The Nature of Planets, Dwarf Planets, and Space Objects
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
THE NATURE OF PLANETS DARF PLANETS AND SPACE ORECTS Moon because the objects appeared to move across the background of the apparently fixed stars In some languages the names of these seven bodies are still connected with the days of the week In time astronomers learned more about how celestial objects move in the sky and they recognized that the Sun not Earth is the center of the solar sys tem The term planet came to be reserved for large bodies that orbit the Sun and not another planet so the Sun and Moon were no longer considered planets With the aid of telescopes modern astronomers have discovered hundreds of thousands of additional objects orbiting the Sun including more planets Although Uranus also is sometimes visible with the naked eye ancient astronomers were unable to distinguish it from the stars Uranus was first identified as a planet in 1781 The eighth planet Neptune was discovered in 1846 THE QUESTION OF PLUTO Pluto was initially considered a planet Indeed its discovery in 1930 was the result of a major search for a distant ninth planet Astronomers found that Pluto was unlike the other planets in many ways including its 12
WHAT Is A PLANET composition of rock and ice and its tilted and very elongated orbit For decades it seemed unique Beginning in the late 20th century however advances in telescope technol ogy allowed astronomers to discover many more small icy objects with elongated orbits that lie beyond Neptune Along with Pluto these numerous objects orbit the Sun in a doughnut shaped zone called the Kuiper belt At first it seemed that Pluto was significantly larger than all these objects But a few Kuiper belt objects were found to be nearly as big as Pluto and one which was later named Eris was slightly larger This raised the question of whether those bodies should be considered planets if Pluto was HARACTERISTICS OF A PLANET To address this issue and the growing con troversy swirling around it the group that approves the names of astronomical objects for the scientific community issued an offi cial definition in 2006 The organization known as the International Astronomical Union IAU defined a planet as a celestial body that meets three conditions First it must orbit the Sun and not be a satellite of another planet Second it must be massive 13
