ffiffi PIaffiffi*mffiW Syst€il?'es
R.Paul Butler
oDERNAsrRoNoMy nEvEALsto us, for the first time in his_ tory, scenesfrom one end ofthe cosmosto theother. 377 We havepicturesque views of planetarysurfaces in our own solarsystem - asthis book amply demonstrates I I - and panoramasof adolescentdeep-field galaxies swarming near the limit of the observableuniverse . Beyond pro- viding pretty pictures, asronomy pracesour worrd and our brief human livesin their true conrexrs:as vanishingly tiny subplotsin a truly enormous cosmicplay. The curtain op*, *iri, a Big Bang synthesisof the chemicalelembnts that evenruallylead to self- replicating, competitivestructures of moleculeswe call *life.,, While we humans play out our brief bit parts, we yearn ro grasp the overall plot. Naturally we wonder whether there are worrds beyond those of our solarsystem. Are they numerousor rare? How many of thcm haveconditions ripe for biologyf These are not new questions. _ In the fourth century BC, the Greek philosopher Epicurus spoke boldly of the infinite worlds that logrcally ,.aroms,, followed from the infinite number of thar he postulated. His contemporary,Aristotle, differed, seeing Earth asthe unique center ofa perfect crystallinesky. Aristotle,s Earth-centered cosmosdominated Westernthought for more than 1,500 years.The notion ofotherworlds took hold "gain on\ after Copernicusyanked Earth from its centralposition and placed it in orbit around rhe Sun with other planets. A computer simulates the birth of a Soon lupitcr-size planet around various thinkers another star. realizedthat the starsmight be diirant sunsand thercfore might haveplanets of their own. For centuries thereafter, detecting these extrasolar plenet! seemed beyond all possibility. Shining by reflected light, such objects should be roughly a billion times (perhap s 22 to 25 magnitudes) fainter than their host stars. Moreover, they would appear separatedby lessthan a few arcseconds,at best, from even the neareststars in our stellar neighborhood. If extrasolarplanets exist, they are lost in the glare surround.inga )78
st.lr'sin).lgc. l-or thcsc rc.ls()ns.tlctcctinq .l l)l.lltct.sr isiLrlcor nlf f.lr-c(lliqltt tlircctlr lt.rsIong [r('('11.1 PiPc t]rc.rnr r I lorr cr cr.. .rstlcscrilrctl l.tter..rstr.ortorl]cr\ ll-c ()n tltc rcr.gc()l .l r.(.\(rlull(,t) ttt Il^111c1sl)( )tlilttl tcrltrt0loqr'. r In thc nrc.rntintc..l nunrirer of irrrlir.cctolrscr-r ing str..lrcqre\ h.rrclrccrr tleriscd ()\'cf the l:lstccr.ltrrr\..rnrl esl.rcci:rllr or1;1]11 llst fc* r'c.rrs,t() ()\'erc(t'tc thc c\trcr)tc tiiflicLrlticsl. dctcett'tl pl,rnetsorbiting c\cn thc rrc.rrcstot thc st.rrr.Irrrlircct tcch nr(lUcs ()lt focUs thr sll1tll[)crtrtrl)tti()ns th.rt .rpl.rrrcr It)p()sc\ on l-i.tttn'tl. visiblc A strrr llrd its rrrrsccnc()nlpilni()n orbit tlrcir shrrrcd 'l'ltc Its h()stst.ir. !cntcr 0l tI.lss. st.stcrrrrrror cs througlr slr.lcc is $,cll. crtrsing tlrc For crlrrplc. \t.Ir )Lrpitcr.rrtlt]rc SLut.rre lockcri inro.r gr.rrrT^r t() trrlcc.r rrohttlirrg p.rth rcross tlrc of thc skr.,()ltscrvers Pl:rrre tr(xrJl(i:rrec scek tri dcttct 'tr.rrarithcir e()rllr)()nccrtcf ()tl)1.lss. srat.c rltc SLrrr c\tris{)lJr plrUlcts .rstx)lr)ctric.rllr, bt. trickltrg thc strlr.s tirrr rs 1.050 rir'cs'r'rc rrroliorrsrr itlr Ireeisc positiotr.rl,,r.,.rrura,,,a,,,r. lr).rssi\cthl. f LrPitcr,tlrcir.tcrrtc*f .rrss is 1,050 tinrcscloscr -.r to thc Surr l.roint.rbour50,000 knr out sitlcthc sol.rrsprhcre . Sintph,bv niovirrgrkrng its orbit, Juprtcr crruscs thc Sun t WOBI]LIN(; PI-AN ET DI-.TECTI0NS ( )r cr tite l.rst l0 r'c.rr.solrrcrr crs lt.rrc nrost olicn sc.rrt.lrt.ritor c\trJs()l.lr pl.rr)cts Usinq I)0!!/t:t.rltttl.ti.ir.rr1rr,..\ontctir.)lr.s 1.1111.1j r-.rtii.rlrckrcjtr sl)cctr()se()l)\. rhis rntlircet,.lctcetiotrnrt.tlrrirl h,rs p.rid ofi'spcct.rcLrl.trl\'. JLlst.ts ,r lclshcti rlos ..111jcrk rrs Irc.rricr ou ncr Jrorrntl rrt crrclcs,a grl rrltiorrlllv borrnd pl.lnct \\ ill s* ing its stilr.lr()rrrd thcir sh,rretlccrter ()f lr),lssin l snr.rll,rirr.r inr.tlrcot'rts crrrn orbir. SLrch.r stcll.rrrro[rblc. 0t.1.r.llL..\ utt)ut)tt. l)ctrrvs -l.lrc thc cxistcnceot rrlrLrnsecrt orbitirrg lrotlr sizc ot.rhc urrbblc vields thc p1.11111'5nt.rss. lhe tintc rlrc \t.lr t.rkc\ro e()nr plctr onc u olrtrleis thc pl;rrrcr.sorlrir.rl pcrirxj. 'l'lrc clr.rllcrrqc rcstsin tlctciriltg thr lln\ srcll.rr.ntor cn.tcllrs. rvhich .rrc bcst tietcctctl irv thc l)opplcr-cfli.ct thcr inrposc rirr.r stlr's I ilturt .i. A st:rr.tppro.rchcs ilnd light t Fryurr.i). .