stages and placed the strips in order from first to last, call on a team to read off their order, or have a rep- Post- Exploration Debriefing resentative of a team come to the board and place the After completing this activity, have students visit NASA’s magnetic strips in the order his or her team found. Kepler website to research the latest discoveries at htt p:// Correct order according to the text (*coeval events): kepler.nasa.gov/Mission/discoveries. 1. Cloud of interstellar gas starts to collapse under the force of its own self- gravity. Instructions 2. *Gravitational potential energy of collapsing inter- Complete instructions are included on the student stellar gas cloud is converted into heat and radiative -1— worksheet. energy. 0— +1— 577-53231_ch01_4P.indd 2 3/9/13 8:16 AM Chapter 7 The Birth and Evolution of Planetary Systems ◆ 3 3. *Cloud of interstellar gas rotates faster and faster • Investigate the claim that a Jupiter- like planet could as it collapses because of conservation of angular not exist at the location of 51 Pegasi b. momentum. • Chapter learning goal(s) addressed: Describe how 4. *Inner parts of flattening cloud begin to fall freely astronomers find planets around other stars and what inward, raining down on growing object at the center. those discoveries tell us about our own and other solar 5. Material makes its final inward plunge, landing on a systems. thin, rotating accretion disk. hn hk io il sy SY 6. Motions push smaller grains of material back and Required Materials hn hk io il sy SY forth past larger grains; smaller grains stick to larger • Scientific calculator hn hk io il sy SY grains.