Comparing Solar Systems

Unit 8: Comparing Solar Systems This material was developed by the Friends of the Dominion Astrophysical Observatory with the assistance of a Natural Science and Engineering Research Council PromoScience grant and the NRC. It is a part of a larger project to present grade-appropriate material that matches 2020 curriculum requirements to help students understand planets, with a focus on exoplanets. This material is aimed at BC Grade 6 students. French versions are available. Instructions for teachers ● For questions and to give feedback contact: Calvin Schmidt [email protected], ● All units build towards the Big Idea in the curriculum showing our solar system in the context of the Milky Way and the Universe, and provide background for understanding exoplanets. ● Look for Ideas for extending this section, Resources, and Review and discussion questions at the end of each topic in this Unit. These should give more background on each subject and spark further classroom ideas. We would be happy to help you expand on each topic and develop your own ideas for your students. Contact us at the [email protected]. Instructions for students ● If there are parts of this unit that you find confusing, please contact us at [email protected] for help. ● We recommend you do a few sections at a time. We have provided links to learn more about each topic. ● You don’t have to do the sections in order, but we recommend that. Do sections you find interesting first and come back and do more at another time. ● It is helpful to try the activities rather than just read them. ● Explore the “Ideas for extending this section” and “Resources” sections at the end of each topic in this Unit - they aren’t just for teachers! Learning Objectives ● The BC curriculum requires students to learn: ○ about the “Components of our solar system”: studying the orbits and scale of our solar system helps meet this requirement. ○ “[How the] exploration of extreme environments on Earth and in space changed in the last decade?” and also “Canada’s contribution to exploring extreme environments” (in the context of planets) Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 1 ● Students will not be limited to talking about planets in our solar system and will learn about planets around other stars in order to expand the range of environments and see our solar system as one of many. Learning Outcomes ● Students will: ○ understand the scale of our solar system and how exoplanet orbits compare ○ understand how orbits can affect the environment, sometimes leading to extreme environments on other worlds. ○ understand two geometric shapes, ellipses and hyperbolas, with an emphasis on ellipses ○ understand how our understanding of our solar system’s configuration changed over time Materials and tools needed for the activities ● Activity 1 - Stellarium Installed ● Activity 2 ● Activity 3 - This link : https://www.exploratorium.edu/ronh/age/ Time Required ● Lesson time - 90 minutes ● Activity time ○ Activities 1 and 2: 15 minutes each ○ Activity 3: 5 to 10 minutes Contents The activities are marked in yellow. ● Families of worlds ● Do planets really "wander"? ○ Activity 1: Retrograde Motion ● Orbit shapes ○ Activity 2: Making ellipses ● Orbit Sizes ● How long is a year on different planets? ○ Activity 3: How old are you by different planet years? ● Different kinds of solar systems ● The Goldilocks zone Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 2 F amilies of worlds If you watch science fiction like Star Wars or Star Trek you take for granted that there are planets going around other stars. But we’ve only learned that there really are other solar systems in the last thirty years, which is within the lifetime of most of the parents of grade 6 students. As we’ve mentioned, 3,176 solar systems have been found at the time of writing in August 2020, and more will soon be found. Are these families of worlds, each born together, like ours with small rocky planets close to their star and gas giant planets further out? How are these other solar systems different from ours, and how are they the same? If our solar system is unusual, do we know why? You’ll start by learning a little about how astronomers figured out that we are living in a solar system, then you’ll learn about orbits, and then compare known solar systems, many of which are quite different from our own. D o planets really “wander”? In the last unit we talked about how the word “planet” is the Greek word for “wanderer”, and how they were called that because they looked like stars that moved around. Think for a minute about what “wander” really means. If you are wandering, are you following a plan, or a specific route? When people wander they don’t follow a set path, and instead make it up as they go along, seldom going the same route twice. That’s not what planets do. The Greeks and others who watched carefully noticed that the planets move in ways that have special shapes and paths, and those paths are quite predictable and usually repeat. It’s the opposite of wandering. Most ancient peoples thought the Earth was at rest: it did not give the impression of wandering or having any sort of motion at all. There was no effect like a wind that blew from one direction constantly, like they expected for a moving Earth. That’s what they experienced when running fast or riding a horse: air rushes past your head. The stars and planets, as we discussed in Unit 1: Stars and Distances, appeared to be of the same distance. The stars seemed to turn together, but people noticed that the Sun and Moon seemed to move at a slightly different rate than the stars but did not collide even though they passed each other in the sky once a month. To explain this, a Greek academic named Anaximander who lived around 2,600 years ago, argued that they must be something like spinning Chariot wheels, one inside the other, with us at the centre. He suggested these wheels turned at different rates. Anaximander thought that the light of the Moon and Sun came from a little window showing the fire on the inside of each wheel. Even before we could prove that the Sun and Moon were at different distances from us, which Hipparchus did 500 years later, the Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 3 idea that they were at different distances was suggested by Anaximander to explain sky motions. Later Greeks imagined that these wheels were instead thin transparent rings or spheres, one inside the other, made out of something similar to the clear crystal, a material they knew about. To explain the planets, which moved more slowly than the Moon and Sun, they put them further out from the Earth, which they put at the centre. If you look at a cross-section of this, as you can see in Figure 1, the circular paths of the planets look a bit like orbits, but they didn’t imagine that the planets were even worlds, let alone zipping through empty space. Notice that the stars are near the outside, with the Earth in the middle. Figure 1: Celestial Spheres (Engraving from Peter Apian's Cosmographia, 1524) This was a good start, but it didn’t explain how things moved in the sky precisely. While it seemed to explain some things about the speeds at which things moved across the sky, it ran into problems with trying to explain the details of motions of the planets, especially Mercury and Venus, which we now know are closer to the Sun than us. Mars would also, for example, Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 4 change directions in where it moved among the stars every two years, doing a loop over a period of a couple of months. A Greek named Philolaus who lived about 2,400 years ago tried to solve the problem by suggesting that there was a central fire surrounded by the planets and our Sun, Moon, and Earth orbited it. In this complicated idea, Philolaus imagined that we couldn’t see the central fire because another Earth blocked our view of it! Of course, he had no evidence for another Earth or a central fire that was always invisible to us. On the other hand, it was the first time someone suggested that the spinning of the Earth gave us day and night, and that the Earth orbited something. It still didn’t fully explain the motions of the planets, however. A Greek astronomer who lived about 100 years later, Aristarchus, built on Philolaus’ ideas and made the Sun the “central fire”, and also put each planet in its correct order from the Sun. This was the first “heliocentric” system, “helio” being the Greek word for “Sun”, and “centric” meaning “centred”. He realized that this would explain the motions of each celestial object much better, and he said that the Earth moved around the Sun, which was a new idea. Aristarchus, who could prove the relative distances of the Moon and Sun, realized that the solar system must be large, and reasoned that the stars were other suns seen at a great distance. Aristarchus was correct, but most people were not convinced. Seleucus, who lived about 100 years later, is thought to have found more proof by using Aristarchus’ theory to predict the motions of planets with greater accuracy. But most people supported the idea that the Earth was at the centre and crystalline spheres revolved around us, an idea promoted by Ptolemy and by the influential philosopher Aristotle.

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