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The idea that biological material—and even living organisms—can be exchanged between planetary systems is more than one century old, but it has not been part of the mainstream discussion in astrobiology for long [2.28, 2.43]. Historically, skepticism on the early proposals of panspermia, put forward at the beginning of the 20th century [2.3], was at least in part motivated by an incorrect understanding of planetary formation mechanisms, which was dominated by the “catastrophic” theories of Buffon, Chamberlin, Moulton, and Jeans [2.9]. However, even many decades after such theories were abandoned, panspermia failed to regain a place in the scientific discourse.

This has changed recently, at least in the version of panspermia—more appropriately called “lithopanspermia”—which posits that life can travel across space carried by meteoroids and other minor bodies. There is now well-established evidence that rock fragments have indeed been exchanged between nearby planets in the Solar System, such as Mars and the Earth [2.32]. Available data on the survivability of radio-tolerant organisms in deep space, as well as experimental tests on hypervelocity impacts, make it conceivable that extremophiles trapped in rocks can be expelled by an inhabited planet and reach other locations unharmed [2.18, 2.19, 2.33]. This has led to speculation that nearby planets in the Solar System could have cross-contaminated in the past [2.30] and that other, more densely packed planetary systems, such as the one around TRAPPIST-1, might be even more conducive to the accidental spreading of life from one habitable location to another [2.20, 2.23]. The possible occurrence of panspermia within the Solar System would have obvious direct consequences for the problem of the origin of life on Earth.

Enlarging the scope to galactic scale, the observation of the first interstellar asteroid visiting the Solar System [2.27] has confirmed that the exchange of material between stellar systems is feasible. In light of this, the idea that life can be disseminated by natural processes over interstellar distances cannot be dismissed. In this short chapter, I will argue that the mere possibility of interstellar panspermia should be given careful consideration, as it would have relevant consequences on the assessment of galactic habitability and on the interpretation of future exoplanet observations.