The Nature of Planets, Dwarf Planets, and Space Objects
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
THE NATURE OF PLANETS DARF PLANETS AND SPACE OnTECTS enough for its gravity to have pulled it into a nearly spherical shape Third it must have cleared the neighborhood around its orbit meaning that it must be massive enough for its gravity to have removed most rocky and icy debris from the vicinity of its orbit Pluto is not a planet under this definition It orbits the Sun and is nearly round but it has not cleared away many small Kuiper belt objects from the area around its orbit The IAU created a new category called dwarf planet for those nearly spherical celestial bodies that orbit the Sun but that have not cleared the area around their orbits and that are not moons The first bodies to be des ignated dwarf planets were Pluto Eris and Ceres which is also the largest asteroid The IAU definitions were themselves con troversial Some scientists welcomed them as appropriate recognition that Pluto is one of the larger objects in the Kuiper belt Other scientists criticized the definitions lack of precision and scientific rigor and called for their revision CLASSIFICATION OF THE PLANETS The eight planets of the solar system can be divided into two groups inner planets and 14
WHAT Is A PLANET An ultraviolet image taken by the Cassini spacecraft shows the A ring of Saturn The ringlets shown in turquoise have a greater percentage of water ice and less dirt than those shown in red The reddish area at left is the large gap called the Cassini division it forms the inner bound ary of the A ring The red band at right is the Encke gap NASA JPL University of Colorado outer planets according to their basic phys ical characteristics and positions relative to the Sun The four inner planets are Mercury Venus Earth and Mars They are also called the terrestrial or Earth like planets These 15