{s ,r \t.lr rPf r(,il(.lr(\ , rhsrl.r rccedcs fi.onr Flarth u,hile orbiting the tht, cr. 11sli.h1 ctrttcr 01 rrtrss it sharrs .flris \\ll\cs becolncVcrv rvith its Utrsccltc0nrp.1piqy115. induccs slrghtlVc()ntlll.csse(1. shorlcrtilrg tllc rr.rrc l)(r'r{)(jicl)opPlcr slrifts in the spcctrrrl lcttgthstorr lrd ( lincs th.rt cntcrgc fronr the stitr,s blrrcrcolors orrrcrsclt . .i. rltc st.trr.ccc(lc\ fi( )ll .ttrnrisplrc|c. I r:tcking thcse firiltute rr.,rYclcngthshifis I-.,rrtlr.tlrc rr:rrclcnqths.irc s.ith stilie-()f.the sligittltlcns,thcrrc(1 . ()l.r-c\t\Jrnfrt. .irl \l)citt ()of.tl)hrerr.thlt.s .I\tr()n()nlcr\fi) (letefntinc tltc st.tr.rrtfler Ihcst l)opgrlly.slrilis.rr.cr\(.fualltiltql\ 'il1,tt()n.llt(j tinr IltcSLrrrrrrrlrtrir.slrr tn tlrfD l() r\titll.ltt tltc rtr.trscsOl rrrtscen pl.lttets. .lirotllll.i nt lrqr\!Ll)lt(1. sil.l rlr.tr.relr.rLrlt (i!tcrtt()n (,1 .1 \ ii i l( lli .cd lLrpitcrrntlrr rro[r[llc r-ctlrrir.csnrc.l\urcntcnt prccisron of 379 .rlrout .l nr pcr sccorrri,()r.c.l\\ J()ur:ings1-rcct1. l.lris c:rusesthc u.rrclcngrhsot-sttriisht to chlnrrc br.I rncreonc pJfr in 100 r,-nl- li.rr. L'.til rcccrltl'sLrclr prccisi.'\\.es fir bcr'.(i rcach, bLrtit rs rrou p1r55iblc. 'l'lte itrstrrrnrcnteihc.rrt of thc l)opplcl._sprctroscopvtcch- rritltrcis.t hiqh rcsolutr()n spcctr()nrcter.l-hc spcctrLrr.rrof.rr Surr_ lrkc st.rris tillcd uith rich tlctrriliu rhc firrnt,rf..r[rs,rrprrorrlincs r:rtt.illrilrrk gitps llt tlre othcrrr isc c()ntinLt()usrlirtLrrt\. 5pgi_ t|Lrrnr lhcsc lirtcse,rrrcV intirrrrlrti.n.r[r.rrt tirc chcntrc.trc.rtt. tcr)tPcrilfrtrc, ir(,\lIl()rt. l)rCssUt-c,nlllgltctie.tctivrt\..lt1d sPin rirtc ot rlrc rt.rr,.rntj thcr' .rlso provitie thc rr.rvclcrrqthnrrrrkcrs tirr l),ppl('r arc.lsLlrcnrerts. -\ cir.rrrr:ci. thc P'siti.n .f-tircsc li'cs rctlccts,rth.urqc irr thc linc of:siehtre krcitv of.thc st.u.rr)5pJcc. For scrcr.rl tir:cittics tht. pr.ceirjonr rf J\tr(,n(,n)i(.tll)o;lplcr nrc.r-\Lrrcntrnts ,l.lrc h.trl bccn st:rllcd.rt rrbotrti00 rl pcr scconti. \\.JS l)r()[rlclI thirt VcrV,\.erV sn].rll clrtnues iIt .r sl.lcctr () I IJH{ I't.,.\\t:i.\t{\ s\ \ I 1,,\l\ lfl 160 planets Known of Sunlike Stars stor norne Distonce spectrol Stor orbitot orbitot (light'yeors) orbitol Doppter Minimum ryPe mos', semlmoior period eccentricity semiomplitude ,noss (sunt axis(AIJ) 14Herculis (doys) ,rrjj, se Kov o.8o 2.s r,srs o"\::.ri, 47Ursa€ Majoris 46 o.3s 6 OV t.| 2.t 1,09g |6 CygniB 0.tO 46 2.4 72 G2.SV | .O | .7 70 802 0.67 44 Virginis 59 G4V |.7 0.95 0.47 | 16.6 O.4O HD I t4762 90 3OB 6.8 Fsv t.t5 0.36 83.9 Gliese876 0.34 5t3 | t.6 t5 M 9V 0.32 0.20 6 t,I Bt RhoCoronae Borealis 57 220 t.89 GOV 1.0 0.23 39.6 RholCancri 0.04 67 l. I 44 GgV O.g5 O.l I 14.64 0.03 71 UpsilonAndromedae 57 F7V 0.93 1.25 0.056 4.61 O.lO 5| Pegasi 50 14 0.55 G2.5V t.0 0.05| 4.23 TauBootis 0.0| 55 0.45 49 F7V t.25 0.045 3.3| 0.006 4G9 ? 7 'lnbh l.'l-ht plrrrre r00 ts firurrdlrourrd rrcr.rrhr..sol.rr,fi.pc st:rrs thr().gh nrid' 1998 cxhibit .r u,icrerlrrgc 47 Ursae Majoris .f .rbitrr crrlr,rctcristics and likell nt:lsses.Not included a hcrc ;rc thc thrcc planets O surrounding o the puls:rrdesignated pSR I 957+ I 2. i- r.ttiio tclescope U rrc.rr.{rccil.ro, Irrrcrto l{ico.,\lcr \\irlszcz.tn o fourrel ! thlt .t trir.l of'sntlll pl.rncts rirlrits tltc pLrls;rr.I)Sll I 257+ I 2 ( ltqtrrt J). lruls.rrs.rrc rlc.rtj \t.rrs. thc r.tpitilvspin U ttiltr.:..ttt trltrrtI \l.lt- l.(.t)lI U trc.rrr z i.re. se,.,,.,,.,,, .,.I''.ll,t],;';:i,:;I;: i:::i] :,:' ;Ir ] ::] x fiotn our pcrspccti\c thcse r,rclio\\...tvcs \J ilppcar to blrrrk orr lnd of}, likc .r lighrhoLrsc, -50 .ls rhe pulsirr spirrs. Llitr.rprccisc rirnjnq .f l'SR 1257+t2's sig..tls .r{-{r,'stittontic cl.cks r.tliertcri r988 | 990 c\lrcnlcl\' \nl.lll \\i rltlrlq.l t991 1994 t996 | 998 11.,its rttotiort.rltr, rcllt.rlr, sirrrr.ilrrn,ol orllitins conrp.r11j1y115IrLrls,rr tirrringis rltc orrlv tcchrrrtlrrccrrr 50 ttltllt .tLrlr'trr q|q.1.., l-.y.ljr.rrr.rs, l6 €ygni B lrltrrr.t. I-hc l)oppicr.rcclrnrtlrrcsrrcccctjctl a4n _ soon tlrcrclficr.Jn(1 srr)cc 1995 crrr'.rsol,rrpl.rncls lt.rr c lrccrr Lrn.rnr[rigLroLrslrtjetce tcrl : .l .r'orrrrti I I .\Lrnlikcst;\r\ | f !)n iqurL.5, altlt 11.\ljchcl ,\ll.or .rrti l)idie r QLrclozrjiscorcrct.i tlrc fir.stpllrrq orbitinq un .r nor.nr..rlstar., 5I I'cg.rsi.