Panspermia is the hypothesis that life exists throughout the universe and is spread between worlds by space dust, meteoroids, asteroids, and comets. Rather than life arising independently on Earth, panspermia suggests that the seeds of life, or life itself, may have arrived here from elsewhere.
The notion is ancient, traceable to Greek philosophers, but it became a scientific hypothesis only as we learned how hardy some organisms can be and how readily material moves between planets. In its most modest form, it proposes that microbes or the chemical building blocks of life could ride through space inside rocks blasted off one planet by an impact, eventually landing on another.
A key support for the idea is the discovery that some living things can endure the brutal conditions of space.
Certain microbes, such as the bacterium Deinococcus radiodurans, and tiny animals called tardigrades, can survive intense radiation, the vacuum of space, and temperatures near absolute zero, at least for a time.
Several other findings make panspermia at least plausible. Complex organic molecules, including amino acids, the components of proteins, have been found in meteorites and in the clouds of gas and dust between the stars. The raw chemistry of life appears to be widespread in the cosmos rather than unique to Earth.
Crucially, rocks really are exchanged between planets. Meteorites that originated on Mars, thrown into space by ancient impacts, have been recovered on Earth. This shows that the basic delivery mechanism panspermia requires, material traveling from one world to another, genuinely occurs within our own Solar System.
For all this, panspermia is still very much a hypothesis. No living organism has ever been shown to have come from beyond Earth. Surviving a brief exposure in a laboratory is very different from enduring a journey of millions of years across interstellar space, followed by a fiery landing.
Even if life did arrive from space, panspermia would only move the question of life's ultimate origin to some other world, not answer it. The deeper puzzle of how non living matter first became living would still remain, simply relocated to a different address in the cosmos.
Panspermia remains an intriguing, partly testable idea. The search for life on Mars, the study of organic molecules in comets and meteorites, and experiments exposing microbes to space conditions may all, over time, strengthen or weaken the case.
