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in　atmospheres　and　icy　surfaces　of　all　these　worlds　are　being　irradiated—by　cosmic　rays；　if　nothing　else　and　nitrogen—rich　organic　pounds　are　being　formed。　It　looks　as　if　the　stuff　of　life　is　sitting　not　just　on　Titan；　but　throughout　the　cold；　dimly　lit　outer　reaches　of　our　planetary　system。

Another　class　of　small　objects　has　recently　been　discovered；　whose　orbits　take　them—at　least　part　of　the　time—beyond　Neptune　and　Pluto。　Sometimes　called　minor　planets　or　asteroids；　they　are　more　likely　to　be　inactive　ets　（with　no　tails；　of　course；　so　far　from　the　Sun；　their　ices　cannot　readily　vaporize）。　But　they　are　much　bigger　than　the　run…of…the…mill　ets　we　know。　They　may　be　the　vanguard　of　a　vast　array　of　small　worlds　that　extends　from　the　orbit　of　Pluto　halfway　to　the　nearest　star。　The　innermost　province　of　the　Oort　et　Cloud；　of　which　these　new　objects　may　be　members；　is　called　the　Kuiper　Belt；　after　my　mentor　Gerard　Kuiper；　who　first　suggested　that　it　should　exist。　Short…period　ets—like　Halley's—arise　in　the　Kuiper　Belt；　respond　to　gravitational　tugs；　sweep　into　the　inner　part　of　the　Solar　System；　grow　their　tails；　and　grace　our　skies。

Back　in　the　late　nineteenth　century；　these　building　blocks　of　worlds—then　mere　hypotheses—were　called　〃planetesimals。〃　The　flavor　of　the　word　is；　I　suppose；　something　like　that　of　〃infinitesimals〃：　You　need　an　infinite　number　of　them　to　make　anything。　It's　not　quite　that　extreme　with　planetesimals；　although　a　very　large　number　of　them　would　be　required　to　make　a　planet。　For　example；　trillions　of　bodies　each　a　kilometer　in　size　would　be　needed　to　coalesce　to　make　a　planet　with　the　mass　of　the　Earth。　Once　there　were　much　larger　numbers　of　worldlets　in　the　planetary　part　of　the　Solar　System。　Most　of　them　are　now　gone—ejected　into　interstellar　space；　fallen　into　the　Sun；　or　sacrificed　in　the　great　enterprise　of　building　moons　and　planets。　But　out　beyond　Neptune　and　Pluto　the　discards；　the　leftovers　that　were　never　aggregated　into　worlds；　may　be　waiting—a　few　largish　ones　in　the　100…kilometer　range；　and　stupefying　numbers　of　kilometer…sized　and　smaller　bodies　peppering　the　outer　Solar　System　all　the　way　out　to　the　Oort　Cloud。

In　this　sense　there　are　planets　beyond　Neptune　and　Pluto—but　they　are　not　nearly　as　big　as　the　Jovian　planets；　or　even　Pluto。　Larger　worlds　may；　for　all　we　know；　also　be　hiding　in　the　dark　beyond　Pluto；　worlds　that　can　properly　be　called　planets。　The　farther　away　they　are；　the　less　likely　it　is　that　we　would　have　detected　them。　They　cannot　lie　just　beyond　Neptune；　though；　their　gravitational　tugs　would　have　perceptibly　altered　the　orbits　of　Neptune　and　Pluto；　and　the　Pioneer　10　and　11　and　Voyager　1　and　2　spacecraft。

The　newly　discovered　etary　bodies　（with　names　like　1992QB　and　1993FW）　are　not　planets　in　this　sense。　If　our　detection　threshold　has　just　enpassed　them；　many　more　of　them　probably　remain　to　be　discovered　in　the　outer　Solar　System—so　far　away　that　they're　hard　to　see　from　Earth；　so　distant　that　it's　a　long　journey　to　get　to　them。　But　small；　quick　ships　to　Pluto　and　beyond　are　within　our　ability。　It　would　make　good　sense　to　dispatch　one　by　Pluto　and　its　moon　Charon；　and　then；　if　we　can；　to　make　a　close　pass　by　one　of　the　denizens　of　the　Kuiper　et　Belt。

