Genre • Expository Text

Essential Question How can scientific knowledge change over time? Read about how knowledge about our has changed over time.

Go Digital! (bkgd) Hubble Heritage Team/NASA, (c) NASA/JPL Team/NASA, Hubble Heritage (bkgd)

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Excerpt from WHEN IS A PLANET NOT A PLANET: The Story of Pluto by Elaine Scott. Copyright © 2007 by Elaine Scott. Reprinted by permission of Houghton Mifflin Harcourt Publishing Co mpany. All rights reserved. 404-419_CR14_SA5_U5W4_SEL_119223.indd 406

NASA/JPL Vendor: SRM Program: CR14 406 they arelikeisthekindofactivitythatmakes scienceexcitingandfun. the planets.Learninghowplanetsform,where theyarelocated,andwhat remember theirnamesandorder. orbiting theSun.Perhapssomeonewillcreate anewsentencetohelpus didn’t workanymore,becausenowthereareonlyeightmajorplanets not aplanet.Suddenly,“Myveryeagermotherjustservedusninepizzas” around theworld,madeanannouncement.TheydeclaredthatPlutowas (IAU), agroupofindividualastronomersandastronomicalsocietiesfrom Names areimportant,buttheynottheonly thingstoknowabout However, onAugust24,2006,theInte rnational AstronomicalUnion Grade: Component: SA 5 PDF Pass 12/12/11 6:10PM PLUTO’S PROBLEMS There are two groups of planets in our solar system. The planets closest to the Sun—Mercury, Venus, Earth, and Mars—have a solid surface made of a mix of rocks, dirt, and minerals. The planets farthest away from the Sun—Jupiter, Saturn, Uranus, and Neptune—don’t have a solid surface. They are made up mostly of gas, with a rocky core. Scientists have a theory about why some planets are terrestrial, or made of rocks and dirt, and why some are composed primarily of gas. Most scientists believe that our solar system began as a space cloud, called a nebula. The nebula was made up of bits of space dust, rocks, ice, and gas. A tiny star, not yet ready to give light, began to form in the center of the nebula. The star was our Sun. As years passed, the Sun grew big enough that high temperatures and extreme pressure caused hydrogen at the center of the Sun to begin to fuse into helium and release energy as light—sunshine! Meanwhile, the nebula continued to orbit the new Sun until it formed a large flat ring around it. Scientists call this ring a “protoplanetary disk.” The disk, or ring, was hottest where it was closest to the Sun, and coolest at its outer edge. As the disk swirled around the Sun, the Sun’s gravity went to work. It pulled and tugged at the bits of rock, dust, ice, and gas until they came together in clumps of material we now call the planets.

An artist’s conception of a protoplanetary disk forming around a star.

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NASA/JPL-Caltech/T. Megeath(University of Toledo) & M. Robberto(STScI) Vendor: SRM Program: CR14 408 our solarsystemformedinjustthisway. swirling aroundthem,formingnewsolarsystems.Scientists believe Earth. inthisregion haveprotoplanetarydisks At least153 stars away lightyears from A smallportion of theOrionNebula,1,500 became thegasgiants—Jupiter,Saturn,Uranus, andNeptune. of gas;theydidn’tburnaway.Theplanetsfarthest fromtheSun cooler. Theclumpsofrockanddirttherestill hadtheirthicklayers outer edgesofthedisk,farawayfromSun’s heat,itwasmuch terrestrial planets—Mercury,Venus,Earth,andMars.Butonthe spheres ofmatter,withonlyalittlegas.Thosebecamethe of theirgas.TheSun’sheatblasteditaway,leavingbehindsolid The planetsthatwereclosesttotheSundidn’tkeepmuch Grade: Component: SA 5 PDF Pass 12/12/11 6:11PM 404-419_CR14_SA5_U5W4_SEL_119223.indd 409 Vendor: SRM Program: CR14 crust means ithasasolidinnercore. Earth isaterrestrial planet,which all—made ofrock? were mostlymadeofgas,whywasPluto—themostdistantplanet to theSunwererockyandplanetsfarthestawayfrom core. Thatnewinformationraisedquestions.Iftheplanetsclosest instruments toldthemthatPlutowasdenseandmusthavearocky were abletostudyPlutoandthelightthatreflectedoffit.Their year orbitandscientificinstrumentshadimproved.Astronomers Pluto hadmovedintoapositionthatonlyoccurstwiceinits248- gas becauseofitsgreatdistancefromthesun.However,by1987, discovered in1930,astronomersassumedPlutowasmadeoficeand planets formed,theyhadaproblemwithPluto.Whenitwasfirst Grade: Component: SA Because astronomersstillbelievedthistheoryabouthowour 5 core inner solid liquid outer core mantle back to to thetext findtheanswer. of gasandothersmostlymaderock? Go theory, why are someplanets mostlymade Ask andAnswer QuestionsAccording to STOP AND CHECK PDF Pass mantle solid core. and othergas giantshaveasmall ofgas,entirely oflayers Jupiter Though theyaremadealmost core hydrogen metallic metallic molecular hydrogen 409 12/12/11 6:11PM

