Fogg, 1992

estimating one such planet per 18-39 stds ofmefalicity > 0.3Table 5 reveat how tus conclusion ru$ counter to previoustrends. It lists estimates of the abundance of tbEe previousstudies of the preva.lence of "habitable" planets. These estimatesvary enormously, due partialy to diffedtrg definitions ofthe parameter space defiDing a habitable plaDet Dotre [2], forinstance, iDcluded multiple sta$ in his analysis, whercas BoDdand MarliB [31] and Pollard[32] adopted the ecosphere of Hart[4] and sel out, deliberately, to come to a conservative conclusion.Tbe prcsent study coDculs with the data i! Table 5 in thatit suggests lhat specifically Earthliko, habitnblq planetl arelikely to be quite rarc (1 in 188 - 413 stars) but cont€nds tiatsuch planets ale only a subset of a geafly more abundant set ofpotentially lifebearing biocompatible planets.We may ttrus requirc aootler explanation for "The creat Silerce"other than the lack of suitable sites for the orisin of tife.ACKNOWLEDCEMENTSThe author would like to tiant Salvatore Santoli and JimKastins for their intercslTABLE 5:RefDole [4]Bond & Mantu [5]Potard [6]n(HP)N1n,o1/6000 - l/r2000105 - 10rd(LY)80 - 100200 - 1000Di Schwartzt:m dd T. Volk, Notue, 340, 457-460 (1989\, h^yeshown that chemical weathering of basalt is gready feilitaten by ficlresence of pldts and oiqooiganisos. A ge@hemicsl cdbor cycledrkled by biology would theEforc act d m evetr morc rcbustplmetary th.mostot. This eff@t hd &t b@n included in thir paperbeause theprcbability ofthe oriain oflift tu comlletely un&lowD andb@ause the Seochemical cdbon cycle can olerate abiotically.REFERENCF^S2,3.c sag6 ed I. N, Leonsrd, '?k ,,r", t if€ scl@e Libffy, TiFLif€Intemalioml Nenrhd) Nrv 0966).S.H. Dole, "Hobitdblt Pbnat fur M@". Blaidcl Plbiisiidg Co,, NewS.l. Ra@l dd C, d. Be€h, 'tte R@My Cfted@s. dd theAcuniaton of cO? j!th€ vetu Atlm{Frer, Natte,7,t6, 7037 -1039Zuctemd, pp, 135-141, PdCrmon h6s, Elmsfod New Yort (1982).17, B.A. TE@g, "The Ch.nical Eyoiuti@ ofthe Sold Nejghbounood tr.the Age-Metaliciry Relatioo ed the rIliory of Srar !tuation in rheGalA ic Disc", Atttophlt J., 242, 242-259 (1980).18. l.ltca, Aa. Rq. Attr, Astruph}'.,5,571 (1961).l9- D'o. Coush, solr lnterie Srru.:M dd L@jlosit' vdiarids", ,toi,t5,1.8.M.H- Har! "Tbe Evolutionofthc Aln6phde ofrhe Eanh",IcarN,33,23-29 (1974].C. Sa*rq, "Smilivitt dd Slabiljg of clobal Climre lrodeb", tdtu,56, r35-139 (r984)_J,C.c, wdte4 P.B. qays ed t,F, Ksring. A Negalive !.edbqckMeclEnin for lhe Lng,Tetu Sllbiliatim of Esth s sudaceTmpentre", J. Oropt n Rd&, 46,9176-9732/J9A1),J.l.