-l'hcsclrrh c()n(luctcd L.J [rl Licof]l.cr.l\1.r,,,.,,,u r,,. JLrthofrc\ c.rlcd tltc rre jr 3 -zo rt s trcrrpl,rncts, onc ot.u hich i .rrounrl l 6 ( 'r gnr ll ) * rs fi.,Lrrtr irrtlcpcrrdcrrrrrbr \\'iili,rrr (..crrrerr .rrd \) ,{rtic z --ao Il.rtzcs.'l'hc pl.rnctcire]inq l{ho (_oron.rc -r llorc.rlrss,.rs firLrrrtlb'ir tc.lrr ()t rilrc ilstr()r()r)crsrctr b'l{.bcrt \.r.cs. oac -6U Orbital plane nearly edge-on | 988 | 990 t992 t994 t9g6 | 998 ir.r' , Actual J \ mo[ton f igtrrt 6 (nbovr).yer - I-irltrrc (ri.qht).An urravoidlblc lntbiguitv occursin spectroscopic rstinrrrtcsot pl.rnetarv nrasscsbecause the sameradial_velocin,signal crn bc producedeithcr br..r Iou,cr {t nrassplanct orbiting in rrplarre rrcrr.tlrciirrc of'sight. or bv l rrrtr()re nl;tssivcplanet irr rllt orbit scctr r)(.trl\ (Il. l.l(c ll(.i.lu\( (,1 lll .,)t)r r).r. i( ),) \ r.,r' rrc'r.* rrJ,liil ;::i::il:::::::ifil,;' Orbital plane nearly face_on l cl)t'c\(t)ts I elocifr,. rri,\\i\l\ ll(iill r:tri.4i1iir.:t!i:l{;tri1t!*8*r-!{ria:i4s*riqr,.fr!itrq.ws*:sl4ig*rimg & additiona.lobject, discoveredby David latham and othcrs, orbits 381 the srar HD 114762 in Coma Berenices.It has a large mass, more rhan l I Jupiters,which suggestsrhar it is perhapsnot a planet but instead a substellarbrown dwarf (seeChapter l4). The orbital period of eachplanet is the wobble cycle time. From that period and Kepler's Third l^aw,the orbit,s semimajor axis (the planet's averagedistance from its star) can easilybe dete rmine d. If the velocity varies like a perfect sine wave) we know the . orbit is circular; ifnot, the skewnessofthe velociw curve can be analyzedto find the eccentricityofthe elliptical orbir (Figare 6). The amplirude of the Doppler variationstell us the planer,s minimum mass, not its acrual mass(Figure 7).If theorbit were seen nearly edge on, the Doppler shifts would yield the star,s full orbital velociry. However, the viewing "ngl. ,, unknown, and orbits rhar are strongly tiltcd with ..rp.* to our line of sight will produce Figne 8.The planets in our solar a subdued Doppler shift. Thus, we can only system have nearly circular orbits, derermine but nearly halfofthe known extrasolar planet, the massof the planet multiplied by do not, The most the sine of the extrcme caseis orbital the planet of l6 Cygni B (eccentricity 0.57). inclination(Msini), where I remainsunknown. Its orbit Our is shown superposed on the inncr inferred masswill solar svstem. most likely be smallerthan the real one. Luck- ily rhis problem is nor roo grear.For randomly oriented orbit planes, the true mass of a planet will be, or, .u.rrg., %n times the minimum masswe determine. So tle true massvery proba_ bly lies within twice t}le value we measure. A PLANETARY ZOO The extrasolarplanets discoveredaround sunlike starshave surprised more than a few astronomers. They all have roughly /upite rlike masses,with values of M sini berween 0.4 and 12 Of Jupiters. course these stars could also have smaller planets that are currently beyond our detection limits, but larger ones are unlikely. Any companion having a massof l0 to g0 Jupiters would have been very easily detected. Apparently, nature rarely makesplanets much larger than Jupiter.Why that is so remains unclear. What's more astonishing is that six of these /upiter-mass orbit remarkably {T.j: closero their host stars,within 0.25 AU. This is much closer than Mercury,s mean distanccfrom the Sun (0.39 AU). The Doppler searchtechnique Fig*re 9-Likely protoplanetary preferentially disks in the Great orion Nebula, identifies such tightly bound photographed by the planets, becausetheir orbital Hubble Space Tetescope. The largest blob velocitiesare very coltail a &rk, nearly fast. No one expectedto find any giant planet edge_on disk about the size of pluto,s orbit. so close to a star, The bright part ofthe blob is gas that and no one knows for sure howlhey could has been evaporated from the disk's surfacc have goEen there. by radiation from hot sta"s outside thc framc. At lcast two other By comparison, circumstellar disks dot this small field. the planet orbidng 47 UrsaeMaioris seems more similar to our |upiter. It has a minimum of 2.4 fupiter in_their calculations ,,predictions,, masses' to yield circular orbits as its orbit is armostcircular, and its orbital for ndius of 2.1 AU solar systemseverywhere. correspondsto the inner cdge ofour asteroidbelt. Such a planet, When the new discoveriesshowed that ifplaced in the solarsystem, would ellipseshappen, theo_ look like Jupiter,sbig brother. rists scurried back to their blackboards"nd soon Also puzzling are the six known planetary developed companions that some beautifully havedramatically sensibleelaborations on standardplanet-for- elliptical orbirs(Figare g). The companions to mation theory. Pawel futymowicz patrick 16 Cygni B, 70 Virginis, g76, and Cassenhave Gliese and 14 Herculis have reanalyzedthe birth ofplanets in the eccentriciriesof 0.67, 0.40, kind ofprotoplanetary 0.37,and 0.35, respectively.