Michelle Gengaro-Kokmen 404-419_CR14_SA5_U5W4_SEL_119223.indd 410

Michelle Gengaro-Kokmen Neptune’s orbit Vendor: SRM Program: CR14 410 Pluto’s orbit crosses Neptune’sorbit. the Sunoutsidetheirindividualorbits.ExceptforPluto.Pluto don’t runoutsidetheirindividuallanes,planetstravelaround orbits, takentogether,makeupthe“orbitalplane.”Justasrunners lanes togethermakeuptheracetrack.Forplanets,alltheir planet hasitsownorbitaroundtheSun.Forrunners,all racetrack. Justasrunnershavetheirownlanesonthetrack,each from theorbitsofplanets.Thinkanorbitasalaneon Neptune’s orbitfrom1979 to1999. making Neptunefarther fromthesunthanPluto.Plutowas inside For ofeach 20years orbit,Plutomoves insidetheorbit of Neptune, 248Earth tocomplete,bringsitoutsidetheorbitalplane. takes years an imaginaryflatdiskcalledtheorbitalplane. Pluto’s orbit,which orbit aroundt he Sun. Their orbitslineupwitheach other, creating All oftheplanets,comets,andasteroidsinsolarsystemare There wereotherquestionsaswell.Pluto’sorbitisdifferent Grade: Component: SA 5 PDF Pass 12/12/11 6:11PM 404-419_CR14_SA5_U5W4_SEL_119223.indd 411 Vendor: SRM Program: CR14 Were theyplanets,too? wondered, Weretheremoreobjectsout thesizeofPluto? too smalltobeaplanet?Andsincetheyhad found Charon,they (Charon’s diameteris790miles.)Theybegan toask,IsPluto they hadoriginallythought.Plutoisonly1,440 milesindiameter. separate fromPluto,theyrealizedthatPlutowasevensmallerthan Charon (CARE-en).Onceastronomersdiscoveredthatwas thought wasPlutoreallytwoobjects:hadamoon— astronomer JamesChristydiscoveredthatthetinydoteveryone light inthenightsky.Thentelescopesimproved.In1976,American it washardtoseetheplanetclearly.Plutoappearedasatinydotof tiny whenitwasdiscoveredin1930.Butbecausesofaraway, astronomers wondered,CouldPlutobeacomet? with theSun.Pluto’sistilted.Comets’orbitsareoftentilted,so more ofastretched-outoblong.Theotherplanets’orbitsarelevel travel aroundtheSuninanoval-shapedorbit.Pluto’sorbitis Earth whentheHubbleSpace Telescope tookthisphoto. Pluto anditsmoon,Charon.was 2.6billionmilesfrom And ofcoursethereisPluto’ssize.AstronomersknewPlutowas The shapeofPluto’sorbitisdifferent,too.largerplanets Grade: Component: SA 5 PDF Pass

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Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility; NASA FINDING PLANETS

In 1992, astronomers made an amazing discovery: 9.3 billion miles away from our sun is another region of space, shaped like a disk. Astronomers believe it contains approximately 70,000 icy objects, including Pluto. This area of space was named the , after the Dutch-American astronomer Gerard Kuiper (KI-per) who lived from 1905 to 1973. In 1951, more than forty years before its discovery, Kuiper actually predicted that a region like this might exist. Michael Brown, , and David Rabinowitz are planetary astronomers who study Kuiper Belt Objects, or KBOs. People often call these men “the Planet Finders.” Together, they hunt for planets at the outer edges of our solar system using the at the in . The Oschin telescope is a wide-field telescope, which means it views broad regions of the sky at once. When paired with a camera at the observatory, it can take pictures of these large areas. In the past, astronomers had to spend their evenings peering through telescopes in order to study the night sky. Now things have changed. Robots control the Oschin telescope and its camera. In the evenings, the cameras in the telescope at the Palomar Observatory are at work. They take three photographs over three hours of the part of the night sky the men want to study. Any object moving across the background of billions of stars and galaxies will be captured in pictures. The pictures are then sent from the telescope’s cameras to a bank of ten computers at the California Institute of Technology. Next, the computers decide which objects appear to be moving and therefore might be a planet. Usually, the computers select about 100 objects; when the men arrive at work each morning, the pictures are ready for them to view.

