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it；　and　all　the　nations　responsible　seem　mitted　to　the　project。　Perhaps　it　will　actually　e　about。　Perhaps　winging　across　the　billion　miles　of　intervening　interplanetary　space　will　be；　in　the　not　too　distant　future。　news　about　how　far　along　the　path　to　life　Titan　has　e。




CHAPTER　8　THE　FIRST　NEW　PLANET

　

　

1　implore　you；　you　do　not　hope　to　be　able　to　give　the　reasons　for　the　number　of　planets；　do　you？　This　worry　has　been　resolved　。　。　。

—　JOHANNES　KEPLER；　EPITOME　OF　COPERNICAN　ASTRONOMY；　BOOK　4　/　1621

Before　we　invented　civilization；　our　ancestors　lived　mainly　in　the　open；　out　under　the　sky。　Before　we　devised　artificial　lights　and　atmospheric　pollution　and　modern　forms　of　nocturnal　entertainment；　we　watched　the　stars。　There　were　practical　calendrical　reasons；　of　course；　but　there　was　more　to　it　than　that。　Even　today；　the　most　jaded　city　dweller　can　be　unexpectedly　moved　upon　encountering　a　clear　night　sky　studded　with　thousands　of　twinkling　stars。　When　it　happens　to　me　after　all　these　years；　it　still　takes　my　breath　away。

In　every　culture；　the　sky　and　the　religious　impulse　are　intertwined。　I　lie　back　in　an　open　field　and　the　sky　surround　me。　I'm　overpowered　by　its　scale。　It's　so　vast　and　so　far　away　that　my　own　insignificance　bees　palpable。　But　I　don't　feel　rejected　by　the　sky。　I'm　a　part　of　it；　tiny；　to　be　sure；　but　everything　is　tiny　pared　to　that　overwhelming　immensity；　And　when　I　concentrate　on　the　stars；　the　planets；　and　their　motions；　I　have　an　irresistible　sense　of　machinery；　clockwork；　elegant　precision　working　on　a　scale　that；　however　lofty　our　aspirations；　dwarfs　and　humbles　us。

Most　of　the　great　inventions　in　human　history—from　stone　tools　and　the　domestication　of　fire　to　written　language—were　made　by　unknown　benefactors。　Our　institutional　memory　of　long…gone　events　is　feeble。　We　do　not　know　the　name　of　that　ancestor　who　first　noted　that　planets　were　different　from　stars。　She　or　he　must　have　lived　tens；　perhaps　even　hundreds　of　thousands　of　years　ago。　But　eventually　people　all　over　the　world　understood　that　five；　no　more；　of　the　bright　points　of　light　that　grace　the　night　sky　break　lockstep　with　the　others　over　a　period　of　months；　moving　strangely…almost　as　if　they　had　minds　of　their　own。

Sharing　the　odd　apparent　motion　of　these　planets　were　the　Sun　and　Moon；　making　seven　wandering　bodies　in　all。　These　seven　were　important　to　the　ancients；　and　they　named　them　after　gods　not　any　old　gods；　but　the　main　gods；　the　chief　gods；　the　ones　who　tell　other　gods　（and　mortals）　what　to　do。　One　of　the　planets；　bright　and　slow…moving；　was　named　by　the　Babylonians　after　Marduk；　by　the　Norse　after　Odin；　by　the　Greeks　after　Zeus；　and　by　the　Romans　after　Jupiter；　in　each　case　the　king　of　the　gods。　The　faint；　fast…moving　one　that　was　never　far　from　the　Sun　the　Romans　named　Mercury；　after　the　messenger　of　the　gods；　the　most　brilliant　of　them　was　named　Venus；　after　the　goddess　of　love　and　beauty；　the　blood　red　one　Mars；　after　the　god　of　war；　and　the　most　sluggish　of　the　bunch　Saturn；　after　the　god　of　time。　These　metaphors　and　allusions　were　the　best　our　ancestors　could　do：　They　possessed　no　scientific　instruments　beyond　the　naked　eye；　they　were　confined　to　the　Earth；　and　they　had　no　idea　that　it；　too；　is　a　planet。*

