pale blue dot -carl sagan-第28章

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Spaceborne　techniques　will　be　much　more　sensitive。　A　Jovian　planet　going　around　a　nearby　star　is　about　a　billion　times　fainter　than　its　sun；　nevertheless；　a　new　generation　of　ground…based　telescopes　that　can　pensate　for　the　twinkling　in　the　Earth's　atmosphere　may　soon　be　able　to　detect　such　planets　in　only　a　few　hours'　observing　time。　A　terrestrial　planet　of　a　neighboring　star　is　a　hundred　times　fainter　still；　but　it　now　seems　that　paratively　inexpensive　spacecraft；　above　the　Earth's　atmosphere；　might　be　able　to　detect　other　Earths。　None　of　these　searches　has　succeeded　yet；　but　we　are　clearly　on　the　verge　of　being　able　to　detect　at　least　Jupiter…sized　planets　around　the　nearest　stars—if　there　are　any　to　be　found。

A　most　important　and　serendipitous　recent　discovery　is　of　a　bona　fide　planetary　system　around　an　unlikely　star；　some　1；300　light…years　away；　found　by　a　most　unexpected　technique：　The　pulsar　designated　B1257＋12　is　a　rapidly　rotating　neutron　star；　an　unbelievably　dense　sun；　the　remnant　of　a　massive　star　that　suffered　a　supernova　explosion。　It　spins；　at　a　rate　measured　to　impressive　precision；　once　every　0。0062185319388187　seconds。　This　pulsar　is　pushing　10；000　rpm。

Charged　particles　trapped　in　its　intense　magnetic　field　generate　radio　waves　that　are　cast　across　the　Earth；　about　160　flickers　a　second。　Small　but　discernible　changes　in　the　flash　rate　were　tentatively　interpreted　by　Alexander　Wolszczan；　now　at　Pennsylvania　State　University；　in　1991—as　a　tiny　reflex　motion　of　the　pulsar　in　response　to　the　presence　of　planets。　In　1994　the　predicted　mutual　gravitational　interactions　of　these　planets　were　confirmed　by　Wolszczan　from　a　study　of　timing　residuals　at　the　microsecond　level　over　the　intervening　years。　The　evidence　that　these　are　truly　new　planets　and　not　starquakes　on　the　neutron　star　surface　（or　something）　is　now　overwhelming—or；　as　Wolszczan　put　it；　〃irrefutable〃；　a　new　solar　system　is　〃unambiguously　identified。〃　Unlike　all　the　other　techniques；　the　pulsar　timing　method　makes　close…in　terrestrial　planets　paratively　easy　and　more　distant　Jovian　planets　paratively　difficult　to　detect。

Planet　C；　some　2。8　times　more　massive　than　the　Earth；　orbits　the　pulsar　every　98　days　at　a　distance　of　0。47　astronomical　units*　（AU）；　Planet　B；　with　about　3。4　Earth　masses；　has　a　67…Earth…day　year　at　0。36　AU。　A　smaller　world；　Planet　A；　still　closer　to　the　star；　with　about　0。015　Earth　masses；　is　at　0。19　AU。　Crudely　speaking；　Planet　B　is　roughly　at　the　distance　of　Mercury　from　our　Sun；　Planet　C　is　midway　between　the　distances　of　Mercury　and　Venus；　and　interior　to　both　of　them　is　Planet　A；　roughly　the　mass　of　the　Moon　at　about　half　Mercury's　distance　from　our　Sun。　Whether　these　planets　are　the　remnants　of　an　earlier　planetary　system　that　somehow　survived　the　supernova　explosion　that　produced　the　pulsar；　or　whether　they　formed　from　the　resulting　circumstellar　accretion　disk　subsequent　to　the　supernova　explosion；　we　do　not　know。　But　in　either　case；　we　have　now　learned　that　there　are　other　Earths。

*　The　Earth；　by　definition；　is　1　AU　from　its　star；　the　Sun。

The　energy　put　out　by　B1257＋12　is　about　4。7　times　that　of　gun。　But；　unlike　the　Sun；　most　of　this　is　not　in　visible　light；　but　in　a　fierce　hurricane　of　electrically　charged　particles。　Suppose　that　these　particles　impinge　on　the　planets　and　heat　them。　Then；　even　a　planet　at　1　AU　would　have　a　surface　around　280　Celsius　degrees　above　the　normal　boiling　point　of　water；　greater　than　the　temperature　of　Venus。

These　dark　and　broiling　planets　do　not　seem　hospitable　for　life。　But　there　may　be　others；　farther　from　B1257＋12；　that　are。　（Hints　of　at　least　one　cooler；　outer　world　in　the　B1257＋12　system　exist。）　Of　course；　we　don't　even　know　that　such　worlds　would　retain　their　atmospheres；　perhaps　any　atmospheres　were　stripped　away　in　the　supernova　explosion；　if　they　date　back　that　far。　But　we　do　seem　to　be　detecting　a　recognizable　planetary　system。　Many　more　are　likely　to　bee　known　in　ing　decades；　around　ordinary　Sun…like　stars　as　well　as　white　dwarfs；　pulsars；　and　other　end　states　of　stellar　evolution。

