The Nature of Planets, Dwarf Planets, and Space Objects
CONTENTS INTRODUCTION CHAPTER 1 WHAT Is a PLANET 1ol CHAPTER 2 MoTIONS OF THE PLANETS 29 CHAPTER 3 DWARE PLANETS 39 CHAPTER 4 ASTEROIDs 58 CHAPTER 5 COMETS 66 CHAPTER 6 METEORS AND METEORITES CONCLUSION 88 89 91 GoSSARY For MORE INFORMATION BIBLIOGRAPHY INDEX 94 95
Nhe skies hold many marvels Among the most fascinating are planets dwarf planets and space objects such as asteroids comets and meteors Astronomers view these celestial bodies through powerful telescopes and send out space probes and satellites to record data and take photographs These pages will take you on your own exciting journey among planets and space objects Planets have long been known to be large natural bodies that orbit around the Sun but the specifics of the definition have changed over the years The most recent definition includes eight planets in the solar system Mercury Venus Earth and Mars known as inner planets and Jupiter Saturn Uranus and Neptune outer planets From 1930 until 2006 Pluto was considered a planet too But after scientists discovered several other dis tant bodies similar to Pluto orbiting the Sun Pluto was reclassified as a dwarf planet Most astronomers believe that the solar system started developing around 4 6 billion years ago from a large cloud of gas and dust called the solar nebula Gravity caused the matter to contract and the cloud began to spin flattening into a disk The Sun formed
INTRODUCTION in the center and the planets formed from remaining material spinning around the Sun Collisions between the forming planets and smaller bodies may have caused impact cra ters on their surfaces and even the formation of Earth's Moon Dwarf planets are smaller than planets and have not cleared away icy and rocky debris from around their orbits Cold dark distant Pluto named after the Roman god of the underworld is probably the best known dwarf planet It is part of what is known as the Kuiper belt a remote region with many small icy bodies orbiting the Sun The dwarf planets Eris Makemake and Haumea also belong to the Kuiper belt Ceres another dwarf planet orbits the Sun from within the main asteroid belt Asteroids are large pieces of rock and metal the Sun in that orbit usually the zone between the orbits of Mars and Jupiter Small asteroids regularly strike Earth s surface but large asteroids crash much less frequently In the past such impacts may have caused earthquakes or sea waves and maybe even the extinction of the dinosaurs Comets are another type of space object The only permanent part of a comet is its