*　There　was　one　moment　in　the　last　4；000　years　when　all　seven　of　these　celestial　bodies　were　clustered　tightly　together。　Just　before　dawn　on　March　4；　1953　B。C。；　the　crescent　Moon　was　at　the　horizon。　Venus；　Mercury；　Mars；　Saturn；　and　Jupiter　were　strung　out　like　jewels　on　a　necklace　near　the　great　square　in　the　constellation　Pegasus—near　the　spot　from　which　in　our　time　the　Perseid　meteor　shower　emanates。　Even　casual　watchers　of　the　sky　must　have　been　transfixed　by　the　event。　What　was　it—a　munion　of　the　gods？　According　to　the　astronomer　David　Pankenier　of　Lehigh　University　and　later　Kevin　Pang　of　JPL；　this　event　was　the　starting　point　for　the　planetary　cycles　of　the　ancient　Chinese　astronomers。　There　is　no　other　time　in　the　last　4；000　years　（or　in　the　next）　when　the　dance　of　the　planets　around　the　Sun　brings　them　so　close　together　from　the　vantage　point　of　Earth。　But　on　May　5；　2000；　all　seven　will　be　visible　in　the　same　part　of　the　sky—although　some　at　dawn　and　some　at　dusk　and　about　ten　times　more　spread　out　than　on　that　late　winter　morning　in　1953　B。C。　Still；　it's　Probably　a　good　night　for　a　party。

When　it　got　to　be　time　to　design　the　week—a　period　of　time；　unlike　the　day；　month；　and　year；　with　no　intrinsic　astronomical　significance—it　was　assigned　seven　days；　each　named　after　one　of　the　seven　anomalous　lights　in　the　night　sky。　We　can　readily　make　out　the　remnants　of　this　convention。　In　English；　Saturday　is　Saturn's　day。　Sunday　and　Mo＇o＇nday　are　clear　enough。　Tuesday　through　Friday　are　named　after　the　gods　of　the　Saxon　and　kindred　Teutonic　invaders　of　Celtic/Roman　Britain：　Wednesday；　for　example；　is　Odin's　（or　Wodin's）　day；　which　would　be　more　apparent　if　we　pronounced　it　as　it's　spelled；　〃Wedn's　Day〃；　Thursday　is　Thor's　day；　Friday　is　the　day　of　Freya；　goddess　of　love。　The　last　day　of　the　week　stayed　Roman；　the　rest　of　it　became　German。

In　all　Romance　languages；　such　as　French；　Spanish；　and　Italian；　the　connection　is　still　more　obvious；　because　they　4　derive　from　ancient　Latin；　in　which　the　days　of　the　week　were　named　（in　order；　beginning　with　Sunday）　after　the　Sun；　the　Moon；　Mars；　Mercury；　Jupiter；　Venus；　and　Saturn。　（The　Sun's　day　became　the　Lord's　day。）　They　could　have　named　the　days　in　order　of　the　brightness　of　the　corresponding　astronomical　bodies—the　Sun；　the　Moon；　Venus；　Jupiter；　Mars；　Saturn；　Mercury　（and　thus　Sunday；　Monday；　Friday；　Thursday；　Tuesday；　Saturday；　Wednesday）—but　they　did　not。　If　the　days　of　the　week　in　Romance　languages　had　been　ordered　by　distance　from　the　Sun；　the　sequence　would　be　Sunday；　Wednesday；　Friday；　Monday；　Tuesday；　Thursday；　Saturday。　No　one　knew　the　order　of　the　planets；　though；　back　when　we　were　naming　planets；　gods；　and　days　of　the　week。　The　ordering　of　the　days　of　the　week　seems　arbitrary；　although　perhaps　it　does　acknowledge　the　primacy　of　the　Sun。