Eventually；　we　will　have　a　list　of　planetary　systems—each　perhaps　with　terrestrials　and　Jovians　and　maybe　new　classes　of　planets。　We　will　examine　these　worlds；　spectroscopically　and　in　other　ways。　We　will　be　searching　for　new　Earths　and　other　life。



ON　NONE　OF　THE　WORLDS　In　the　outer　Solar　System　did　Voyager　find　signs　of　life；　much　less　intelligence。　There　was　organic　Matter　galore—the　stuff　of　life；　the　premonitions　of　life；　perhaps　but　as　far　as　we　could　see；　no　life。　There　was　no　oxygen　in　their　atmospheres；　and　no　gases　profoundly　out　of　chemical　equilibrium；　as　methane　is　in　the　Earth's　oxygen。　Many　of　the　worlds　were　painted　with　subtle　colors；　but　none　with　such　distinctive；　sharp　absorption　features　as　chlorophyll　provides　over　much　of　the　Earth's　surface。　On　very　few　worlds　was　Voyager　able　to　resolve　details　as　small　as　a　kilometer　across。　By　this　standard；　it　would　not　have　detected　even　our　own　technical　civilization　had　it　been　transplanted　to　the　outer　Solar　System。　But　for　what　it's　worth；　we　found　no　regular　patterning；　no　geometrization；　no　passion　for　small　circles；　triangles；　squares；　or　rectangles。　There　were　no　constellations　of　steady　points　of　light　on　the　night　hemispheres。　There　were　no　signs　of　a　technical　civilization　reworking　the　surface　of　any　of　these　worlds。

The　Jovian　planets　are　prolific　broadcasters　of　radio　waves—generated　in　part　by　the　abundant　trapped　and　beamed　charged　particles　in　their　magnetic　fields；　in　part　by　lightning；　and　in　part　by　their　hot　interiors。　But　none　of　this　emission　has　the　character　of　intelligent　life—or　so　it　seems　to　the　experts　in　the　field。

Of　course　our　thinking　may　be　too　narrow。　We　may　be　missing　something。　For　example；　there　is　a　little　carbon　dioxide　in　the　atmosphere　of　Titan；　which　puts　its　nitrogen/methane　atmosphere　out　of　chemical　equilibrium。　I　think　the　CO2　is　provided　by　the　steady　pitter…patter　of　ets　falling　into　Titan's　atmosphere—but　maybe　not。　Maybe　there's　something　on　the　surface　unaccountably　generating　CO2　in　the　face　of　all　that　methane。

The　surfaces　of　Miranda　and　Triton　are　unlike　anything　else　we　know。　There　are　vast　chevron…shaped　landforms　and　crisscrossing　straight　lines　that　even　sober　planetary　geologists　once　mischievously　described　as　〃highways。〃　We　think　we　（barely）　understand　these　landforms　in　terms　of　faults　and　collisions；　but　of　course　we　might　be　wrong。

The　surface　stains　of　organic　matter—sometimes；　as　on　Triton；　delicately　hued—are　attributed　to　charged　particles　producing　chemical　reactions　in　simple　hydrocarbon　ices；　generating　more　plex　organic　materials；　and　all　this　having　nothing　to　do　with　the　intermediation　of　life。　But　of　course　we　might　be　wrong。

The　plex　pattern　of　radio　static；　bursts；　and　whistles　that　we　receive　from　all　four　Jovian　planets　seems；　in　a　general　way；　explicable　by　plasma　physics　and　thermal　emission。　（Much　of　the　detail　is　not　yet　well　understood。）　But　of　course　we　might　be　wrong。

We　have　found　nothing　on　dozens　of　worlds　so　clear　and　striking　as　the　signs　of　life　found　by　the　Galileo　spacecraft　in　its　passages　by　the　Earth。　Life　is　a　hypothesis　of　last　resort。　You　invoke　it　only　when　there's　no　other　way　to　explain　what　you　see。　If　I　had　to　judge；　I　would　say　that　there's　no　life　on　any　of　the　worlds　we've　studied；　except　of　course　our　own。　But　I　might　be　wrong；　and；　right　or　wrong；　my　judgment　is　necessarily　confined　to　this　Solar　System。　Perhaps　on　some　new　mission　we'll　find　something　different；　something　striking；　something　wholly　inexplicable　with　the　ordinary　tools　of　planetary　science—and　tremulously；　cautiously；　we　will　inch　toward　a　biological　explanation。　However；　for　now　nothing　requires　that　we　go　down　such　a　path。　So　far；　the　only　life　in　the　Solar　System　is　that　which　es　from　Earth。　In　the　Uranus　and　Neptune　systems；　the　only　sign　of　life　has　been　Voyager　itself。

As　we　identify　the　planets　of　other　stars；　as　we　find　other　worlds　of　roughly　the　size　and　mass　of　the　Earth；　we　will　scrutinize　them　for　life。　A　dense　oxygen　atmosphere　may　be　detectable　even　on　a　world　we've　never　imaged。　As　for　the　Earth；　that