Jowl ofTh. Bdkh ltterylanaarysocietr vol. 45,w. 3-12,1992. AN ESTIMATE OF THE PREVALENCE OF BIOCOMPATIBLE AND IIABITABLE PLANETS MARTYN J. FOGG. Pfobability ResearchGroup, clo 44 Hoganh Coart, FoLntain Driye, Londotl SE19 I Uy, United Kingdotn- A Monte Carlo comtuter model of extla-solarplanetary fomration ard evolution, which includesthe planetarygeochemical carboncycle, is prcsented,The lesults of a run of onemillio! galactic disc sta$ arc showDwbere the aim was to assesstie possibleabundarce ofboth biocompatibleand habitable pla[ets. (Biocompatibleplanets are defined as worlds wheie the long- term prcseoceof suface liquid water providesenvircnmental conditions suitable for the origitr andevolution oflife. Habitable planetsare those worldJ witn more specmcallyEadnike conditioDsr.Tbe model gives atr;sdmate of 1 biocompatiblepla{et per 39 stars.with the subsetof habitableplanets being rnrch rarer at I suchplanet per 413 stars.The nearestbiocompatible planetmay thus lie -14LY distant andthe nearesthabitable planet -31 LY away.If planet! folm in multiple star systemsthen the aboveplanerstar ntios may bemore tlan doubled.By applyingthe rcsultsto st rs in the solarneighbourhood, it is possible to identify 28 staN at djJtancesof< 22 LY with a non-zeroprobability ofpossessinga biocompatibleplaoet 1. INTRODUCTION It is likely &at life was alreadypresent by a billion yearsafter averageglobal surfaceteinperature fes between a ]ittle less the forfiation ofthe Earth.Unless our planetwas'\eeded" with thaDOC up to somevalueatwhich annaway greenhouseeffect life ftom without, a hypothesisooly tal(enseriously by a small occurs.This zooe, the ecospfule, has inner and outer bounda- minority of scientists,ils genesismust have occuned spontane- ries defitring the valuesat which envimDmentalconditions are ously within a few bun&edmillion yearsfollowing the Earth's respectivelyeither too hot or too cold to permit the genesisor accretion.One inte4)retation ofthis factleadsto the speculation sustenanceof life. that life may originate comparativelyftpidly in any planetary Beforethe spaceage, it wasthought quite possiblethat both envlonment similar to that of the early Earth and that the Venusand Mars wereeBviroBmeDtally benign and inhabited by Gala.\ymay b€ strewnwith nurnerouslife-beadng planet!. The life. Somemodels of Venuspainted the picture of vast steamy odgin ofterrestrial life is still anunsolved problembutthe most swampsor a wodd gtdliDg seltzerocean, while obsenationsof populartheory tnkes,as its startiDgpoint,Darwh's "warmlit e the changingcolou$ of Ma$ suggestedthe Fesence of pladt pond", containing an aqueoussolution of o{ganic chemicals Siowth, fluctuating with the seasonsUl. Thus, the Sun's producedftom reactionsbetween atnospheric gases, triggered ecosphercwould have exteDdedat least betweenthe orbits of by eoergysomces such as lightning andultra-violet light. Thus, these platretsgiving the distarces ftom the Sun of the inrcr for a planet to give rfue to and rnahtain ao indigenousbio- boundaryrr = 0.71 AU andtheouter boundary r, = 1.52AU (ng. sphere,one of ib prime featurcsmust be the prcsenceof large and stablebodies of liquid water on its surface.Thjs requirc- Dole has presenteda model of an Eartlilike planet with ao ment setJstrong corsuaints oDsudace tempenrtrc and atmos- optically thio atmosphercwhich was subjectedto vaiiations itr phericpressure, ensudng thal similar environmentalcoEditions illumination and axial itrclinatioa [4. Assuming 6at such a will prevail as do o! lhe Earth. planetwas "habitable" ifatlea$ l07o ofthe sudacehad ayerage How aburdalrtarcsuch biocompafible "water wodds" likely to be in the Milky Way galaxy? Although therc is, as yet, no unequivocalevidence for a plaoetary systemabout ally o$er slar lhan our owo, it is expectedthat extra-solarplaDets rnay be Knowledgeof modem astophysics and planetologyis now sufficient to permit a reasonableestiDate of the rnassrange of starsabout whicb biocompatibleplanets are likely to exirt this is donebelow by using a siople computermodel of plateta.ry systemfornation and evolution. A! estimateofthe ftequency per fteld star of biocompatibleplanet! tien allows a surdyto be madeof stals within iie near-solarneighbourhood to identify candidatesupon \r/hichthe searchfor extratenestriallife might be concentrated. 