(The disks that are observed 4round newborn, Suntit. largesteccentricities in our solar system, stars(Fcgure for Mercury and pluto, 9). Their calculationsshow that protoplanets exert gravirational are abour 0.2.) Why do some starshave planets in such strongly fo1c1son thc elliptical disk that trigger spiral dcnsity wavef not unlike orbitsl Indeed, how commo n arecircularplanetary tighdy wound arms in spirar orbitsl The arrangement galaxies.These spiral wavesin turn of our own solar system has always exert subtle gravitadonalforces back seducedtheoriss into including just on the planet, pulling it enough physicalprocesses away from pure circular motion. In rhe courseof , rnillion y..r, 1.+-rl OTHER PLANETARYSYSTEMS $ q*t{f.i1i!$€:;:llii.l!::::]t*11:eil:'ts!i;;.: 382 the departuresfrom circular motion can build up to be quite The real planetary pool table, however, is not flat but rather significant,leading ro eccenuic orbits. It is not yet clear whether hasa cenua.ldepression produced by the graviry ofthe host srar. spiralwaves can induce eccentricitiesas high as0.6. Nonethe_ The scatteredfupiters careenaround the depressionfor a while. less,this theory echoesthe wild historicalpast of planet hunting. Eventually the situation stabilizeswhen planets collide and The firsr photographsof.,spiral nebulae" (now known to be merge, fall into the stiu, or are flung out of the systementirely. galaxies),taken by |ames Keelerat Lick Observatory in the early What's left will be just a few planets in eccentric orbirs rhar are 1900s,were originally inrerpreted as planetary systemscaughr in far enough apart not to affect eachother much. the act of formation. Narure may haveprovided us with an excellentexample of a A second theory for the large orbital eccentricitres seems chaoticplanetary system in the making. The southern-skystar equally likely.Suppose, for a moment, rhar Saturn had grown ro Beta Pictorisis surroundedby a largedisk ofdust and gasrhat a largermass as it condensedaround the infant Sun. Indeed, all appearsnearly edge on ro us. First seenin 19g4, the disk hasa fbur giant planetsmight haveamained larger massesif the proto- clearingin its center roughly 8 billion km (50 AU) across. planetarydisk had containedmore materialor had lastedlonger. Astronomersbelieve one or more planetsmay residein this In that cascour solarsystem would havestarted out with four clearing,and rhey may be sizable. The inner parr of rhc disk is rrue hear.yweights,each exerring greater gravitational florce "warped" - on it occupiesa diff-erentplane rhan the part flrther tlrc othersthan it docsnow. They would perturbeach other,s out (Figure 1,1).The re is strongcircumstantial evide orbits nce rharrhe until thev overlapped.The planetswould then gravrtarion_ warping is due to gravitational force of a planet with a nass anv_ allv scatterone anorherand assume chaotic,unpredltable final where from t/zoto 20 times - the massof ]upiter, depending on its orbits almost like the breakof ballsin billiards (Figure l0). exactlocarion within the disk (or the central clearing).This body 40 X 30 20 = t0 qJ= 0 0.8 p Figare 10. Survival ofthe fittest. In this computer simulation (nppar years. Eventually one planct panek), is usually thrown out ofthc sysrem three |upitcr-mass planes start in nearly coplanar orbits S,0, (right)' leaving the othcr two in eccentric 7.3, and f0.2 orbits. The rooer ponels show AU from the Sun. As they perturb cach other, their how the semimajor axcs, eccentricities, and inclinations orbits begin to cross of the three and evolve chaotically after only about f00,000 orbits change drastically and unpredictably before_one planet is lost. CHAPTERTWE^\TY.EIGHT jjrl!&,:ii:.]:jFf+tiiijgit*it5&Mti*iiw*ll j8j 'l'hc lilturt I I Hrrbble sPrrccl'elcsc.Pc.trt:ri'rd t\.lpl).lrfntlI n()t \cr\.!l()sc this z2 rrcscc.rril *,itie t() thc st.lritscif. lrcc.rrrsc rrc rjo not inrageot'thc dusn disk;trorrlrdBcta I'ictoris,r tr.pe,.4 strrr6.1 lishr tlctcrt l)opplct.