A pair of Harvard astrophysicists have proposed a wild theory of how life might have spread through the universe

Millions or billions of years ago, back when the solar system was more crowded, a giant comet grazed the outer reaches of our atmosphere. It was moving fast, several tens of miles above the Earth’s surface — too high to burn up as a fireball, but low enough that the atmosphere slowed it down a little bit. Extremely hardy microbes were floating up there in its path, and some of those bugs survived the collision with the ball of ice. These microbes ended up embedded deep within the comet’s porous surface, protected from the radiation of deep space as the comet rocketed away from Earth and eventually out of the solar system entirely. Tens of thousands, maybe millions, of years passed before the comet ended up in another solar system with habitable planets. Eventually, the object crashed into one of those planets, deposited the microbes — a few of them still living — and set up a new outpost for earthly life in the universe.

You could call it “interstellar panspermia,” the seeding of distant star systems with exported life.

We have no idea whether this ever actually happened –.and there’s a mountain of reasons to be skeptical. But in a new paper, Amir Siraj and Avi Loeb, both astrophysicists at Harvard University, argue that at least the first part of this story — the depositing of the microbes into a comet that gets ejected from the solar system — should have happened between one and a few dozen times in Earth’s history. Siraj told Live Science that although a lot more work needs to be done to back up the finding, it should be taken seriously — and that the paper may have been, if anything, too conservative in its estimate of the number of life-exporting events.

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