This　collection　of　seven　gods；　seven　days；　and　seven　worlds　the　Sun；　the　Moon；　and　the　five　wandering　planets　entered　the　perceptions　of　people　everywhere。　The　number　seven　began　to　acquire　supernatural　connotations。　There　were　seven　〃heavens；〃　the　transparent　spherical　shells；　centered　on　the　Earth；　that　were　imagined　to　make　these　worlds　move。　The　outermost—the　seventh　heaven—is　where　the　〃fixed〃　stars　were　imagined　to　reside。　There　are　Seven　Days　of　Creation　（if　we　include　God's　day　of　rest）；　seven　orifices　to　the　head；　seven　virtues；　seven　deadly　sins；　seven　evil　demons　in　Sumerian　myth。　seven　vowels　in　the　Greek　alphabet　（each　affiliated　with　a　planetary　god）；　Seven　Governors　of　Destiny　according　to　the　Hermetists；　Seven　Great　Books　of　Manichaeism；　Seven　Sacraments；　Seven　Sages　of　Ancient　Greece；　and　seven　alchemical　〃bodies〃　（gold；　silver；　iron；　mercury；　lead；　tin；　and　copper—gold　still　associated　with　the　Sun；　silver　with　the　Moon；　iron　with　Mars；　etc。）。　The　seventh　son　of　a　seventh　son　is　endowed　with　supernatural　powers。　Seven　is　a　〃lucky〃　number。　In　the　New　Testament's　Book　of　Revelations；　seven　seals　on　a　scroll　are　opened；　seven　trumpets　are　sounded；　seven　bowls　are　filled。　St。　Augustine　obscurely　argued　for　the　mystic　importance　of　seven　on　the　grounds　that　three　〃is　the　first　whole　number　that　is　odd〃　（what　about　one？）；　〃four　the　first　that　is　even〃　（what　about　two？）；　and　〃of　these　。　。　。　seven　is　posed。〃　And　so　on。　Even　in　our　time　these　associations　linger。

The　existence　even　of　the　four　satellites　of　Jupiter　that　Galileo　discovered—hardly　planets—was　disbelieved　on　the　grounds　that　it　challenged　the　precedence　of　the　number　seven。　As　acceptance　of　the　Copernican　system　grew；　the　Earth　was　added　to　the　list　of　planets；　and　the　Sun　and　Moon　were　removed。　Thus；　there　seemed　to　be　only　six　planets　（Mercury；　Venus；　Earth；　Mars；　Jupiter；　and　Saturn）。　So　learned　academic　arguments　were　invented　showing　why　there　had　to　be　six。　For　example；　six　is　the　first　〃perfect〃　number；　equal　to　the　sum　of　its　divisors　（1　＋　2　＋　3）。　Q。E。D。　And　anyway；　there　were　only　six　days　of　creation；　not　seven。　People　found　ways　to　acmodate　from　seven　planets　to　six。

As　those　adept　at　numerological　mysticism　adjusted　to　the　Copernican　system；　this　self…indulgent　mode　of　thinking　spilled　over　from　planets　to　moons。　The　Earth　had　one　moon；　Jupiter　had　the　four　Galilean　moons。　That　made　five。　Clearly　one　was　missing。　（Don't　forget：　Six　is　the　first　perfect　number。）　When　Huygens　discovered　Titan　in　1655；　he　and　many　others　convinced　themselves　that　it　was　the　last：　Six　planets；　six　moons；　and　God's　in　His　Heaven。

The　historian　of　science　I。　Bernard　Cohen　of　Harvard　University　has　pointed　out　that　Huygens　actually　gave　up　searching　for　other　moons　because　it　was　apparent；　from　such　arguments；　that　no　more　were　to　be　found。　Sixteen　years　later；　ironically　with　Huygens　in　attendance；　G。　D。　Cassini*　of　the　Paris　Observatory　discovered　a　seventh　moon—Iapetus；　a　bizarre　world　with　one　hemisphere　black　and　the　other　white；　in　an　orbit　exterior　to　Titan's。　Shortly　after；　Cassim　discovered　Rhea；　the　next　Saturnian　moon　interior　to　Titan。