\.lri.ltions r.crrs in tlrc s;rcctrutrrof.lJct.r Irictoris. au'lv' A subtlewarp (fronr r,n'er Hou.- Ieft t. tr''cr right) in thc crisk'silrrt,r z.lrt c\ cr. rltc rrlr|'s tilt nt.lv i)c tcllinr{ us mlv betral thc gravitationirl thilr thc plinct ()(cuprcs.tn ef'fcctsofa lrrpitcr,nr;rssplirnct closrr in. rn.lincrl orbit. lt tlt.rt p11r1.c5trLtc, thts svstcntnt.l\.[rc in thc tlttocr r,t.r "Ij.rrrcr.rr.r \lt.)kc,rul'.Irkc th.tr,'|.p,.,..1 ut litlrtn I(t g'rlrrrrcts.If'tltis sccn.rrio ir eorrccr. Ihc qrrrrit.rtiorr.ri lt,ru tlid rlrcscsir crtr..rsol.rr sctrtcringnrotlcl cln crpl.rjnc,a,.t thc ll()st It,i.ttttpl.trtcts .rr.l-ir c so ,rirsrrrtllvqlosc (\tter)lc cecclttncttics to thcrr st,trs.rr ]tcrc rers.rrc \c(.n.lnt()nqthe ettrlrsol,trpl.rncts. If.lirir is not st.rirlc)l lrcscpl.rrtt.tr cirlter lirrnreil rrlr.rt rr.rlli (.\'q.r r.iu,lrtrrltt,rr.tlrr,i rrorr h.r|Pc11(,iJt i() lJ .r..1tlrc 'tlrcr cercrrrrri c.r' rc\t('i(.\\itlt.tl,tut.l.trtr r.o.[1 q1..1,,,.,rlrrr.itrng,.l]t\ i).ul()nsrrc sltoitltj r.lr.(.rJI,rt t\(\ crcrrtLr.rllrtlclcct otltcr rcrtt.tirtingpllnet.trr oI titcr tirrrrrctll.rr-tltcr ()ut .lr)(llt)lgf.ltc(l lrilli.rrtlb.rlls 'l-hc l]]\\Jt.(1. rt tllcscsvstcnts. prcsirrnrrblv orlrrting nrLreh trrrthcr lc.idirrethe orr eorncr tl tr.ornl), )ugl.l\ I_l). l,ctcr ljoticn otrt t.tch st.lf..\ \.trjrlti()rr()lt thjs theorv is th:rt.r [rrn.trr.st.rr. ltcinrer-. .rnti I)cr.clillrh.rrdsrrn. rr lto irorrou ctl hc.rr \\ \tLnt \\ ()ulrj plrrncts ilr fronr tlrc Pcl-tlu.b orbitrng tiqltth. .rroLrrrtlcrthcr slilf. rolk of \\rilli.rrtr\\'.rrtl. In thcir ricrr..r pr.otopi.rrct lndcr',1,l6 (.r grri Il is .rccontp.rrrrcti rn rrsrr.u.ri .rt .rgrclrt tlisr.rnee tilt lc.lst tlisk l,irrl rl)0reirrrr.rrrl. li)r f\\() l-c.t\()lt\.l,ir.:t. tltc lifl0.'\L br tltt,nror.rnt,rssrrc st.rl ( riislint.rtcr.r.tl l(r rgni .\. rrltreltrrr.rr.lrc ttscll tr sp11-.11111qrilt(){lrc \{)ung \l.t1.,lrrr.t,, rr\rr)\1\ i r'sPollsjlrl1tirr. rltc i.rr-qc(.cc(.ntnrjt\ ll'r(llr.r( 0t tlrc irl.rnct.lrl)ril)(j \t.lr. lJ. tiott rrrtltin tirt tlr:li i lrrr9111..;..,1 llori rrrll,lr.tgpr.,11,p1.11q..1. lll\\.t1.(1.\'ott;1. 51y1.1, I\irf \l.ll.\tlo rltrrrr!l(.tr \l)r!ll.lisil1.,,l i\l\'S]-ERIOL'S .rc!r('ti()nin tltcir SHORT PERIOD PLANET-S crecsstrjtr.rr rojr.t .rntl inli.rr.cLi a,,,irr,,,,,, \,,. irtr'J'tl rrriqr.tti{)r!Jr) h.lr(li\ lrc.ri'rrlcti. Iltc tttost ntvstcriorrs \ce'atl, l)r_()t()l)l.rr(.i\ .llt(l c()1tr()\,cl.sillpcq. pl.lncts i1 thc rrrll h'rrc rcs()'.lat,r'lrit.rr ri,trs rr Illcl).lqcricitrc "hot rre ilrr trr.sc ,r'thc :'ir.rr *.rr t.s thc si\ JrrPitcrs"in circuler or[rits Lt()scr ntrul thet sct Ltpin thc tlisk.'lhis (..lusc\ 0 15 .\L' ljtc i)l-()l()l)l.utctslo losr .rngrr ti'1. rhcrr.sr.rrs.'l''cse trc thc c()l.ltp.lltio's of..l-ilLl l:tr ntOrncttttr|ttcr cn lloLiti:. il t]tc disli \,rs nOt itscll rlr.Unrrrr-1tnri.tr.tj 51 Itcg,.rsi.L'lt.il,,n .\nrlr()nrc(1.1c,l(h,, t .rrrcrr,l{lro Intrir.r[rlr.tlrcl ui]l ( ( )r.()..1c LreJr.rr.geLl t()\\.lr(i tlre rrclJ.rr.lirln.rrc Iirr-e.rlis .r.tl (ilicsc ri7(r. l ircir ,rLrir,rl 'l pcrirtls .rrc 3..i. itisirrrr.rrti *.1. ltl.trtctrrrrqr.ttrort r.l\e\ clt{)l.n)()l\(.(}l)fltrls..l\ \cl {.6, l-1.(r,.39.(r. .rntl 6l . I tlirvs,rcsp.rccrivclr.. eonrpirrccl r Giant parked field sweeps ()llll:li I'j...\\lt.\ti\ \\\tl \r. Ic\|ecirllv tltc ions .rnt1clce trons i, cithcr flirrqrrrr:ir orrtr',rrtl or hrrcirrgit ro llorr .rlong nr.turrctrciicl.1 ()nt() lincstiou'n tltc st.rr.'lhe holc providcs.rs.rit, pirkirrg spot firr thc in\\.lr(llvlniqr.ltir)q p].lncr ( )ncc it eltrcrg,csfiont thc inncr ctluc of tltc tiisk.the is rrrr lonqcr.tir.rq{cd Pl.rnct tirrhcr.irru.rr.tl. .\ Ploro pl.l)ct ll.l\ bc tirr.trrn.uccrrtirr!,lt to rcr.rr.rinrn tltc cic.lnnq until rhc rliskdispcr.scs. tirrrs .rlklr irrg. ir to ()rt)itrllrrc h.rPPihr-1 cl-.tir'1-. 'l'hc rreqnctic q.lp [r]rkillq nrcch.rnisnrcrnrrot crpllin thc pl.rrrcrs.rround l{ho' (..rncrior. ltho (.oron;rc llorcalis.u ltosc orlrifltl r.rdir.rrc 0. I I .rntl 'l'lrcse 0.2.1 ALr. r-csPcs1i1('lq, drst.rnecs .rrcu cll ,tltlri,lt llt(.l)r'\llti.ll((l .lr..rr.utg, \,, \\(. .lr.ilell s ith .l ln\.\tcr\.: ll or[.rr.rllniqritti()r] t.rotrg,lrt tltcst, r:lriccts iuu.rr.r1,l hv rlitlrr'trltcr continuci Il.n lrc t ltcr rrt rt. It.li .t r-.rn,lc,irr lr,.rrt ltt.l)t.(,1 r)l)l.lr)(.t.lr.