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Downloaded by guest on September 29, 2021 www.pnas.org/cgi/doi/10.1073/pnas.1703517114 to systems. planetary applicable other also in are results and plan- exoplanets These habitable multiple system. TRAPPIST-1 on the in by ets initiated was evalu- life- for whether metrics ating of observational propose number We increased immigration. the the of of because rates and higher be transferred to likely show also species are we bearing of , number solar theoretical the the from with that models compared adopting planets of By TRAPPIST-1 probability system. the the on that enhanced argue case. is we -to- consequence, the a with As compared system TRAPPIST-1 occur to the likely in more that magnitude of find orders and potentially TRAPPIST-1 is panspermia seven dwarf of ultracool system the discovered orbiting 2017) recently planets inter- 1, of the March probability in review the for panspermia estimating (received planetary for 2017 model 10, simple May a approved present and We NJ, Princeton, University, Princeton Bahcall, A. Neta by Edited 02138 MA Cambridge, , for Center Harvard–Smithsonian a Lingam Manasvi system TRAPPIST-1 the in panspermia interplanetary Enhanced u nig n xedoraayi oohrpaeaysystems. planetary other to analysis our support extend to ecology and theoretical findings from our models on a draw to also We leads abiogen- esis. of panspermia probability the of in probability increase significant higher correspondingly much the We system. that TRAPPIST-1 show the within panspermia for model tative be Thus, would panspermia Mars. system. that and this hypothesize in Earth to enhanced between inclined distance be would the one than less by of HZ only the separated of in are planets The system (9–13). TRAPPIST-1 Earth trans- the the for from mechanism or potential to a life investi- as the porting system widely solar through been own our has (panspermia) in Panspermia gated another material? rocky to of life transfer could raises one consequences: possibility from profound this spread with planets, question TRAPPIST-1 immediate the an of one on exist (8). planets even these perhaps, and whether determine carrying possess TRAPPIST- to for the observations opportunity Hence, unique additional (7). a out represents Earth system are the that transiting of radius 1 a those and mass to a equal has them nearly reside of planets planets each seven also, seven of HZ; the three of within because is remarkable, discovery latter more The the the (6). of TRAPPIST-1 (ii) all star and nearest dwarf ultracool (5), the the system transiting b, solar Centauri namely the Proxima year, to of past discovery the the over (i) advances remarkable two witnessed easier (4). comparatively analyze and are M detect they to have orbit because dwarfs studied, M which the- extensively —of region of been liquid (HZ)—the supporting many zone of exist habitable capable Way, the there oretically in Milky that sup- Planets the well-known (3). of dwarfs now capable in is (i.e., planets of habitable It habitable planet understanding (2). a life) better make porting a that by factors accompanied the been This (1). has thousands the progress in numbering now exoplanets discovered T onA alo colo niern n ple cecs avr nvriy abig,M 23;and 02138; MA Cambridge, University, Harvard , Applied and Engineering of School Paulson A. John ee eepoeti osblt ypooigasml quanti- simple a proposing by possibility this explore we Here, (abiogenesis) life of origin the for favorable conditions If has low-mass nearby around exoplanets for search The dacsi h attodcds ihtettlnme of number total the with decades, two remarkable past witnessed the has in research advances exoplanetary of field he | panspermia a,b,1 | n baa Loeb Abraham and rgno life of origin | b ∼ 0.01 .. tens a.u., ∼ 10 10 ilipc lntY eas nrdc h vrg rni that transit X average the Planet introduce from also τ ejected We Y. rocks Planet of impact will fraction cumulative the as gravita- of at Eqs. effects arrive Combining we the etc. resonances, for secular account focusing, tional to introduced factor fication and respectively, where In (14). influence of sphere provided gravitational model, Y its this Planet within by fall captured they sim- are strictly a that rocks use the is we wherein Instead, magnitude cases. model, which most of ple in orders estimate, value many actual be the this than would smaller impact, However, direct for Y. only valid Planet of radius frac- event. the per represents captured (RHS) rocks side of right-hand tion the on factor second where hsatcei NSDrc Submission. 