A rock knocked off one planet by an asteroid impact hurtles through space and lands on another. If microbial spores are hiding inside, they might survive the cold, the radiation, and the rough landing. That's panspermia — the idea that life travels between worlds. But imagining such a journey is one thing; calculating every step is quite another.

The authors of the new study combined all stages into a single mathematical model, from the ejection of rock from the donor planet to the survival of microbes after impact. It turns out that early Mars offered the gentlest conditions. It was warm and wet, with low gravity, and its rocks did indeed reach Earth. Moreover, rare fast transits lasting hundreds or thousands of years give microbes a chance to survive under the rock's protection.

Beyond the solar system, however, panspermia doesn't work. For a rock from another star system to be gently captured by Earth, it would need to arrive at a very low speed, and even the hardiest spores cannot survive millions of years in transit. Even accounting for the Sun's birth in a stellar cluster, the expected number of life-bearing events is a tiny fraction of one. So a terrestrial origin of life remains the most reliable explanation, and 'soft' panspermia, delivering only chemical ingredients rather than ready-made microbes, looks far more plausible.