\ 'l'hc ,iisk .1r.srlr.s1('rl.ru.tr l-t,t1ril.t ,\l.rrIrt. t lrcr t,,r.rlr.tlnu,lrr 1.1. birth ol'.r JrrPiter.Irr this conrputer simulrrtion,gas.continues to pour onto tvlrclcthcr' 'r rcr Ir c..dt'nscrr_giant'r;rrret .trc.u ithorrt lrrr orbitll lt)iqr,ttr()lt.lt c'en lfter the prarrethls su,ept a raggedgap in rrstar.s .rll. I)crh.rps irt'trtopl.riletlrrrdisk, c\pcci.lll\ tl.rssirc yrr.otoPl.r;111.11.1 ,it.kr (.lt(,uojl i.11,r t((.lt(.(. tlt.ll(.t.r.lj .t,,(L .ju\r .ltd iron tlrrst qi.rrrt ) to lirrrl i)l.lnet.lt.\e()rcs \\'hl rlocrn't oLu.sol.tl \\ \tr.nt ( lt.r\ .rrlr.rrtt Il,rlrrt in .lose to rltc Srrrril-crh.rlrr Extremely ltrPitcrlirrnrcrl nt..rr tlrc rrrri of.thr.lrlt.trn.rroi .rrr-pr.,11;Pl.111ct,r'r incomplete tlisk. .l\ tlrc \ isc'sitr rr.rrrcti.rrrtl rlictl .ut l)crh.rpsour t.iisl< ncvcr hltl cnorrgh g.rs.rnt.l tlrrst to cxhi[rit nrtrch viscrisirl irr the first pi.rce s , so rt st.rvcr.lI)ut until thc irc.rt of.thc ne\\'born Sttrt[rlcu it li,rr.. Intlcetl, l.rotol.l.rnctlr.r.t]rskscorcr. .r l'iclc rlnqc - of'rn.rsscs frorn I fcrv lupiters up t() ltun(ife(is..r\ ticdLrcedfionr UJ thc r.rdrocntission l.r.thcir.rlLrst. So f]rc tlrrersrrr of obscn cci pl.lrlcts nrJv r.cprcsent ,l scrlucnccof protolrl.rrret.rri "Planers" rlisk rn.rsscsor IJ lrfL'tintcs,.rntl Pcrhlpl cltcntic.rlc()ntp()srtl()ns. A chellcnuct() thc vcrv { +______+ cxistcrrctot'thc sjr shortcst_Pcrrotj "Brown ertr:isolar pline { ts c;1ntcin elrrlt,l9c)7. High rcsolutiorr spccfr:r o dwarfs" of 5l Pcsisi sLrggcstctl rh.rrthc -,/.raptsot'rhcsr:rr,s sp1g11.1l lr11cs. n()I fltclr.ll)p.lre nt rr':rvclenqths..rrc n h.rt clt.rngcl ith .t _l.l.d.rr. lhis uoLrld l.crrotj. inrPlrrltrt thc plrol,isPhgr_(,ottlre .r.iris llll(lcrq()itt1I.t tontlrl(.rosr'ill.ttrolt ol .rsorl n(.\(.T\ccn rn tltr.\Ltn u ) |r{)r' .lt)f t( ii).ltc(l tltr'ort l.,tllr,\lor.r. rrttlroil.tnt t0 t5 20 25 30 lr. rt strqqt.rte,l tlr.tt tltr'l),'1'1ri11l)(r'r(,,1r\rlr r,l il l,r..t.rri,r,rrJLl MINIMUM MASS lrr cri,1.,,,,,.r, (upitersl rr rtlrotrt iril okilrg ,t Ii.rnct. I-i.tttrt'tl-!.I' Hol cvcr. ntorc .tdcliti.n to [)li'cis hr'irrg r.ughl'r'e c.rrctirlstrrriv rcr c.rictlrro sLrrlrose rll.rtorr. r'irss.f Jupitcr, s.nrc "()\'cft()ncs 'r\tr()r()r)lcrs .rrc clisc.r'cri'g rrruch in tltc rrighcr-nrrss co'.rpilili()ns orbiting stlrs. l)opplcr lltcilsLtr.cl)tcrrrsot. 5I i,cr.r.rrr ()hjtcts i' the ra'ge.l'10 to ll0 fLrpitcr nlilsscs Instt.rrl.thc slr.rPcsof'thc arc.ficn crlsscd ils brown spcetr.rilincs rcnr.rinc()ltstJnr..ln(r llte .rr.rrts. F.r crr tlr.ugh tlrc' ir.c ,rue' c.l\ier t() dct.et, put:ltiVe t.cr. ;rrc p.^.ilrg t. bc Pi.Itct nc.rtlVerpl1j115 thc singlc.clc.rr l)oPi.l(.r_|cl_11;,1 r'.lt\'l l llJlt l0rrCr nt.tssstCll.t 'f 'l-lLr u'c;tresceing of'-1.2d.rr s. hc ,ritri'rcrrrir,rsscs ,,r.bjcct, ,;,::]ili;l'i.i:: sl.cctrLrrlot Iloiitis under\\ cnt sllrl.lr il:,::::|',.'iilrr.t scrLrtilt\'.l his st,rr .tPpclirst0 host thc n.rostnt.tssir c .llt(t cl()stst. orbitinq "hot JLrpitcr."Its l)opplcl-sllifi cr clt,rs rhc srror.rgcsr rr' srlrlirgs'r.r c irr*.trrli Il.r'lrc tlrc' tiidl O.e rlre()r.\.sr.rggcsts vct clctcctecl- s()strong, irr t.rrt. rh.rrit r.rrlrl l.t.rrc lrecn ttscor tlt.rt .r nrrrli-er of'gi.rnt prolol.l.rncts tirntretl .rt lcrrst5 AU r>trt, crcti rlrrnr' \'c:lrs 'l..ru ..rg()hlri rrnvone bccn lookirrc: liri)tis nrigr.rtcriirrulrtl..rrrtl ruct thcir.firtcin tlre.SLnr. Jupitcr rntl thc shoricd no r.rrirrtions in thr slr.rpcsof its spcctr.rllincs.rt .rll. lc\t oi tltc Srrn'sfintili.tr.plJncrs \\erc onlr tlrc l.rstrn linc, lcfi J'hus, oscillrti<)ns c.lnnot crpllin I stlr's I)oPpl11-shifis..rntt \tr.lrt(icdllrcn thc I'cstol rlrt.pr,,t,,l.l.r,r..r.rrl, tnc tjirk clc.rrccl6ut. existcncc of'.r pl111c1rcnr.rir.rs thc orrlr vi.rblcconclrrsion \or:1c r,rsoltrliotrs P1-111111j11.rrr ro tltis ntigr.rtir)n(ii]enIt).1 trc enr( r'qing. I lrc 111.o11.;11l.11at.rrrriisks.ir{)un(l \ {)unqSrtnlrkc st.l.s ,icicr.P.r PLANETS |111[r'1lrl1 hrre .r,r g.rpirr thci'ecrtcr.s *irrh l ..'li.r'rctcr OR BROWN DWARTSI 'rlr.LrrI 0 'r'rris rirrcs r.rrqc'rh.rrthc st.rrirscrt. cc'rr.rr h.lc isclc.rrccr lJorr e.rrrue bc srrrc '\ th.tt the ciqht conrP.rrriorrstjiscorcre.j lrr Irr fltc s1.;1111111g\t.lf n).lgnf tie fjcltl,n.hiclr cntr.linstlrc hOt q.rs thc l)opplcr.ll)pr.().rclr.rrc rc.rjlrpl.rrret:.rs,Lrclr. r..rrlrcr th.r,r . : .\'.r.l\ll(,|{i : :ar',jij' rr :, riiJ:' :ii;r:'-!i-i:- j:i!"l.ii4.ilfir.'