Radial Variation of Panspermia Probability in Milky Way–like Galaxies and Its Limited Dominance over Prebiotic Evolution

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Gobat, Hong, Snaith, and Hong show that in Milky Way–like galaxies the probability of interstellar transfer of organic materials between stars hosting habitable planets varies by orders of magnitude from the inner disk to the outer disk, and because only a small fraction of stars reach high transfer probabilities, panspermia is far less likely to dominate life's origin than simple probability would suggest.

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Panspermia in a Milky Way-like Galaxy

We study the process of panspermia in Milky Way-like galaxies by modeling the probability of successful travel of organic compounds between stars harboring potentially habitable planets. To this end, we apply the modified habitability recipe of Gobat & Hong to a model galaxy from the McMaster Unbiased Galaxy Simulations suite of zoom-in cosmological simulations. We find that, unlike habitability, which only occupies a narrow dynamic range over the entire galaxy, the panspermia probability can vary by orders of magnitude between the inner (R, b = 1-4 kpc) and outer disk. However, only a small fraction of star particles have very large values for the panspermia probability and, consequently, the fraction of star particles where the panspermia process is more effective than prebiotic evolution is much lower than from naïve expectations based on the ratio between the panspermia probability and natural habitability.