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Astronomy - The Solar System

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    Study Guide Astronomy – The Solar System 1. Introduc Ɵ on to the Solar System 1.1 What Is the Solar System? The solar system is made up of the Sun , nine planets , about 60 moons , and many smaller objects. These smaller objects include asteroids, meteoroids, comets, dust, and gas . Even though planets look big in pictures, all of these objects are very small compared to the huge distances between them . Space is mostly empty! 1.2 A Helpful Way to Imagine the Solar System To understand just how spread out the solar system is, let’s imagine a scale model . Picture a 10 - kilometer - long hiking trail . Along this trail, we place models of the Sun and planets at the correct distances from each other. On this scale: • The Sun is only 2.3 meters in diameter (about 7.6 feet ). • The planets are tiny and placed hundreds or thousands of meters apart . • This shows how far apart the planets really are, even though textbooks often draw them close together. 1.3 Understanding the Scale Model Table The table above shows three important things for each object in the solar system: 1. Position along the trail 2. This tells you how far each planet would be from the Sun on the 10 - km trail. 3. Diameter of the model sphere 4. This shows how big each planet would be in the scale model. 5. Comment 6. This compares each planet’s size to something familiar, like a marble , baseball , or soccer ball , to make it easier to imagine.

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    Study Guide 1.4 What the Table Helps Us Learn • Inner planets like Mercury, Venus, Earth, and Mars are very small and close together compared to the outer planets. • Giant planets like Jupiter and Saturn are much larger , but still very far apart. • Pluto is extremely small and located near the end of the trail . • Comets can travel incredibly far — up to 25,000 kilometers on this scale — and are made of tiny dust - sized particles. 1.5 Why This Model Ma Ʃ ers This scale model helps students understand two key ideas: • Planet sizes are tiny compared to space distances • The solar system is vast , much bigger than it appears in diagrams It reminds us that space is not crowded — it is wide, open, and mostly empty.

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    Study Guide 2. Origin and Evolu Ɵ on of the Solar System 2.1 When and Why the Solar System Formed The solar system formed about 4.6 ± 0.1 billion years ago . Astronomers observe many features of the solar system that cannot be explained by simple laws like gravity alone. Because science assumes that every observable feature has a cause , these characteristics must be linked to how the solar system originally formed . 2.2 Key Observed Features of the Solar System Scientists have identified several important patterns: • Almost all planets orbit the Sun in one flat plane (the solar system is nearly flat). • Pluto is an exception. • The Sun’s equator lies in nearly the same plane as the planets’ orbits. • Most planetary orbits are nearly circular . • Mercury and Pluto are exceptions. • The Sun and planets revolve in the same direction , called direct motion (west to east as seen from Earth). • The Sun and most planets also rotate in the same direction , and their axial tilts are usually small. • Exceptions include Venus, Uranus, and Pluto. • Most planets and asteroids have similar rotation periods , except Mercury, Venus, and Pluto. • The planets are regularly spaced , a pattern often described by Bode’s law . • Major moons orbit their planets in a way that mirrors the solar system: • flat planes, circular orbits, and uniform direction of motion . • 99.8% of the solar system’s mass is in the Sun, but 99.8% of its angular momentum is in the planets. • This explains why the Sun rotates relatively slowly. • Chemical composition changes with distance from the Sun:

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    Study Guide o Inner planets are dense and metal - rich (terrestrial planets). o Outer planets are gas - rich , made mostly of hydrogen and helium. • Meteorites have compositions similar to planets but are not exactly the same , showing early variation. • Comets exist far beyond the planets in a large spherical cloud around the solar system. 2.3 Early Ideas About Solar System Forma Ɵ on Older theories suggested catastrophic events , such as a near collision between the Sun and another star. Modern science rejects these ideas because they assume the solar system is unique or special . Instead, scientists support the Solar Nebula Theory , also called the planetesimal or condensation theory , which explains solar system formation using basic laws of physics . 2.4 The Solar Nebula Theory: Step by Step 1. The Original Nebula Before the Sun and planets existed, all solar system material was part of a huge cloud of gas and dust , called a nebula . • Mostly hydrogen and helium • About 2% heavier elements Normally, gas pressure balances gravity, keeping the cloud stable. However, a small disturbance — such as a shock wave from a nearby exploding star — can trigger collapse. This instability is known as Jeans Instability . 2. Gravita Ɵ onal Collapse and Hea Ɵ ng As the nebula collapsed under gravity (called Helmholtz contraction ): • Particles fell inward and heated up • Collisions converted motion energy into heat • About half the energy escaped as radiation , preventing pressure from stopping collapse The center collapsed faster, becoming hotter and denser .

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    Study Guide 3. Birth of the Sun Eventually, the central temperature and pressure became high enough for thermonuclear reactions to begin. At this point: • Gas pressure balanced gravity • A new Sun was born at the center of the nebula 2.5 Role of Angular Momentum Angular momentum depends on mass, rotation speed, and radius . As the nebula shrank, its rotation sped up , similar to a spinning skater pulling in her arms. 2.6 Why the Sun Slowed Down Although the young Sun rotated rapidly at first, two processes slowed it: 1. Magnetic braking 2. The Sun’s magnetic field linked the fast - spinning center to the slower outer nebula, transferring angular momentum outward. 3. Strong solar wind 4. The early Sun lost angular momentum through powerful outflowing particles. 2.7 Forma Ɵ on of the Protoplanetary Disk As rotation increased: • Gravity balanced motion in the equatorial plane • Material above and below fell inward • The nebula flattened into a rotating disk

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    Study Guide 2.8 Temperature Di ff erences in the Disk The disk cooled outward from the Sun: • ~2000 K near the center • ~10 K at the outer edge This temperature gradient controlled what materials could condense . 2.9 Condensa Ɵ on of Materials As temperatures dropped: • Above 2000 K : all elements stayed gaseous • Below 1400 K : iron and nickel condensed • Below 1300 K : silicate minerals formed • Below 300 K : gases froze into ices (water, ammonia, methane, carbon dioxide) Meteorites called carbonaceous chondrites provide direct evidence of this early grain formation.

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    Study Guide 2.10 Forma Ɵ on of the Planets Inner Solar System • Only heavy elements condensed • Dust grains stuck together → planetesimals • Planetesimals merged into terrestrial planets • Leftover debris formed the asteroid belt Outer Solar System • Contained ices + gas • Protoplanets grew larger • Gravity caused rapid gas collapse , forming gas giants • Surrounding material formed moon systems , similar to miniature solar systems 3. Terrestrial Planets, Gas - Giant Planets

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    Study Guide 3.1 Understanding Planets Through Planetary Astronomy Planetary astronomy focuses on studying the planets and other major objects in our solar system. The main goal is to understand how planets are similar and how they are different . This comparison is called comparative planetology . Scientists compare many features, such as: • Atmospheres • Surface conditions • Internal structures • Magnetic fields They also study the order of events and time periods that explain how the solar system formed and changed over time. 3.2 Two Main Types of Planets in the Solar System When we look at the planets in the solar system ( see Figure 1 ), a clear pattern appears. The planets naturally fall into two major groups based on their characteristics: 1. Terrestrial (Earth - like) planets 2. Gas - giant (Jupiter - like) planets These differences are clearly shown in Table 1 , which compares their properties.

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