1 Direct PNAS a is article This interest. of conflict no declare authors The paper. the wrote and data, average an at Y) (Planet planet of target distance that the rocks of is impact number the The event successfully isotropically. from single emitted ejected are a rocks rocks during the of that X) number (Planet total planet the host that suppose us Let Model Simple Lithopanspermia—A uhrcnrbtos ..adAL eindrsac,promdrsac,analyzed research, performed research, designed A.L. and M.L. contributions: Author owo orsodnesol eadesd mi:[email protected]. Email: addressed. be should correspondence whom To xy h RPIT1sse n h ieiodo iei u . our in life of of likelihood studies the future and system for TRAPPIST-1 - the consequences profound observational has work and our retical Thus, mul- possess planets. may life-bearing system tiple planetary We oper- this being material. that of implying probability rocky high ational, a of least has transfer process at can this the that planets paper conclude which through these Our of others of life. one to on supporting Earth, spread life of that the capable possibility the be of addresses may those them masses to of with three similar planets rocky radii seven host and to TRAPPIST-1 exciting discovered the most was Recently, the star . of present-day one in is life frontiers extraterrestrial for search The Significance avl paig emyexpect may we speaking, Naively hti ie by given is that a σ y y and steefciecosscinlae fPae .The Y. Planet of area cross-sectional effective the is D P xy xy M PNAS rmPae is X Planet from = y M r h eiao xsadms fPae Y, Planet of mass and axis semimajor the are N N ? σ x y stems ftehs star; host the of mass the is | y N = 0 = ue2,2017 27, June y η b = τ nttt o hoyadComputation, and Theory for Institute xy .16 xy N π = η a x xy y 2 · D hv  4π  xy | i 2M σ D M D a σ , y o.114 vol. xy y y xy y 2 ? =   , 3 2 2 π  R , M M | y 2 where , o 26 no. y ? N η  x xy 3 2 n assume and , sa ampli- an is | R 6689–6693 1 y and sthe is [1] [3] [2] [4] 2,

ASTRONOMY where hvi is the average velocity of the ejected rocks. Consequences for Abiogenesis We can now attempt to calibrate this model in the solar We now turn our attention to quantifying the implications of system by choosing X and Y to be Earth and Mars, respec- panspermia in promoting abiogenesis by drawing on an equation tively. Substituting the appropriate values, we obtain PEM ∼ similar to that developed by in the context of the −5 0.75 ηEM × 10 . On comparison with the older simulations car- Search for Extraterrestrial Intelligence. The parameter ried out in refs. 10, 15, and 16, we find that ηEM ∼ 20 − 200. Sim- of interest is the probability λ of life arising per unit time (22, ilarly, if we use the recent value for PEM from table 4 of ref. 23). According to the Drake-type equation proposed in ref. 24, λ 13, we obtain ηEM ≈ 270. Next, we consider the TRAPPIST-1 is computed as follows: system with X and Y being TRAPPIST-1e and TRAPPIST- Nb 1f, respectively. On substituting the appropriate parameters (7), λ = · fc · Pa , [6] we find hn0i

  where Nb is number of potential building blocks, hn0i is the mean ηef P ≈ 0.04 , [5] number of building blocks per , fc is the fractional avail- ef η EM ability of these building blocks during a given timespan, and Pa is the probability of abiogenesis per unit time and a suitable set which implies that a nontrivial percentage of the rocks ejected of these building blocks. All of the factors, apart from Pa , are from TRAPPIST-1e during each will land on dependent on complex biological and planetary factors, which TRAPPIST-1f. As a result, the fraction of rocks that are trans- we shall not address here. ferred between planets would be comparable with (although Instead, we mirror the approach outlined in ref. 24, where Pa smaller than) the fraction of rocks that fall back on the sur- can typically be enhanced via panspermia by a factor face of the originating planet. [Gravitational perturbations by the n densely packed planets could also disperse the “” of rocks Pa = P0 E , [7] out of the (caused by unstable orbits).] This where n is the number of panspermia events, P0 is the probabil- conclusion is fully consistent with the previous numerical sim- ity in the absence of panspermia (i.e., with n = 0), and E is the ulations undertaken by ref. 17. Thus, the TRAPPIST-1 system enhancement arising from each event. A few clarifications are in is expected to be