*ei**9,F.Bi@*@rB.3rffiwd&&,ww,&w#ffid&K*{*ffiFmwffiffi &1 extremely low-mass 385 brown dwarfs) Sometimes considered situations where "failed the orbits could nor have been circularized bv stars," brown dwarfs are objects with too little massto tides from the system,sstar. shine by their own nuclcar power, a threshold achieved at 0.0g solar mass(about 80 fupiten). In today's parlance,the distinc_ tion "brown *planet" THOUGHTS beween a dwarfl' and a with the same ON PLANETARY DIVERSITY massin rhe sameorbit dependson how the body formed. A With only 14 exrasolar planets confirmed, we lack a good sta- planet grows from dust and gas accreting in a circumstellar disk tistical sampleagainst which to compare our own (Figure solar system. 13). A brown dwarf forms the way a star does, by a frag_ For example, all the extrasolarplanets found so far by the ment of a gascloud collapsing in on itself before the gashas the Doppler technique haveorbital periods of lessthan years. chance 5 This to develop a protoplanetary disk around another star. is not a reflection ofplanetary systemsin generalbut rather due Some good evidencebearing on this question comesfrom the to the limited duration (about l0 years)of the hunrs distribution to date. of the newly-discoveredobjects' masses(Figure t4). With time and improved Doppler precision,more planetsmay The most numerous companions have the lowest masses,below be found in slower, longer orbits fhrther from their stars. 5 Jupiters.This is absolutelyremarkable, because such objects However, the meagersample is alreadysending us a warrung are the most difhcult ro derect with the Doppler technique (or that our solarsystem may not be the norm. Supposethat gravita- any other). In ftct, we are probably overlooking many compan- tional scatteringof planetsis common in newborn solar systems. ions with masses below 5 Jupiten becausethey induce Doppler After all, many signs give testimony to heary bombardment by shifu in their starstoo small to be detected. In contrasr, we are bodies in crisscrosing orbits when our solar hardly systemwas young. missingany companionsbetween 20 and 70 mass- ]upiter The cratered facesof the Moon and other bodies, the Moon,s es, becausetheir Doppler amplitudeswould be so strorrg. very formation, and the extreme axial rilt of Uranus Thus, two populations all tell of a of low-massobjects are found around violent, impact-rackedinfancy. The near, racerrackorbirs Sunlike stars. There of is a spotry population having masses today's middle-agedsolar system are the crashsurvirors from its between I0 and Z0 /upiters; some asffonomerswould call these recklessyouth. Our systemmay be unusualin bror.r'n dwarfs. that small planets The striking concenftadon in mass below 5 like Earrh werc nor flung away by giants falling into eccentric lupiters suggests a separate class that may have formed by a dif- orbits, asmay have .planerary', happenedaround l6 Cygni B. ferent method. The term seemsquite appropriate An outcome of tidy, circular orbits may require flor these. Three specialstart- of them (the companions of4Z Ursae Majoris, ing condidons. The near-circular orbit ofJupiter actually pro_ Rho Coronae Borealis, and RhoI Cancri) have circular orbits in motes rhe stabiliry of circular orbits among the other eight OREIIATPERIOD(yeors) ORBITALPERIOD(yeors) 0.001 0.01 0.00t 0.01 ?' Ptr ru !l S z nr Z o o z r0 2 { { = E o o \) t{ o.otY o o \^ I E E 0.01 0.1 I l0 0.01 0.1 | ORBITALMDTUS (ostronomico! lo units) ORBITALMD/US (astronomicol units) 15.Complementary lil*y meansfor dctecting the reflcx motion of a (f 0 parsea) away, This technique favors planets in Srrnlike star in a planetary systcm. For cur.erriDoppler-bascd wide, and therefore scarchcs long-period, orbits. While out (left panel), the limiting sensitivity of reach todaS wobbles seen in the is shov,,nas " Aiigo"A linc, along positions of HD 114762, 70 Virginis, and 47 Ursae Majoris with the five best finds to datc. This technique favori plancts may soon in short_ bc dctecable by ground-bascd intcrferometers. period, high-velocity orbits. Note that, in principlc, In all likelihood, a ptenct with howcver, ncither Doppler |upiter's massand orbit could spectroscopy nor astrometry will be able to be disccrnJ. ffr.righa-lrand panel rweal shows analogues of Barth. the sensitivity ofastromctric searchesfor stars 33 lighi_years OTH ER PLANETARYSYSTEMS 386 planers. Simulationsshow that, were Jupiterin an eccenrric bodiesorbiting in ellipsesaccording to Kepler'slaws. IsaacNew_ orbit, Earth and Mars u,ould likely havebeen