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Study Guide Our Solar System Student Note: The upcoming test on these topics will be given on ________________. This sheet should serve as a list of main topics but it is expected that you should also use class notes, labs and checked homework to review as well. Models of the Solar System Ancient observers noticed that the positions of the stars did not change over time. They believed that Earth was at the center of the universe. An Earth-centered model is known as a geocentric model. In a geocentric model, the Earth is at the center of the revolving planets and stars. About A.D. 140, Greek astronomer Ptolemy developed the geocentric model. It was widely accepted for almost 1,500 years after his death. In a heliocentric system, Earth and the other planets revolve around the sun. In 1543, Nicolaus Copernicus further developed the heliocentric model. Copernicus was able to work out the arrangement of the known planets and how they move around the sun. After years of detailed calculations, Johannes Kepler found that the orbit of each planet is an ellipse, which is an oval shape. For many years, people continued to believe in the geocentric model. However, evidence collected by Galileo Galilei gradually convinced the others that the heliocentric model was correct. In 1610, Galileo discovered four moons orbiting Jupiter, proving that not everything in the sky travels around the Earth. He also discovered that Venus goes through phases similar to the moon. Introducing the Solar System Our solar system consists of the sun, the planets, their moons and a variety of smaller objects. The sun is at the center with other objects orbiting it. Gravity holds the solar system together. About 99.85% of the mass of the solar system is contained within the sun. Distances in the solar system are so large that they are measured in astronomical units. One astronomical unit (AU) equals the average distance between the Earth and the sun, about 150,000,000 kilometers. A planet is round, orbits the sun and has cleared out the region of the solar system along its orbit. All except Mercury or Venus have at least one natural satellite, or moon. A dwarf planet is an object that orbits the sun and has enough gravity to be round but has not cleared the area of its orbit. Scientists think the solar system formed about 4.6 billion years ago from a cloud of hydrogen, helium, rock, ice and other materials pulled together by gravity. Eventually, the temperature and pressure in the cloud became so high that, through nuclear fusion, hydrogen atoms were pressed together to form helium, a process that produces light and heat. Away from the sun, planets began to form as gravity pulled rock, ice and gas together. The rock and ice form small bodies called planetismals. Over time, planetismals collided and stuck together, eventually combining to form all the other objects in the solar system. The Sun The sun has an interior and an atmosphere. The interior includes the core, the radiation zone, and the convection zone. The sun produces a large amount of energy in its core through nuclear fusion, a process where hydrogen atoms join together to form helium. The energy produced in the sun’s core moves outward through the radiation zone. The outermost layer of the sun’s interior is called the convection zone. In this zone, energy moves towards the sun’s surface by convection. Like the sun’s interior, its atmosphere is composed mainly of hydrogen and helium. The sun’s atmosphere includes the photosphere, the chromosphere and the corona. Each layer has unique properties. The photosphere is the inner layer which you see when you look at the sun. The chromosphere is the middle layer. The corona is the outer layer. The corona gradually thins into streams of electrically charged particles called the solar wind. Features on or just above the sun’s surface include sunspots, prominences, and solar flares. Sunspots are areas of gas on the sun’s surface that are cooler than the gases around them. Sunspots usually occur in groups. Huge looks of gas called prominences link different parts of a sunspot region. When loops in sunspot regions suddenly connect, they release large amounts of magnetic energy. These eruptions are called solar flares. The Inner Planets The inner planets are small and dense and have rocky surfaces. They are often called terrestrial planets. Although the four inner planets have many features in common, they differ in size and composition as well as distance from the sun. Mercury is the smallest terrestrial planet and closest to the sun. It has virtually no atmosphere because its gravity is so weak. Venus has a thick atmosphere, an unusual pattern of rotation and the hottest surface of any planet. It is so hot because its atmosphere traps heat in a process called the greenhouse effect. It rotates slowly so its day is longer than its year. Oddly, Venus rotates from east to west, the opposite direction from most other planets and moons. Earth has liquid water and a suitable temperature range and atmosphere for living things to survive. Earth has enough gravity to hold onto most gases that make up Earth’s atmosphere. Earth is the only planet with an oxygen-rich atmosphere. Like Venus, Earth experiences a greenhouse effect. Although Mars is too cold for liquid water, it does have water ice now and had liquid water in the past. Mars’ atmosphere is more than 95 % carbon dioxide. Some regions of the planets have giant volcanoes. The Outer Planets The four outer planets are much larger and more massive than Earth and they do not have solid surfaces. Because these four planets are so large, they are often called gas giants. The gas giants all contain hydrogen and helium. In addition, all have moons and are surrounded by a set of rings. A ring is a thin disk of small particles of ice and rock. They do not have solid surfaces. If you could visit them, you wouldn’t have a solid place to stand. Jupiter is the largest and most massive planet. Its mass is about 2.5 times that of all the other planets combined. One notable feature of Jupiter’s atmosphere is its Great Red Spot, a storm that is larger than Earth. Saturn has the most spectacular rings of any planet. The rings are made of chunks of ice and rock, each traveling its own orbit around Saturn. Saturn’s largest moon, Titan, is larger than the planet Mercury and has features that look as if they were formed by flowing liquid. A few scientists thing that Titan might support life. Uranus is twice as far from the Sun as Saturn, so it’s much colder. It looks blue-green because of methane traces in its atmosphere. Its five largest moons have icy, cratered surfaces. Uranus’s axis of rotation is titled at an angel of about 90 degrees from the vertical. Uranus rotates from top to bottom- not side to side. Neptune is a cold, blue planet. Its atmosphere contains visible clouds. Neptune’s blue color comes from methane in its atmosphere. Its interior is hot from energy left from its formation. As the energy rises, it produces clouds and storms in the atmosphere. Small Solar System Objects The solar system contains many small objects that, like the planets, orbit the sun. Scientists classify these objects based on their sizes, shapes, composition and orbits. The major categories include dwarf planets, comets, asteroids and meteoroids. Most small solar system objects are found in three areas: the asteroid belt, the Kuiper belt and the Oort cloud. The asteroid belt is a region between Mars and Jupiter. Beyond Neptune is a region called the Kuiper belt, which extends to about 100 AU. Further out is the Oort Cloud, extending more than 1000 times the distance between the sun and Neptune. Dwarf planets orbit the sun and have enough gravity to pull themselves into spheres but they have other objects in the areas of their orbits. There are five known dwarf planets, including Pluto. Except Cere, all dwarf planets orbit beyond Neptune. Comets are loose collections of ice, dust and small rockey particles whose orbits can be long, narrow ellipses. Most comets originate in the Oort cloud. When a comet gets close to the sun, the ice melts, releasing gas and dust. Clouds of gas and dust form a comet’s flowing, fuzzy outer layer, or coma. The nucleus is the solid inner core of a comet. Some of the gas and dust streams outward, forming two tails – a gas tail that points away from the sun and a dust tail that points along the path the comet has taken. Asteroids are rocky objects, most of which are too small and too numerous to be considered planets or dwarf planets. Asteroids are leftover pieces of the early solar system that never came together to form a planet. Meteoroids are chunks of rock or dust smaller than asteroids. When a meteoroid enters Earth’s atmosphere, friction with the air creates heat and produces a streak of light called a meteor. Most meteoroids burn up completely. But those reaching the Earth’s surface are known as meteorites. Meteorite impacts can leave craters. Meteor showers occur as Earth passes through areas with many meteoroids, and are often named for the constellations from where they appear to come.