more efficient than the Earth-to-Mars case in order: by “enhancement,” we refer to the fractional increase in facilitating panspermia. the number of molecular building blocks (not biological species) It should be noted that Pxy only quantifies the fraction of rocks transferred per event. This condition is a less stringent require- that impact the target planet and does not quantify the total num- ment, possibly extant even in our own (25), implying ber. The latter is dependent on Nx , which is governed by the fre- that this mechanism of “pseudopanspermia” has a higher chance quency and magnitude of impacts (10, 18); the impact rate of being effective. itself is regulated by the properties of the planetary system under At this stage, it is equally important to highlight the limitations consideration (19). Hence, we shall not attempt to quantify Nx of the above ansatz. The exponential gain represents an idealized (or Ny ) for the TRAPPIST-1 system, because there are too many scenario, namely the most positive outcome possible. In reality, unknown factors involved. We reiterate that Pxy is the fraction the scaling with n would be much weaker, possibly being alge- of rocks transferred per event, and to obtain the total estimate, braic (or even logarithmic). In addition, the introduction of these a knowledge of the total number of impacts over the plane- new molecular “species” does not necessarily enhance the prob- tary system’s lifetime is required, which cannot be quantified at ability of abiogenesis, because they could have landed in a habi- this stage. tat on the planet that is inimical for their survival and growth. Moreover, panspermia does not merely depend on the fraction Moreover, the reaction networks (protometabolic or otherwise) of rocks transferred but also depends on other factors. Many of contributing to abiogenesis and subsequent could have them have to do with the survival of during the been primarily engendered by the multitude of environmental processes of ejection from X, transit from X to Y, and reentry planets on the planet (26) as opposed to external contributions on Y (20). Because of the biological characteristics of the - through panspermia. isms in question, one cannot quantify these probabilities; even The value of n varies widely from study to study (15, 20), for Earth-based microbes, there exist uncertainties (21). How- because it depends on the minimum size of the that ever, if we assume that v¯ is the same for Earth and most of the is capable of sustaining life (or molecular material) and many TRAPPIST-1 planets, we see that the transit time in Eq. 4 among other biological and dynamical considerations. However, even the TRAPPIST-1 habitable planets is about 100 times shorter for the conservative choice of E = 1.01 and n ∼ 103, it follows than that of Earth to Mars. A higher value of v¯, albeit one much 4 that Pa /P0 ∼ 10 . If we compare the relative probabilities for lower that the , could reduce the transit time even any of the TRAPPIST-1 habitable planets and the Earth, assum- further (e.g., becoming four to five orders of magnitude lower ing E to be the same in both instances, we find than the Earth-to-Mars value). Hence, if the probability of sur- τ (T) viving the transit is inversely proportional to xy , the survival Pa n(T)−n(E) n(T) probabilities of microbes on the TRAPPIST-1 system could be (E) = E ≈ E , [8] several orders of magnitude higher. Pa We emphasize that the above model does not explicitly where the last equality follows from our argument that pansper- take into account the architecture of a given planetary system, mia events are likely to be much more common on TRAPPIST-1. thereby neglecting effects arising from gravitational focusing, Although we cannot hope to estimate E or n(T ), a robust qual- resonances, eccentricity, and mutual inclination to name a few itative conclusion can be drawn: the presence of an exponential (1). We have also implicitly assumed that the planets settled into scaling on the RHS of Eq. 8 ensures that the probability of abio- their present orbits quickly and that there exists a sufficiently genesis via panspermia can be orders of magnitude higher than high number of to cause spallation (12). Many of on Earth in the optimal limit. these aspects can only be studied through detailed numerical models, which fall outside the scope of this work. Despite these Analogies with Ecological Models simplifications, our conclusions are in agreement with recent The close proximity of the TRAPPIST-1 planets is reminiscent numerical simulations (17), which had incorporated most of the of an analogous environment (albeit at much smaller scales) on aforementioned factors. the Earth, namely islands. If we look on the habitable planets of

