Astronomy - The Solar System - Quick Guides | Astronomy

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Study GuideAstronomy–The Solar System1. IntroducƟon to the Solar System1.1What Is the Solar System?Thesolar systemis made up of theSun,nine planets, about60 moons, and many smaller objects.These smaller objects includeasteroids, meteoroids, comets, dust, and gas.Even though planets look big in pictures,all of these objects are very small compared to the hugedistances between them. Space is mostly empty!1.2A Helpful Way to Imagine the Solar SystemTo understand just how spread out the solar system is, let’s imagine ascale model.Picture a10-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:•TheSunis only2.3 meters in diameter(about7.6 feet).•Theplanetsare tiny and placedhundreds or thousands of meters apart.•This shows howfar apartthe planets really are, even though textbooks often draw themclose together.1.3Understanding the Scale Model TableThe table above shows three important things for each object in the solar system:1.Position along the trail2.This tells you how far each planet would be from the Sun on the 10-km trail.3.Diameter of the model sphere4.This shows how big each planet would be in the scale model.5.Comment6.This compares each planet’s size to something familiar, like amarble,baseball, orsoccerball, to make it easier to imagine.

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Study Guide1.4What the Table Helps Us Learn•Inner planets likeMercury, Venus, Earth, and Marsarevery smallandclose togethercompared to the outer planets.•Giant planets likeJupiter and Saturnare muchlarger, but still very far apart.•Plutois extremely small and located near theend of the trail.•Cometscan travel incredibly far—up to25,000 kilometerson this scale—and are made oftiny dust-sized particles.1.5Why This Model MaƩersThis 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 diagramsIt reminds us that space is not crowded—it is wide, open, and mostly empty.

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Study Guide2. Origin and EvoluƟon of the Solar System2.1When and Why the Solar System FormedThe solar system formed about4.6 ± 0.1 billion years ago.Astronomers observe many features of the solar system that cannot be explained by simple laws likegravity alone. Because science assumes thatevery observable feature has a cause, thesecharacteristics must be linked tohow the solar system originally formed.2.2Key Observed Features of the Solar SystemScientists have identified several important patterns:•Almost all planets orbit the Sun inone flat plane(the solar system is nearly flat).•Pluto is an exception.•TheSun’s equatorlies in nearly the same plane as the planets’ orbits.•Most planetary orbits arenearly circular.•Mercury and Pluto are exceptions.•The Sun and planetsrevolve in the same direction, calleddirect motion(west to east asseen from Earth).•The Sun and most planets alsorotate in the same direction, and their axial tilts are usuallysmall.•Exceptions include Venus, Uranus, and Pluto.•Most planets and asteroids havesimilar rotation periods, except Mercury, Venus, and Pluto.•The planets areregularly spaced, a pattern often described byBode’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 massis in the Sun, but99.8% of its angular momentumis inthe planets.•This explains why the Sun rotates relatively slowly.•Chemical composition changes with distancefrom the Sun:

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Study GuideoInner planets aredense and metal-rich(terrestrial planets).oOuter planets aregas-rich, made mostly of hydrogen and helium.•Meteorites have compositions similar to planets but arenot exactly the same, showing earlyvariation.•Cometsexist far beyond the planets in alarge spherical cloudaround the solar system.2.3Early Ideas About Solar System FormaƟonOlder theories suggestedcatastrophic events, such as a near collision between the Sun andanother star.Modern science rejects these ideas because they assume the solar system isunique or special.Instead, scientists support theSolar Nebula Theory, also called theplanetesimal or condensationtheory, which explains solar system formation usingbasic laws of physics.2.4The Solar Nebula Theory: Step by Step1. The Original NebulaBefore the Sun and planets existed, all solar system material was part of ahuge cloud of gas anddust, called anebula.•Mostly hydrogen and helium•About2% heavier elementsNormally, gas pressure balances gravity, keeping the cloud stable. However, a small disturbance—such as ashock wave from a nearby exploding star—can trigger collapse.This instability is known asJeans Instability.2. GravitaƟonal Collapse and HeaƟngAs the nebula collapsed under gravity (calledHelmholtz contraction):•Particles fell inward andheated up•Collisions converted motion energy into heat•Abouthalf the energy escaped as radiation, preventing pressure from stopping collapseThe center collapsed faster, becominghotter and denser.

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Study Guide3. Birth of the SunEventually, the central temperature and pressure became high enough forthermonuclear reactionsto begin.At this point:•Gas pressure balanced gravity•A newSun was bornat the center of the nebula2.5Role of Angular MomentumAngular momentum depends onmass, rotation speed, and radius.As the nebula shrank, itsrotation sped up, similar to a spinning skater pulling in her arms.2.6Why the Sun Slowed DownAlthough the young Sun rotated rapidly at first, two processes slowed it:1.Magnetic braking2.The Sun’s magnetic field linked the fast-spinning center to the slower outer nebula,transferring angular momentum outward.3.Strong solar wind4.The early Sun lost angular momentum through powerful outflowing particles.2.7FormaƟon of the Protoplanetary DiskAs rotation increased:•Gravity balanced motion in theequatorial plane•Material above and below fell inward•The nebula flattened into arotating disk

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Study Guide2.8Temperature Differences in the DiskThe disk cooled outward from the Sun:•~2000 K near the center•~10 K at the outer edgeThis temperature gradient controlledwhat materials could condense.2.9CondensaƟon of MaterialsAs temperatures dropped:•Above2000 K: all elements stayed gaseous•Below1400 K: iron and nickel condensed•Below1300 K: silicate minerals formed•Below300 K: gases froze into ices (water, ammonia, methane, carbon dioxide)Meteorites calledcarbonaceous chondritesprovide direct evidence of this early grain formation.

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Study Guide2.10FormaƟon of the PlanetsInner Solar System•Onlyheavy elementscondensed•Dust grains stuck together→planetesimals•Planetesimals merged intoterrestrial planets•Leftover debris formed theasteroid beltOuter Solar System•Containedices + gas•Protoplanets grew larger•Gravity causedrapid gas collapse, forming gas giants•Surrounding material formedmoon systems, similar to miniature solar systems3. Terrestrial Planets, Gas-Giant Planets

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Study Guide3.1Understanding Planets Through Planetary AstronomyPlanetary astronomy focuses on studying the planets and other major objects in our solar system. Themain goal is to understandhow planets are similar and how they are different. This comparison iscalledcomparative planetology.Scientists compare many features, such as:•Atmospheres•Surface conditions•Internal structures•Magnetic fieldsThey also study theorder of events and time periodsthat explain how the solar system formed andchanged over time.3.2Two Main Types of Planets in the Solar SystemWhen we look at the planets in the solar system (see Figure 1), a clear pattern appears. The planetsnaturally fall intotwo major groupsbased on their characteristics:1.Terrestrial (Earth-like) planets2.Gas-giant (Jupiter-like) planetsThese differences are clearly shown inTable 1, which compares their properties.

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