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Dan Durda, Fellow, International Association of Astronomical Artists 404-419_CR14_SA5_U5W4_SEL_119223.indd 414

(l) Courtesy of Caltech, Palomar Observatory, (c) Courtesy of Chad Trujillo, (r) Courtesy of Caltech, Palomar Observatory Vendor: SRM Program: CR14 414 planet, ornot? since therewasalotofstrifeanddiscordsurrounding .Wasita Greek goddessofstrifeanddiscord.Itseems anappropriatename, discovered celestialbodyofficiallybecameEris(AIR-is),forthe takes XenamorethantwiceaslongPlutotoorbittheSun. and isapproximately400milesbiggerindiameterthanPluto.It most distantobjecteverseeninorbitaroundtheSun.” calculation ofhowbigitwasandfarawaywas.Xenaisthe was somethingwrong.Ittoobigandbright.ThenIdida “The veryfirsttimeIsawXenaonmyscreen,thoughtthatthere (ZEE-nah), asitwasnicknamed,onOctober21,2003.Mikesays, That’s howMikeandhisteamdiscovered2003UB313,orXena will getveryluckyandsomethingnewexcitingappear. the telescope’scamera.Buteveryonceinawhile,anastronomer screen arenotplanets.Manycausedbysomekindofflawin Xena wasalwaysanickname.OnSeptember13,2006,thenewly Pluto is3.6billionmilesaway,butXena10away Mike Brownsaysmostoftheobjectshelooksatonhiscomputer Grade: Component: SA 5 the Oschin telescope(right)todiscover Eris. Brown(center) and Mike “the PlanetFinders.” The teamused They callDavidRabinowitz (left), Chad Trujillo (upperleft), PDF Pass 12/12/11 6:11PM 404-419_CR14_SA5_U5W4_SEL_119223.indd 415 Vendor: SRM Program: CR14 contains 200 billion stars. billionstars. contains 200 andtheirsolarsystems. of billionsstars The Milky Way isaspiralgalaxy that An artist’s conceptionoftheMilky Way, ourhomegalaxy. A galaxy isagroup Union decidedtocreatenotonedefinitionbut three. planet?” neededananswer,andtheInternational Astronomical and—at thetime—itwasaplanet,too.Thequestion “Whatisa in diameter,anditisaplanet.Plutoonly1,440 milesindiameter body”? Jupiter,thelargestplanetinoursolar system,is88,700miles definition hadproblems.Foronething,what ismeantby“large that circlesaroundtheSunoranotherstar.”Forascientist, of course,butmostsaidsomethingsimilarto“Alargecelestialbody had adefinitionfortheword“planet.”Dictionariesdefinitions, What, exactly,isaplanet? over theworld.Andargumentcamedowntoonequestion. disagreed. Soonanargumentwasragingamongastronomersall thought thesolarsystemhadgaineditstenthplanet.Butothers their information.Whentheyrevealeddiscovery,manypeople of ErisuntilJanuary5,2005,aftertheyhadachancetoverify Brown andhisteamofastronomersdidn’tannouncetheirdiscovery WHAT ISAPLANET? It seemssurprising,butuntilAugust24,2006,sciencehadnever Because scientistsalwayscheckandrechecktheirwork,Mike Grade: Component: SA 5 PDF Pass 415 12/12/11 6:11PM

