https://arxiv.org/pdf/1703.00878.pdf Enhanced interplanetary panspermia in the TRAPPIST-1 system: I. INTRODUCTION The field of exoplanetary research has witnessed remarkable advances in the past two decades, with the total number of discovered exoplanets now numbering in the thousands . This has been accompanied by a better understanding of the factors that make a planet habitable, i.e. capable of supporting life . It is now wellknown that there exist ∼ 1010 habitable planets in the Milky-Way, many of which orbit M-dwarfs . Planets in the habitable zone (HZ) - the region theoretically capable of supporting liquid water - of M-dwarfs have been extensively studied, as they are comparatively easier to detect and analyze . The search for exoplanets around nearby low-mass stars has witnessed two remarkable advances over the past year, namely (i) the discovery of Proxima Centauri b, the nearest exoplanet to the Solar system , and (ii) the discovery of seven planets transiting the ultracool dwarf star TRAPPIST-1 . The latter is all the more remarkable since three of the seven planets reside within the HZ, and each of them has a mass and radius that is nearly equal to that of the Earth . Hence, the TRAPPIST-1 transiting system represents a unique opportunity for carrying out further observations to determine whether these planets possess atmospheres and, perhaps, even biosignatures . If conditions favourable for the origin of life (abiogenesis) exist on one of the TRAPPIST-1 planets, this raises an immediate question with profound consequences: could life spread from one planet to another (panspermia) through the transfer of rocky material? Panspermia has been widely investigated in our own Solar system as a potential mechanism for transporting life to, or from, the Earth [9–13]. The planets in the HZ of the TRAPPIST-1 system are separated only by ∼ 0.01 AU, tens of times less than the distance between Earth and Mars. Thus, one would be inclined to hypothesize that panspermia would be enhanced in this system. Here, we explore this possibility by proposing a simple quantitative model for panspermia within the TRAPPIST-1 system. We show that the much higher probability of panspermia leads to a correspondingly significant increase in the probability of abiogenesis. We also draw upon models from theoretical ecology to support our findings, and extend our analysis to other planetary systems. I. DISCUSSION AND CONCLUSIONS In this paper, we addressed the important question of whether life can be transferred via rocks (lithopanspermia) in the TRAPPIST-1 system. By formulating a simple model for lithopanspermia, we demonstrated that its likelihood is orders of magnitude higher than the Earthto-Mars value because of the higher capture probability per impact event and the much shorter transit timescales involved. We explored the implications of panspermia for the origin of life in the TRAPPIST-1 system by drawing upon the quantitative approach proposed recently by Scharf and Cronin . If panspermia (or pseudo-panspermia) is an effective mechanism, it leads to a significant boost in the probability of abiogenesis because each panspermia event can transfer a modest number of molecular ‘species’, and the cumulative probability scales exponentially in the best-case scenario. Thus, it seems reasonable to conclude that the chances for abiogenesis are higher in the TRAPPIST-1 system compared to the Solar system. We also benefited from the exhaustive field of theoretical ecology in substantiating our findings. By drawing upon the analogy with the theory of island biogeography, we argued that a large number of species could have ‘immigrated’ from one planet to another, thereby increasing the latter’s biodiversity. As known from studies on Earth, a higher biodiversity is correlated with greater stability , which bodes well for the multiple members of the TRAPPIST-1 system. We also utilized metapopulation ecology to conclude that the possibility of multiple planets being ‘occupied’ (i.e. bearing life) is higher than in the Solar system, given the considerably higher immigration rates. In order to observationally test the presence of life seeded by panspermia, we proposed a couple of general tests that can be undertaken in the future. We reasoned that a ‘smoking gun’ signature for panspermia may require the following criteria to be valid: the detection of (i) identical biosignature gases, (ii) the spectral “red edge” of vegetation occurring at the same wavelength, and (iii) distinctive homochirality. However, we predict that some of these observations may only fall within the capabilities of future telescopes, such as the Large UV/Optical/Infrared Surveyor (LUVOIR).2 Lastly, we extended our discussion beyond that of the TRAPPIST-1 system and presented other scenarios where panspermia, and hence abiogenesis, are more likely than in the Solar system. We identified exoplanetary systems orbiting lower-mass M-dwarfs (and perhaps brown dwarfs), and exomoons around Jovian-sized planets as potential candidates that favor panspermia. It seems likely that exoplanetary systems akin to TRAPPIST-1, with multiple exoplanets closely clustered in the habitable zone, will be discovered in the future. We anticipate that our work will be applicable to these exotic worlds, vis-à-vis the greater relative probability of panspermia and abiogenesis on them.