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Earth Space Science : 08 Our Solar System : 08.04 Objects in Our Solar System

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Earth Space Science : 08 Our Solar System : 08.04 Objects in Our Solar System Objectives What can objects in our solar system tell us about its formation and future? At the end of this lesson, you will be able to: • compare characteristics of inner and outer planets • define the role of “other” solar system objects in our solar system • discuss how knowledge of our own solar system can be applied to future planetary discoveries Objects in Our Solar System My Very Excellent Mother Just Served Us Nine Pizzas Taking the first letter of each word in the above sentence gives the following: M V E M J S U N P This is the order of the planets in our solar system. You can remember the planets using this simple sentence or one like it. • Mercury • Venus • Earth • Mars • Jupiter • Saturn • Uranus • Neptune • Pluto But wait! How did scientists determine the number of planets in the solar system, anyway? What about Pluto? Do scientists still consider it a planet? When Pluto was discovered in 1930, astronomers listed it as the ninth planet in the solar system. In 2005, astronomers detected yet another large body, called Eris, beyond Pluto's orbit. A debate erupted among scientists over the characteristics a heavenly body must have to be considered a planet. To resolve the issue, in 2006, astronomers downgraded Pluto from planet to dwarf planet. (Eris is also a dwarf planet.) Why was there so much debate about how to define a planet? As with any discovery in science, new information leads to new ideas. Use the following activity to investigate the difference between a planet and a dwarf planet. Differences between Planet and Dwarf Planet Interactive Characteristics of planets and dwarf planets: • A planet o is in orbit around the sun o has sufficient mass to assume a nearly round shape o is not a satellite or moon of another object o has enough gravity to clear away smaller objects near its orbit; when a planet interacts with smaller objects, it either consumes them or slings them away with its gravity • A dwarf planet o is in orbit around the sun o has sufficient mass to assume a nearly round shape o is not a satellite or moon of another object o does not have enough gravity to clear away smaller objects near its orbit, so it exists in a zone of other, similarly sized objects The International Astronomical Union changed the definition of a planet so that all astronomers worldwide could classify new objects such as Eris, found beyond Pluto's orbit. However, astronomers today still debate the definition of a planet. In this lesson, you will be reviewing the characteristics of the inner and outer planets in the solar system. You will also investigate other objects in the solar system that orbit the sun. The debate over the definition of a planet illustrates that scientific knowledge constantly changes as scientists discover new evidence and make new observations. Often, scientists must rethink their existing hypotheses and present new ones to better explain the new information. The reclassification of Pluto as a dwarf planet is a good example of how scientists use new information to improve classification systems. In our solar system, astronomers classify planets as inner or outer. Inner and outer planets differ according to the criteria in the table below. Inner Planets Outer Planets Examples Mercury, Venus, Earth, Mars Jupiter, Saturn, Uranus, Neptune Number of Moons Few or no moons Many moons Distance from Sun Closest to the sun Farthest from sun Composition Rocky surfaces Gaseous Presence of Rings No rings Has a ring system Inner Planets Outer Planets Size Smaller size Larger size In the activities below, you will investigate each of the planets. As you investigate each planet, record detailed information in your notes. Keep the following points in mind: • Each of the planets follows an elliptical orbit around the sun. An ellipse has a shape like a flattened circle. The following diagram approximates the orbit of each planet around the sun. Note that some orbits are more elliptical than others. Public Domain • Scientists have investigated some planets more than others. The activities below do not list every mission that has been launched to explore each planet. • There are still poorly understood and undiscovered objects in the solar system. Scientists are constantly discovering new information about the planets and other solar system objects. Inner and Outer Planets Interactive A large, two-tabbed interactive divided into inner and outer planets. Each planet exists as a trading card with a front and back, as previously described The Inner Planets Card 1: Mercury Location: 47–70 million kilometers from the sun. Characteristics: Completes one orbit around the sun every 88 days. Completes one rotation on its axis every 58.65 Earth