(asringrldO,B, TM,"Clinat EvolntionontETercriatPlu.tr",ia Onqi ad Evolulio" olPlMLtry atd Saellle AtMphzr$,EAs,s.K.Aleya, LB. Pollack ddMl. Mathsws, UniL ArizotraPEs, p!,4231149o98q,NLH, Cn, " T h. S wJ@. ol M @t", YatePlmetatt ExplmtidSed.s, New9. J.B. Polact, J,F, K6rin& S,i!,L RiclardsmsdK. Poliatotr,'"nr Ca.fda Wet, Vam Clinate d E&1y Ma11.ara,77,2t3-224 (1987).10. C,P. Mc(ay m.l C& slokd,'Tne EdlyEnvirm.lledir Evofuli@ onMs: rnpli6doN fm Life", R v, c.opbt.,27,l99-274..l9A\.R Krlu, "Ihc Eyolulion od Cq d MG", t@ru, 62, 175-190 (1985).tz. J.F. (asrins, O.B. Td ed J,B: PollrcL, "rlow Cli@te Evolye.l d lh.Teftsdi.l Pld€c", &r /r,, 258(2), 46-53 (1988).13. t.F, Kastirg, R@say ldd Mojst crce.hous. Aldo+hd6 ed dreEvoluLion oC E rih and v6u . I@r8,14, 47244 ,Jgaa),MJ. Fog& "l@f@i!g, a P!ft of a Strzteg fc I eurellEColmtati@", J3tS,44, 183-192 (r99 r).15. J.M. Scalo, "fl E Steld Ma! Sp.afl n", i P rctos t4r t ota AzN B,Bd. T.CehEb, pp, 265-28?, Univ. Arih. Pre$ (1978).v. Limbl.. 'Nlcrorym[.sis.!.1G!]acdc Evolution: Inpliatim ad lheOriE lot\it ",iE*at n6dob: Whae AreTh.t!,EnsM.HsratAB.2n. S.H. Dole,'Coopu&rSinuhtj@ of 0D Fmrion of Pl$ehly Systos",Iqr6,13, 49+504 /J970).21. R, Gladd md C. Sag!4 "Cmput rsimilaiim ofPleetl4?Aderiorry':mie Smitiviiy to hirial Con lni6", I.ua,3l,5rG533 (1971).22. (C, Condie, 'Odgin of lle E rrL's c$t', Pdl@g.o9taplq,P ab @ liaoro lo E, Paltu @ la s! ( C lo b a I ann P hwtary c hznA. s..ri.n),?5,57,31(1939).23. W. GIid., tl J,tui$ @d A,& Upgen, 'Sila pithin 25 PaeB of theSb", ia Iht G4l4tt o.d th" Sols ''/lrrn, Eds, R Snol@hm*i IN,8.161l dd l'4.S. Madh€wr, pt 13-34 Univ, Ariaa Pres (1986).T.A.Iicppaleidea"OnlL.FomaiidolPldetsin Bidarystaisystetu",Atrmn Atiophv., 65, 421-426 llEl 8).25. J.l Lisrdei'.'[l!ichStra ]Ine Pltut 1", ia Th. ForMuoR MEnbdot oJPl4tutu, Srst.m, &t!, lI.A. W@vd @d L, Dd y,pp.304308, Cinblidge Univesitr Pr€s (1989).26. RS, Itdinstor,'?Ie€tarr 6its inBina.ySt6",,4rlroa J.,82,?53-?56I19n\.u. IrR. Matidd, "PDje.t Dleddu: AsEodmi@l Dot! or Nctrbt Slendsts.dr, in pbl.cl D@d,ta,tBIs supel, pp. s8,s18 (1c78).24. !"T. Lawio! ed P, Wrigh! "Ihc Se.r.h fq Gnpmi$ ro EpsiloEidei", J,IS, 43, 556-5580990).DB. Gu.nt[d,'A D;!@$i@ ofEpsilon Bridei\ ACa, Codpdirid ud30.31,32.Oscinatim Spetun'i,&tbpbr.I -, 34 2ll -2t 6 (19?'7).A. C.npben, p@!.1 cottjNidlid (191).A. Bod ud A.R Martin.'A Cdmdv. Erimt of tle Nmbd ofHrbillbL Plm€E in lh. Galarr',./rls, !1, 4l I -415 (19?3,.W,G. Pollad, ,lhc Pr.valac.of Eerthl&. Plaets",Aur.!.r., 67, 6s3-659 (1919\.