*　After　whom　the　European…American　mission　to　the　Saturn　system　is　named。

Here　was　another　opportunity　for　numerology；　this　time　harnessed　to　the　practical　task　of　flattering　patrons。　Cassim　added　up　the　number　of　planets　（six）　and　the　number　of　satellites　（eight）　and　got　fourteen。　Now　it　so　happened　that　the　man　who　built　Cassim's　observatory　for　him　and　paid　his　salary　was　Louis　XIV　of　France；　the　Sun　King。　The　astronomer　promptly　〃presented〃　these　two　new　moons　to　his　sovereign　and　proclaimed　that　Louis's　〃conquests〃　reached　to　the　ends　of　the　Solar　System。　Discreetly；　Cassim　then　backed　off　from　looking　for　more　moons；　Cohen　suggests　he　was　afraid　one　more　might　now　offend　Louis—a　monarch　not　to　be　trifled　with；　who　would　shortly　be　throwing　his　subjects　into　dungeons　for　the　crime　of　being　Protestants。　Twelve　years　later；　though；　Cassim　returned　to　the　search　and　found—doubtless　with　a　measure　of　trepidation—another　two　moons。　（It　is　probably　a　good　thing　that　we　have　not　continued　in　this　vein；　otherwise　France　would　have　been　burdened　by　seventy…some…odd　Bourbon　kings　named　Louis。）



WHEN　CLAIMS　OF　NEW　WORLDS　WERE　MADE　In　the　late　eighteenth　century；　the　force　of　such　numerological　arguments　had　much　dissipated。　Still；　it　was　with　a　real　sense　of　surprise　that　people　heard　in　1781　about　a　new　planet；　discovered　through　the　telescope。　New　moons　were　paratively　unimpressive；　especially　after　the　first　six　or　eight。　But　that　there　were　new　planets　to　be　found　and　that　humans　had　devised　the　means　to　do　so　were　both　considered　astonishing；　and　properly　so。　If　there　is　one　previously　unknown　planet；　there　may　be　many　more—in　this　solar　system　and　in　others。　Who　can　tell　what　might　be　found　if　a　multitude　of　new　worlds　are　hiding　in　the　dark？

The　discovery　was　made　not　even　by　a　professional　astronomer　but　by　William　Herschel；　a　musician　whose　relatives　had　e　to　Britain　with　the　family　of　another　anglified　German；　the　reigning　monarch　and　future　oppressor　of　the　American　colonists；　George　III。　It　became　Herschel's　wish　to　call　the　planet　George　（〃George's　Star；〃　actually）；　after　his　patron。　but；　providentially；　the　name　didn't　stick。　（Astronomers　seem　to　have　been　very　busy　buttering　up　kings。）　Instead；　the　planet　that　Herschel　found　is　called　Uranus　（an　inexhaustible　source　of　hilarity　renewed　in　each　generation　of　English…speaking　nine…year…olds）。　It　is　named　after　the　ancient　sky　god　who；　according　to　Greek　myth；　was　Saturn's　father　and　thus　the　grandfather　of　the　Olympian　gods。

We　no　longer　consider　the　Sun　and　Moon　to　be　planets；　and　ignoring　the　paratively　insignificant　asteroids　and　ets；　count　Uranus　as　the　seventh　planet　in　order　from　the　Sun　（Mercury；　Venus；　Earth；　Mars；　Jupiter；　Saturn；　Uranus；　Neptune；　Pluto）。　It　is　the　first　planet　unknown　to　the　ancients。　The　four　outer；　Jovian；　planets　turn　out　to　be　very　different　from　the　four　inner；　terrestrial；　planets。　Pluto　is　a　separate　case。