Mark Garlick (Space-art) 404-419_CR14_SA5_U5W4_SEL_119223.indd 416

NASA/JPL Vendor: SRM Program: CR14 416 destroyed whentheyhitEarth. other, smaller,objectstowarditanddestroythem,thewayasteroidsare its way.Alargeplanetmightclearorbitbyusinggravitytopull enough, andhaveenoughgravity,togetridofanycelestialobjectsin solar system.Inordertocleartheneighborhood,aplanethasbebig neighborhood” isalittlemoredifficult. a roundshape,areeasytounderstand.Theconceptof“clearingthe planets, dwarfandsmallsolar-systembodies. A cosmiccollision.Planetsoften “clear theirneighborhoods” inthismanner. It mighthelptothinkofplanetsastheschoolyardbullies The firsttwoqualificationsforplanethood,orbitingtheSunand The IAUdecidedthatacelestialbodyisplanetifit: The IAUcameupwiththreeclassesofobjectsthatorbittheSun: Grade: Component: SA 3. is bigenoughandhasgravityto“clear 2. is roundornearlyround,becauseitsgravityhas 1. orbitstheSun 5 the neighborhood”arounditsorbit pulled itintothatshape PDF Pass 12/12/11 6:11PM 404-419_CR14_SA5_U5W4_SEL_119223.indd 417 Vendor: SRM Program: CR14 objects intheKuiperBelt. Charon, Ceres,andErisareallKBOs—orbitingfaroutinspacewithother they’re toosmalltohaveenoughgravitycleartheirneighborhood.Pluto, orbit theSun,theyareround,andnotmoonsofanotherplanet—but Brown’s discovery,Eris,isoneaswell.Theyaredwarfplanetsbecausethey are knownasadouble-planetsystem.Ceresisdwarfplanet,also,andMike moon, isstilllockedinanorbitwithPluto,itadwarfplanet,too.Nowthey following characteristics.Theymust: by itself. clearing, accordingtotheIAUdefinition,aplanetmusttravelinitsorbit different orbitandgetridofitthatway.Butnomatterhowdoesthe then turningthemintomoonsthatremaininorbitaroundtheplanet. By thisdefinition,Plutoisadwarfplanet.AndalthoughCharon,itsformer The secondarycategoryofplanets,called“dwarfplanets,”havethe Sometimes aplanetwillsimplypushsmallerbodyintocompletely Or aplanetmightclearitsorbitbyattractingsmallerobjectstowardit, Grade: Component: SA 5 3. notbeamoonorsatelliteofanotherplanet 2. beround 1. orbittheSun PDF Pass text totext findtheanswer. around it?Lookobjects for detailsinthe size andgravity ofaplanetaffectother How doesthe Ask andAnswer Questions STOP AND CHECK 417 12/12/11 6:11PM

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Johns Hopkins University Applied Laboratory/Southwest Research Institute Vendor: SRM Program: CR14 418 Charon isvisibleinthedistance. Horizons s pacecraft asitarrives atPluto. An artist’s conceptionoftheNew solar system! eight. Andthatisjustinour again. Andnowit’sbackto the numberjumpeduptonine back toeight.Then,withPluto, a while,therewerenine.Thenitwas Then wethoughttherewereeight.For ago, wethoughtthereweresixplanets. as newinformationisdiscovered.Alongtime is agoodoneandwilllast. change againinthefuture.Othersthinkcurrentdefinition are called“smallsolar-systembodies.” members ofthethirdclassobjectsthatorbitSunand Science isexciting,becauseitcontinuallychanges Some astronomersthinkthedefinitionofaplanetwill Everything else—asteroids,comets,meteors—arenow Grade: Component: SA 5 PDF Pass 12/12/11 6:11PM We know our Sun is not the only star that has planets in orbit around it. New planets are forming around other stars, making new solar systems. There are 200 billion stars in the Milky Way galaxy alone. And there are billions of galaxies, full of stars, in the universe. As we study those planets and the stars they orbit, we ask questions. Are there other planets like Earth somewhere in the universe? Does life exist on them? We ask questions as we study the planets in our own solar system, too. Does life exist on one of them, or even one of their moons? Did life ever exist on any of them? Is Earth the only planet with life? Are we alone in the universe? In January 2006, NASA launched the New Horizons mission to Pluto. If all goes well, the New Horizons spacecraft will reach Pluto and Charon sometime in the summer of 2015. Then instruments aboard the spaceship will begin to get a close look at these distant worlds. As the information beams back to Earth, scientists here will study it, trying to learn more about the origins of our solar system and what lies at its outer edges. Pluto still has a story to tell. There are questions that need answers, and the answers will come through science. New information is just waiting to be discovered.

STOP AND CHECK Reread Why do some astronomers think the definition of a planet will change in the future? The strategy Reread may help you.

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