One episode of Cosmos: A Spacetime Odyssey describes a mechanism by which microbes could travel between planetary systems. As a star's orbit takes it around the galaxy, it occasionally passes through dense interstellar clouds. Rocks thrown into space at that time from the surface of a microbe-harbouring planet could carry life to temporarily nearby planets of another planetary system, before cosmic radiation would sterilise such rocks.

It's an interesting idea for TV; but scientifically, how plausible is this mechanism? My main concern is whether the average number of "infected" planetary systems per passage through an interstellar cloud is estimable as non-negligible.

Note: the use of "panspermia" in the title refers only to the question of whether an occasional interstellar transfer of microbes is tenable, not whether Earth's life originated outside this planet. As I understand it, the mechanism described above is lithopanspermia.

  • $\begingroup$ Unfortunately I think this may fall neatly between the expertise of Space.SE and here, but I think the key variable here is the time for which such propagules would remain viable. I suspect the sheer time and distance likely to separate compatible planets would exceed this period by a very significant amount. However, if we can show extensive 'cross-pollination' of planets within our own system then that gives us better data to estimate this likelihood. $\endgroup$
    – arboviral
    May 4, 2018 at 10:32

1 Answer 1


The theory is staggeringly improbable. However, so is life. The probability of life emerging from non-life is the basis of the misguided idea of intelligent design (a thinly veiled repackaging of creationism). Bear with me: the probability of life on Earth is 1. It is here, and this is the only observation of Earth we have. But the thought of complex life appearing out of some molecular soup by chance (abiogenesis) is also staggeringly improbable. But as I said, life is here. How did it get here? There are only three possibilities: Abiogenesis, divine creation, or from panspermia (which kicks the can down the road, because it had to have begun somewhere). God is no explanation at all, because even a mystical imaginary creator would have needed some mechanism besides waving a finger. So in terms of probability, both abiogenesis and panspermia would be astronomically improbable, but since life is here (probability of life is 1), then one of these two is the source of it. We have no hard evidence to choose between them.

  • $\begingroup$ You've missed the point of the question. I'm not asking whether panspermia should replace terran abiogenesis as an explanation for life on Earth (it probably shouldn't); I'm asking whether it's plausible to claim planetary systems "contaminate" each other in the manner Tyson discussed. $\endgroup$
    – J.G.
    May 4, 2018 at 10:19
  • $\begingroup$ No, I have not missed the point. Since it is one of only two possibilities, AND we have no evidence between either, then it is plausible, because something happened here. My point is, that there is no way to answer questions that estimate probabilities that are extremely unlikely, and have no data to directly support them. Thus Tyson's discussion is plausible. $\endgroup$
    – Karl Kjer
    May 4, 2018 at 11:00

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