The　rocky　Earthlike　cores　of　Uranus　and　Neptune　seem　to　have　accreted　first；　and　then　gravitationally　attracted　massive　amounts　of　hydrogen　and　helium　gas　from　the　ancient　nebula　out　of　which　the　planets　formed。　Originally；　they　lived　in　a　hailstorm。　Their　gravities　were　just　sufficient　to　eject　icy　worldlets；　when　they　came　too　close；　far　out　beyond　the　realm　of　the　planets；　to　populate　the　Oort　et　Cloud。　Jupiter　and　Saturn　became　gas　giants　by　the　same　process。　But　their　gravities　were　too　strong　to　populate　the　Oort　Cloud：　Ice　worlds　that　came　close　to　them　were　gravitationally　pitched　out　of　the　Solar　System　entirely—destined　to　wander　forever　in　the　great　dark　between　the　stars。

So　the　lovely　ets　that　on　occasion　rouse　us　humans　to　wonder　and　to　awe；　that　crater　the　surfaces　of　inner　planets　and　outer　moons；　and　that　now　and　then　endanger　life　on　Earth　would　be　unknown　and　unthreatening　had　Uranus　and　Neptune　not　grown　to　be　giant　worlds　four　and　a　half　billion　years　ago。



THIS　IS　THE　PLACE　for　a　brief　interlude　on　planets　far　beyond　Neptune　and　Pluto；　planets　of　other　stars。

Many　nearby　stars　are　surrounded　by　thin　disks　of　orbiting　gas　and　dust；　often　extending　to　hundreds　of　astronomical　units　（AU）　from　the　local　star　（the　outermost　planets；　Neptune　and　Pluto；　are　about　40　AU　from　our　Sun）。　Younger　Sun…like　stars　are　much　more　likely　to　have　disks　than　older　ones。　In　some　cases；　there's　a　hole　in　the　center　of　the　disk　as　in　a　phonograph　record。　The　hole　extends　out　from　the　star　to　perhaps　30　or　40　AU。　This　is　true；　for　example；　for　the　disks　surrounding　the　stars　Vega　and　Epsilon　Eridam。　The　hole　in　the　disk　surrounding　Beta　Pictoris　extends　to　only　15　AU　from　the　star。　There　is　a　real　possibility　that　these　inner；　dust…free　zones　have　been　cleaned　up　by　planets　that　recently　formed　there。　Indeed；　this　sweeping…out　process　is　predicted　for　the　early　history　of　our　planetary　system。　As　observations　improve；　perhaps　we　will　see　telltale　details　in　the　configuration　of　dust　and　dust…free　zones　that　will　indicate　the　presence　of　planets　too　small　and　dark　to　be　seen　directly。　Spectroscopic　data　suggest　that　these　disks　are　churning　and　that　matter　is　falling　in　on　the　central　stars—perhaps　from　ets　formed　in　the　disk；　deflected　by　the　unseen　planets；　and　evaporating　as　they　approach　too　close　to　the　local　sun。

Because　planets　are　small　and　shine　by　reflected　light；　they　tend　to　be　washed　out　in　the　glare　of　the　local　sun。　Nevertheless；　many　efforts　are　now　under　way　to　find　fully　formed　planets　around　nearby　stars—by　detecting　a　faint　brief　dimming　of　starlight　as　a　dark　planet　interposes　itself　between　the　star　and　the　observer　on　Earth；　or　by　sensing　a　faint　wobble　in　the　motion　of　the　star　as　it's　tugged　first　one　way　and　then　another　by　an　otherwise　invisible　orbiting　panion。　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