6690 | www.pnas.org/cgi/doi/10.1073/pnas.1703517114 Lingam and Loeb Downloaded by guest on September 29, 2021 Downloaded by guest on September 29, 2021 igmadLoeb and Lingam ae,rsetvl.Frteeulbimnumber equilibrium the For respectively. rates, whereas planets), diversity species equilibrium the and the for lower be to com- likely Thus, is decreases. rate island system. extinction TRAPPIST-1 the the of Mars, area of with the value pared as smaller increases the it of that because for case expression that Earth–Mars latter The apparent the the is for consider it higher we systems, If and TRAPPIST-1 length. island, and scale the inverse and acteristic source the between occurs, where where that see we to Thus, (31). source the from where that hypothesized had exists 29 Ref. there species. that of number was 29 (I (E and immigration the tion 27 between refs. equilibrium bio- of dynamic island a pansper- basic of that interplanetary The theory possibility via mia.) the the transferred be explore 30, could qualitatively ref. to (In used the 29. was from ref. geography arose in primarily which paper 28), seminal (27, case latter island the of in orbits relevant occupy the planets plane. of and common most a surface share but 2D volume, dif- a 3D only occupy The a islands panspermia. that via is material) ference genetic transfer (or to lifeforms amount essentially of they would plane- degree, immigration In this “mainland.” a terms, the tary from to “immigration” isolated to subject are also are islands readily these are similarities although the apparent: “islands,” as system TRAPPIST-1 the httettlnme fdsic ouain lf-ern plan- suppose (life-bearing We (planets). distinct is of “patches” ets) number different total of the the up that couch made now is us tem) Let 34. of and metapopulations. of 33 “ terms refs. in a in system TRAPPIST-1 found as be to can ecology referred metapopulation concerning details commonly Additional (32). populations” is metapop- a of which concept the ulation, is ecology impor- theoretical in Another paradigm biogeography. tant island beyond extend models ical com- transferred thus system. species solar of the with number pared greater the a char- of in by are panspermia than acterized values by seeded higher the planets TRAPPIST-1 much that the be Hence, provided will case, diversity species Earth–Mars the that lows and higher h pce diversity species The hsaaoyealsu oda nterc n estl field versatile and rich the on draw to us enables analogy This h eta rms sta h eaouain(lntr sys- (planetary metapopulation the that is premise central The ecolog- and system TRAPPIST-1 the between similarities The S P and N N A S ae nteiln,wihdtrie h equilibrium the determines which island, the on rates ) R P hc a oenn qainof equation governing a has which , T x y I steae fte“ore rmwihimmigration which from “source” the of area the is stettlnme fseiscpbeo migrating of capable species of number total the is stettlnme fstsaalbe(ubro HZ of (number available sites of number total the is E stedaee fteisland, the of diameter the is n erae ihrsetto respect with decreases and ssihl oe o h RPIT1sse,i fol- it system, TRAPPIST-1 the for lower slightly is d dt N I I d E dt and S = N ∝ smr miuu,bti ufie osay to suffices it but ambiguous, more is S ? = IN S A = ? E sepesbeas expressible is x I = R N r h mirto n extinction and immigration the are  (S y S T 1 x (−λD exp P P −  S − I 1 N + N D I − T ) S xy E I E ES − ?  S ,  stu ie by given thus is xy ? EN − . nrae ihrespect with increases E ) λ , , D Because . ersnsachar- a represents xy , N I S stedistance the is ? P efidthat find we , ilb much be will n extinc- and ) r similar. are I smuch is D [10] [12] [11] [13] [9] xy . erdcmae ihteErht-asseai sn Eq. trans- using are scenario that Earth-to-Mars rocks obtaining the of thereby with fraction compared relative ferred the that estimate conclude then we can around region, is HZ this the in of of star width 7 a the figure for that From a.u. infer HZ. we the 45, within ref. are of planets multiple where dis- dwarf, conclusions dwarfs. the scope. M broader of a have many herein cussed TRAPPIST-1, around centered TRAPPIST-1. Beyond Looking factors. other and illumination, pressure, the atmospheric as gravity, such conditions, surface initial study to to life used of be