As　the　years　passed　and　the　quality　of　astronomical　instruments　unproved；　we　began　to　learn　more　about　distant　Uranus。　What　reflects　the　dim　sunlight　back　to　us　is　no　solid　surface；　but　atmosphere　and　clouds　just　as　for　Titan；　Venus；　Jupiter　Saturn；　and　Neptune。　The　air　on　Uranus　is　made　of　hydrogen　and　helium；　the　two　simplest　gases。　Methane　and　other　hydrocarbons　are　also　present。　Just　below　the　clouds　visible　to　Earthbound　observers　is　a　massive　atmosphere　with　enormous　quantities　of　ammonia；　hydrogen　sulfide；　and；　especially；　water。

At　depth　on　Jupiter　and　Saturn；　the　pressures　are　so　great　that　atoms　sweat　electrons；　and　the　air　bees　a　metal。　That　does　not　seem　to　happen　on　less　massive　Uranus；　because　the　pressures　at　depth　are　less。　Still　deeper；　discovered　only　by　its　subtle　tugs　on　Uranus'　moons；　wholly　inaccessible　to　view；　under　the　crushing　weight　of　the　overlying　atmosphere；　is　a　rocky　surface。　A　big　Earthlike　planet　is　hiding　down　there；　swathed　in　an　immense　blanket　of　air。

The　Earth's　surface　temperature　is　due　to　the　sunlight　it　intercepts。　Turn　off　the　Sun　and　the　planet　soon　chills—not　to　trifling　Antarctic　cold；　not　just　so　cold　that　the　oceans　freeze；　but　to　a　cold　so　intense　that　the　very　air　precipitates　out；　forming　a　ten…meter…thick　layer　of　oxygen　and　nitrogen　snows　covering　the　whole　planet。　The　little　bit　of　energy　that　trickles　up　from　the　Earth's　hot　interior　would　be　insufficient　to　melt　these　snows。　For　Jupiter；　Saturn；　and　Neptune　it's　different。　There's　about　as　much　heat　pouring　out　from　their　interiors　as　they　acquire　from　the　warmth　of　the　distant　Sun。　Turn　off　the　Sun；　and　they　would　be　only　a　little　affected。

But　Uranus　is　another　story。　Uranus　is　an　anomaly　among　the　Jovian　planets。　Uranus　is　like　the　Earth：　There's　very　little　intrinsic　heat　pouring　out。　We　have　no　good　understanding　of　why　this　should　be；　why　Uranus—which　in　many　respects　is　so　similar　to　Neptune—should　lack　a　potent　source　of　internal　heat。　For　this　reason；　among　others；　we　cannot　say　we　understand　what　is　going　on　in　the　deep　interiors　of　these　mighty　worlds。

Uranus　is　lying　on　its　side　as　it　goes　around　the　Sun。　In　the　1990s。　the　south　pole　is　heated　by　the　Sun；　and　it　is　this　pole　that　Earthbound　observers　at　the　end　of　the　twentieth　century　see　when　they　look　at　Uranus。　It　takes　Uranus　84　Earth　years　to　make　one　circuit　of　the　Sun。　So　in　the　2030s；　the　north　pole　will　be　sunward　（and　Earthward）。　In　the　2070s　the　south　pole　will　be　pointing　to　the　Sun　once　again。　In　between；　Earthbound　astronomers　will　be　looking　mainly　at　equatorial　latitudes。

All　the　other　planets　spin　much　more　upright　in　their　orbits。　No　one　is　sure　of　the　reason　for　Uranus'　anomalous　spin；　the　most　promising　suggestion　is　that　sometime　in　its　early　history；　billions　of　years　ago；　it　was　struck　by　a　rogue　planet；　about　the　size　of　the　Earth；　in　a　highly　eccentric　orbit。　Such　a　collision；　if　it　ever　happened；　must　have　worked　much　tumult　in　the　Uranus　system；　for　all　we　know；　there　may　be　other　vestiges　of　ancient　havoc　still　left　for　us　to　find。　But　Uranus'　remoteness　tends　to　guard　its　mys