The Nature of Planets, Dwarf Planets, and Space Objects
THE NATURE OF PLANETS DARF PLANETS AND SPACE ORECTS Comet Hale Bopp Jamie Cooper SSPL via Getty Images
INTRODUCTION nucleus or core The nucleus made up of ice and dust is commonly called a dirty snowball When a comet is near the Sun it develops a huge cloud of gas and dust called the coma as well as one or more dusty or gaseous tails The comet s head is made up of the nucleus and the coma The tail can be more than 60 million miles 100 million kilometers long The trails of light called falling stars are actually little pieces of metallic or stony material that vaporize upon enter ing Earth's atmosphere Before entering they are called meteoroids and once within Earth's atmosphere they are called meteors Most meteors vaporize completely before reaching the ground Sometimes though a piece survives and hits the ground Then it is called a meteorite Although meteor ites consist of materials found on Earth the proportions are radically different from anything on the planet As you watch for meteors or focus a tele scope on a distant planet you see just a tiny fraction of what is in space Scientists have a battery of technology at their fingertips to study the mysteries of space but they all started like you by gazing into the sky with a desire to
chapter WHAT IS A PLANET The relatively large natural bodies that revolve in orbits around the Sun or otherstars are called planets The term does not include small bodies such as comets meteoroids and asteroids many of which are little more than pieces of ice or rock The planets that orbit the Sun are part of the solar system which includes the Sun and all the bodies that circle it The Sun governs the planets orbital motions by gravitational attraction and provides the planets with light and heat Ideas about what makes a planet a planet have changed over time According to the current definition there are eight plan ets in the solar system In order of increasing mean distance from the Sun the planets are Mercury Venus Earth Mars Jupiter Saturn Uranus and Neptune Pluto was considered the solar system s ninth and outermost planet from 1930 until 2006 when it was reclassified as a dwarf planet The concept of what defines a planet changed in another way in the late 20th cen tury when astronomers began discovering distant planets that orbited stars other than IO
WHAT Is A PLANET the Sun Previously the only planets that had been known were the planets of the solar system DEFINING THE TERM O UN Mercury Venus Mars Jupiter and Saturn all of which can be seen without a telescope have been known since ancient times The ancient Greeks used the term planet meaning wanderers for these five bodies plus the Sun and Earth s B3 The solar system's four immer planets are much smaller than its four outers planets and all eight are dwarfed by the Sun they orbit The sizes of the bod S ies are shown to scale S but the distances between 8 them are not The sixes 3 given are the approximate diameters of each body at a its equator Encyclopaedias Britannica Inc 9 S 32 O CO II B
