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.