sensitivity on also the can detected fact are this Consequently, biosignatures planets. same multiple the whether determining by independent a and as serve panspermia abiogenesis. further between may differentiating planets of multiple means on sensing discov- remote the of via Hence, fea- ery (44). detected universal be polarization can a circular and be using life to biochemical posited of been right-handed ture and has acids Homochirality amino ). left-handed utilize posi- false ing identify edges. spectral to “artificial” taken other on be and than also as wavelengths the must such longer of tives, care to cognizant Due shifted be (43). be must Earth may edge it red that the at possibility is for brightness searches would blackbody any peak it Hence, an its is wavelength, star, TRAPPIST-1 dwarf same Because ultracool panspermia. the for two at case on the planets, strengthen observations different photometric more) through (or detected is edge red on be independently occurring would abiogenesis them of probability atmospheres planets’ The the (8). in biosignatures molecular via typically detect we of Eq. value to a panspermia from out increases abiogenesis of probability the Panspermia. of Existence the Detecting from arising consequences major analysis. the our of some explore we Finally, Panspermia of Implications multiplanetary in abiogenesis our systems. and furthering of panspermia capable of intro- is understanding concepts (35–39) and ecology formalisms theoretical of in here, duced array explored diverse analogies and the promising two a about the Beyond statements system. semiquantitative TRAPPIST-1 or qualitative available deducing of system. solar number the with total compared and HZ) the rates (in immigration planets because higher planets life-bearing is the of system of number TRAPPIST-1 greater the a with that consistent conclude we earlier, presented de fvgtto,wihcrepnst hr nraein increase sharp a to around at corresponds reflectance which the vegetation, of edge” are 41) (40, panspermia context. finite this and in null useful of distinguish- also via cases for abiogenesis the proposed for between metrics ing case statistical the Other strengthens panspermia. planets for more) unity (or than two greater ratio the much Because plausibly other. is each with material exchange I that implies result This osmaie h rnfro ievapnpri a etested be can panspermia via life of transfer the summarize, To liv- (i.e., homochirality by characterized is it know we as Life nte en fdtciglffrsi hog h “red the through is lifeforms detecting of means Another h frmnindmteaia oesaevr sflin useful very are models mathematical aforementioned The n erae oooial with monotonically decreases and k osdragnrcmlilntsse ruda M an around system multiplanet generic a Consider lnt nteTAPS- ytmwt in flife, of signs with system TRAPPIST-1 the in planets 0.1–2 P 0 k hra tcudequal could it whereas , PNAS N 7 M ? fpnpri speet ups that Suppose present. is panspermia if | 0.7

nrae oooial with monotonically increases lhuhms fordsuso was discussion our of most Although ue2,2017 27, June fteeeitmr hnoeplanet one than more exist there If . nteErh(2.Tu,i the if Thus, (42). Earth the on µm D xy ehv ttderirthat earlier stated have We ∼ E sn h information the Using . | a k y o.114 vol. ≥ O ∼ eetn ieon life detecting 1, P a k 10 | falplanets all if −2 o 26 no.  (P 0. P a. N 03–0.05 1. a 0 u. /P T 1 | with- µm. 6691 and We 0 ) 3, k

ASTRONOMY 2 − 2 P  η   M  3  M  3 than the Earth-to-Mars value because of the higher capture xy ≈ 20 xy y ? , [14] PEM ηEM M⊕ 0.1 M probability per impact event and the much shorter transit timescales involved. −3 where PEM = 2 × 10 (13). Thus, for most M-dwarf systems, We explored the implications of panspermia for the origin of the fraction of rocks impacting the target planet is conceivably life in the TRAPPIST-1 system by drawing on the quantitative around an order of magnitude higher than the Earth-to-Mars approach proposed recently in ref. 24. If panspermia (or pseu- value. Using Eq. 4, we conclude that dopanspermia) is an effective mechanism, it leads to a significant τ  D   hv i  boost in the probability of abiogenesis because each panspermia xy ≈ 0.007 xy EM , [15] τ 0.01 a.u. hvi event can transfer a modest number of molecular species, and EM the cumulative probability scales exponentially in the best case with τEM = 4.7 My (13), implying that the transit time is two (or scenario. Thus, it seems reasonable to conclude that the chances more) orders of magnitude