9@d o.7 1.0 AU 1.5 1.8 2. THEECOSPIIERE Fig. 1 The Sun's fluctuaring ecosph@. EstiDated widths are due to, The orbital mdius of a biocompatibleplanet aboutits pdmary fton top: pre-spa.eagc ide€s; DoL [2]; Rasoolsnd de Bergh [3]; Hait mustlie within a zooetlemally coDpatiblewith Me, wherethe I4l and Kasting dr a, t121. ye,nly tempelaturcsbetweeo 0 and30.C, with the mostextreme account the weathedng of silicate rccks by CO, to goduce daily temperature betE€en -10 and 4&C, he was able to carboDaleshedid not modellhe geological recycli-Dg ofca oo estinate the widtb of the ecospherc.AccordiDg to his calcuta- belweengaseous and mircral phasesthat is caried out by ter- lions,lheecospbere lvas sUllquirc wide witb r, - 0.725AU and rcstrial tectonic processes. r, = 1.24AU: vrz closelo tbeorbir of VeDusaod hatl wayout 11is thb cycling of carbon which has rccently bee! put to tie o$it of Mars. forward asa major stabilbhg influenceot the Earth's climate. However, the atinosphereof the Earth is not opticafly thin. Ca$on dioxide dissolvedin water reactswith silicate mcks to Its watervapour and carbon dioxide componentsadd up to exert producecarbonate minelals, such as the forEation of calcite agre€Dbouse eflbct thatmaintainsthe suface about33 K above ftom wollastonite: the planet's effective temperatD.e.Rasool and de Bergh [31 pointed out that a terestrial platet might undergoa runaway CaSiO3+ CO, --> CaCOj + SiO, (1) grcenhouseeffect upo! outgassingif situaiedtoo Be€rthe Sutr. As 40 andCO, build up in the atmospherc,the lempemtuleoi Thus Cq io fte atmosphereof even a lifeless wo d wilh ttre suface is nised too high to pemit the condensatiotrof sudaceliquid water,tends to be consumedgradually by chemi- oceans,They calculaled that the Earth would have suffered cal weathering.On Earth,this prcce8sis not all one-way,due to such a fate if situatedcloser than 0.93-0.95AU ftom trc Sun. the fact that carbonatesediments are not pernanendy deleted Sucha value of rr so close to the orbit of the Earth prompteda f|om the geochemicalcarbon cycle. Tectonic movements within reassessmentof previous optimistic estimalesof the width of tXeclust, suchas subduction, may buly calbonatesdeep enough the ecosphere, lo betbermally decomposed, relensing COr whichmigbt rietr ?he Sun's shriDkingecosphere reached its nanowest with beca-rried back up to the surfacedissolved io o?gmas. Walker the model of Hart [4]. Ambitious in scale,Hari constucted a ?r al [6] preseotedevidence that the CO, weatheringmt€ is compulcr simulation of the evolution of the atmosphereof the temperatureselsitive with higherrates at higher tempemtures. Earth,sbning with the origin of the Earth ald running it to the This, and tle fact that CO2on the Earth is cycled back into the Fesent day. The model included a wide rangeofprocesses, "... atmospheeby volcaDism,prcmpted them to sugg$t the g€o- the rate ofdeSassinslron the inteior aM th? nean conposi- chemical carbon qcle as the basis for a powerful negative tion of juyenile wlatiles; condeksationof water tapoar into feedbackwith influence over long texmclimatic fluc[rations. ocea$; solution of atrnospheficgas$ in the oceans;photodis- When tenp€mtures rise, weathering iBqeases and CO2 is yopow sociation ofwater in the upper afiasphere; escapeof dlawn out ofthe ahosphere, Esulting i! a reduced$eedouse hydrogenfton the ernsphere; chenical reactiow between effect atrd lower temleratures. Convenely, weatheringis re- atmospheric gases; the presence of life, and vtriations in ducedwheD teBperatues fal, resulting in CO, builditrg up io biomass;photosJ thzsis and butial oforyanic setliments;lhe the atmosphercftom volcanic degassiog,increasiDg the green- UfeJ reaction; oidation of surfacemineraLt; j,ariatioks in the houseeffect and walming the planet Such a processoffers an solar luminositt; vriarions in the Earth's albed.o;a d the explamtion ofwhy the Earthhas maintai[ed a surfacetempera- SfeenhoweefecL'1 ture aod atnosphericpressure compatible with the presenceof The regulationofsurface temperature was modelled by three liquid water ftom tle eadiert times discerDablein the geologi- fcedbackloops: cal record.Such conditions would havebeen maintained cyber- netically ftom an initial situation wheaeocea$ were present (1) the ice positive feedback that accentuatedany climatic a[d volcadsm and tectonicswere ongoing. The formation of cooling by hcEasing the amountof high albedoice cover, the Earth'sfirst ocearsrnay tbus have setoe scetrefor the entite (2) the water vaporr greenhousepositive climatic evolutioo of our planet up to now. As the Sun's feedback that - accenfuateda.lly warming tend by inq€asing mass lumiDosity increasedftom its initial value of 0.7 L., the the of partial -l qO itr rbeaDnosphere, and carbon dioxide Fessure might have declined irom bar to its current value of 3 x 10+bar I?1. (3) the cloud-albedo negative feedback that would exert a Geochemicalcalbon cycliDg alsohelps explain someofthe stabilisitrg infl[ence on sudacetempemture by ilcreasing oLlestgeological featurcs oDMars. Theplanet's a.ncientcatercd the arcaof rcflective cioud cover at high tempemturesand terrafu, dnled at > 3.5 Gyr old, fu extensivelycut by numerous "runoff channels" [8] that have been interyreted as