flung out of the ton, were he resurrected,could examinethese Doppler data and solarsvstem long ago. There would be no terresrrilal planet in immediatelyrecognize pure orbital modon the habitable stemmingfiom his zone, and no readersto reflecton this fac. The lawsofgraviry. Planetsare surely orbidng those stars, cxisrcnceof intclligent and they life may dependon both lupiter and have eccentricitiesgreater than thoseofall major planetsin Earth being in mutually our stableorbits. Thr.rswe will find our_ solarsystem. selvesin such a circularizedsystem, no matter how unusual it It took humaniry more than 2,200 yearsro nravbc. develop the tech- nology proving that Epicurus was right about the existenceof The handfulof planetsdiscovered so far around Sunlikestars many worlds. The next round of questions show should be answered orbital properriesprofoundly more diversethan those of much more quickly.What fraction of starshave planetsf thc rvorldsaround What us. Even the relativelynormal-seeming plan- fraction of planetarysysrems resemble oursf Finally,do small, etsof 47 UrsaeMajoris and l6 Cygni B havemasses of at least Earth-sized plane$ commonly occur in a star'shabitab.le zone, 1.6 }upiters,showing rhar our own fupiter is nor the largesr plan_ where temperaturesare right to allow lifef ct thar nature can fbrm. The Doppler shiftsof 70 Virgtnis and L6 Today more than 1,000 srarsare being surveyedby Cvgni B vary in nonsinusoidal the preci_ cyclesthat are clearlyrhe result of sion Doppler technique using the world's most powerful optical telescopes. Meanwhile,several groups are surmounting the technicalhurdles neededto combinethe beamsfrom rwo or more telescopes, known as an interferorneter.Thesedevices are capable of extraordinaryresolution. Within a few yearsinrerfbr_ ometric sysrems should be working with gianr telescopcsin Hawaii, Arizona, and Chile. These systemswill be used to make precrse measurementsof stellarpositions, revealingthe presence ofplanets through asffomerry (Figure lS). The following generarion of interferometerswill be soace based. Getting above the blurring effectsof Earth,s a[mosohere will allow us to measurestellar positions with a much higher level of precision. Among the first of thesewill be the Space Interferometry Mission,or SIM (Figure,l6).While ground_ basedinterferometers should eventuallydetect planemcompa- rable in sizeto Neptune (about %orhe massof |uprter), SIM should be able to detectsomerhing the sizeof Ear*r(about /loo of a |upitcrmass). Ground-based interf'erometersare now servingas techno_ logicaltestbeds for SIM. In rurn, SIM rvill help deine rhe sub- sequentgencradon of spacc-basedinterfbromerers. Two such conceprsare planet NASA's Terrestrial Finder (TpF) and rhe EuropeanSpace Agency's Darwin. Unlike their predecessors, TPF and Darwin are being designed to actu"liy saeplaners orbidng nearby stars.They will combine the beamsfrom.sever- al telescopes ,,nulling in a so-called interferometer.,,Such an instrument cancelsout the central star's light, allowing its faint nearby planets to be seen directly (Figure i,f . This critical achievement will allow us ro take specua ofplanets and identi$, the chcmical consriruenrsin their atmospheres.The detection ofoxygen, carbon dioxide, water vapor, and methane in the F igure. I 6 ( abow N ASA's atmosphere of an Earth-mass ). proposed Space Interferometry Mission invorves planet orbiting another srar several telescopes mounted would on a l0-m_long truss. The telescopes, locations be a discovery ofhistoric proportions. would be adjustable, to keep the light path The technological hurdles for these projects are formidable, stable to within l0 angstroms. This and it is difficult ro predict when any of them mighr see orbiting inrerferometer should resolve fruition. But the questions they may answerare obiects separatedby only 0.01 arcsecond. no lesspro_ found. What is our place in SIM may reach orbit as earlv as 2005. rhe universel Are rhere other Earthsf Is our hearwarming solar systema freakish wist in the Figare (righr). cosmic script l7 A four-planer system or merely some common plot device used over 33 light-years from Earth and over stands out in this throughour the solar neighborhoodl We do not simulated image from a futuristic planet know - but we will. finder. Current designs produce two images of eachplanet. (.HAl'lf.t{l\\f \TY f tcHf {i,:,1;ti{t{!6!iirt;n&fi9qffi @wsffi ffi i