lower compared with the Earth-to- for abiogenesis are higher in the TRAPPIST-1 system compared Mars value. Collectively, Eqs. 14 and 15 would result in higher with the solar system. immigration rates, which imply that our previous ecology-based We also benefited from the exhaustive field of theoretical ecol- results are likely to be valid. ogy in substantiating our findings. By drawing on the analogy Exomoons. A second analogous setup involves a planet with with the theory of island biogeography, we argued that a large multiple exomoons in the circumplanetary HZ. Habitable exo- number of species could have “immigrated” from one planet to cannot exist over long timescales when the star’s mass another, thereby increasing the latter’s . As known is < 0.5 M , because their orbits are rendered dynamically unsta- from studies on Earth, a higher biodiversity is correlated with ble (46). Nonetheless, this case should be evaluated on the same greater stability (50), which bodes well for the multiple mem- footing as M-dwarf planetary systems, because habitable exo- bers of the TRAPPIST-1 system. We also used metapopulation moons may even outnumber habitable exoplanets (47) and will ecology to conclude that the possibility of multiple planets being soon be detectable by forthcoming observations (48). Most of our “occupied” (i.e., bearing life) is higher than in the solar system analysis will still be applicable, except for the fact that the exo- given the considerably higher immigration rates. planets and star must be replaced by exomoons and exoplanet, To observationally test the presence of life seeded by pansper- respectively. mia, we proposed a couple of general tests that can be under- To gain an estimate of the relative increase in probability taken in the future. We reasoned that a “smoking gun” signature with respect to the Earth-to-Mars case, let us make use of for panspermia may require the following criteria to be valid: −2 Eqs. 14 and 15. Suppose that My ∼ 0.1 M⊕, M? ∼ 10 M , and (i) the detection of identical gases, (ii) the spec- Dxy ∼ 0.01 a.u. using parameters consistent with ref. 47; we also tral red edge feature of vegetation occurring at the same wave- assume ηxy ∼ ηEM and hvi ∼ hvEM i. With these choices, we find length, and (iii) the existence of distinctive homochirality. How- Pxy /PEM ≈ 20 and τxy /τEM ≈ 0.007, which equal the character- ever, we predict that some of these observations may only fall istic values obtained for low-mass M-dwarf planetary systems. within the capabilities of future telescopes, such as the Large Thus, systems in the circumplanetary HZ are con- UV/Optical/Infrared Surveyor (https://asd.gsfc.nasa.gov/luvoir/). ducive to panspermia, with all other things held equal. This result Lastly, we extended our discussion beyond that of the also implies a greater degree of biodiversity and a higher number TRAPPIST-1 system and presented other scenarios where of moons seeded by panspermia as per our earlier arguments. panspermia and hence, abiogenesis are more likely than in the Brown dwarfs. As seen from Eq. 3, the capture probability has solar system. We identified exoplanetary systems orbiting lower- −2/3 an M? dependence. If we assume that there exist multiple mass M dwarfs (and perhaps, brown dwarfs) and exomoons habitable planets around a , the low value of M? rel- around Jovian-sized planets as potential candidates that favor ative to the could, theoretically speaking, enhance the prob- panspermia. ability of panspermia. However, the HZ around brown dwarfs It seems likely that exoplanetary systems akin to TRAPPIST-1, migrates inward over time (49), thereby diminishing the chances with multiple exoplanets closely clustered in the HZ, will be dis- for abiogenesis and panspermia to occur. covered in the future. We anticipate that our work will be appli- cable to these exotic worlds vis-`a-vis the greater relative proba- Discussion and Conclusions bility of panspermia and abiogenesis on them. In this paper, we addressed the important question of whether life ACKNOWLEDGMENTS. We thank James Benford, Sebastiaan Krijt, Amaury can be transferred via rocks (lithopanspermia) in the TRAPPIST- Triaud, and Ed Turner for their helpful comments regarding the manuscript. 1 system. By formulating a simple model for lithopanspermia, This work was partially supported by a grant from the Breakthrough Prize we showed that its likelihood is orders of magnitude higher Foundation for the Starshot Initiative.

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