days. Diameter is 4,880 kilometers. Satellites: None. Surface: Covered with rocky cliffs and valleys with many craters from impacts by other objects during the formation of the solar system. May contain water ice at the North and South Poles. Atmosphere: Because there is virtually no atmosphere on the planet to trap heat, temperatures range from 430 degrees Celsius in the day to −180 degrees Celsius at night, which is the largest day and night temperature differential in the solar system. Interior: Contains a liquid metallic core. Exploration: In 1974 and 1975, the Mariner 10 mission photographed Mercury for the first time. In 2008 and 2009, the Messenger mission studied the planet in even greater detail. Fun Facts: Mercury rotates only three times for every two orbits it completes around the sun. That means Mercury’s year is only slightly longer than its day. Card 2: Venus Location: 108.5–108.9 million kilometers from sun. It has a nearly circular orbit. Characteristics: Completes one orbit every 225 days. Completes one rotation on its axis every 243 days. Diameter is 12,100 kilometers. Satellites: None. Surface: The surface is covered with volcanoes. Temperatures are hot enough to melt lead. Atmosphere: The atmosphere comprises thick clouds of carbon dioxide and sulfuric acid, resulting in a powerful greenhouse effect. The atmosphere moves with winds at speeds of 360 kilometers per hour. Temperatures are a constant 470 degrees Celsius. Interior: Contains an iron core but does not have a magnetic field because of its slow rotation. Exploration: In 1962, the Mariner 2 became the first spacecraft to send back information about Venus, which is the first time humans had explored another planet in the solar system. Beginning in 1990, the Magellan spacecraft photographed 98 percent of the planet’s surface. Fun Facts: Venus is the brightest planet in the night sky. If you could observe sunrise on Venus, the sun would rise in the west and set in the east because Venus rotates in the opposite direction as Earth. Card 3: Earth Location: 146–152 million kilometers from the sun. Characteristics: Completes one orbit around the sun every 365.26 days. Completes one rotation on its axis every 23.93 hours. Diameter is 12,756 kilometers. Satellites: One moon, named the Moon (with a capital M). Surface: Water covers 70 percent of Earth’s surface. The land surfaces are covered by a rocky terrain of mountains, valleys, deserts, and glacial regions. Atmosphere: Atmosphere consists mostly of nitrogen (78 percent) and oxygen (21 percent). Temperatures range from 58 degrees Celsius to −88 degrees Celsius. Interior: Contains a molten metallic core made of nickel and iron. Exploration: Scientists are constantly seeking to understand our home planet with missions to the upper atmosphere and the deepest depths of the ocean. Fun Facts: Earth is tilted on its axis approximately 23.5 degrees from the vertical, which causes changing seasons. Card 4: Mars Location: 205–249 million kilometers from the sun. Characteristics: Completes one orbit around the sun every 687 days. Completes one rotation on its axis every 24.6 hours. Diameter is 6,785 kilometers. Satellites: Two moons, Phobos and Deimos, discovered in 1877. Surface: The rocky surface contains many mountains, canyons, volcanoes, ice, and craters. The surface appears red as a result of iron oxide (better known as rust) in the Martian soil. Atmosphere: The atmosphere is composed of carbon dioxide, nitrogen, and argon. Temperatures range from 20 degrees Celsius to −125 degrees Celsius [Source: NASA] Interior: A layer of water ice is thought to exist just under the surface of Mars. The core is likely to be made of iron. Exploration: The Mars rovers, named Spirit and Opportunity, found evidence that liquid water may have existed on the planet billions of years ago or perhaps more recently. The Phoenix rover landed in 2008 to further investigate the existence of water. It confirmed that liquid water exists in the polar regions. Fun Facts: Olympus Mons, located on Mars, is the largest volcano in the solar system, rising 23 kilometers above the planet’s surface. The darkened line across the equatorial region is the Valles Marineris. The canyon system is more than four times deeper than the Grand Canyon. The Outer Planets Card 5: Jupiter Location: 741–817 million kilometers from the sun. Characteristics: Completes one orbit around the sun every 12 years. Completes one rotation on its axis every 9.9 hours. Diameter is 142,800 kilometers. Satellites: Jupiter has at least 63 known moons. The four largest moons—Io, Europa, Callisto, and Ganymede—are called the Galilean moons after their discovery by Galileo in 1610. There are at least three ring systems around the planet, although these were discovered only recently by the Voyager I spacecraft in 1979. Surface: Beneath the atmosphere, the gases are under so much pressure that they form a liquid.
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