There's a good chance life from Earth has already spread to other planets, scientists say - and it could have happened dozens of times.
But it wasn't humans going boldly where they haven't before - but microbes.
"Long-period comets and interstellar objects could export life from Earth by collecting microbes from the atmosphere and receiving a gravitational slingshot effect from the Earth," Harvard University astronomers Amir Siraj and Abraham Loeb write in a new paper.
Alternatively, asteroids that hit with enough power could potentially eject matter into space, which could eventually get sucked in by another planet's gravity.
"Impacts on the surface of a planet can launch debris at above the escape speed of the planet, thereby allowing debris spread throughout the planetary system and constituting a plausible mechanism for exchanging life between planets orbiting the same star."
More than 200 meteorites that have fallen to Earth have been confirmed as coming from Mars, for example. None carried life, but that's because there probably isn't any on Mars
There is on Earth however.
"Life in the Earth's atmosphere has been detected up to an altitude of 77km, constituting a reservoir of microbes that objects grazing the atmosphere could draw from," Siraj and Loeb write.
Spreading life from one planet to another is called panspermia.
"Long-period comets... can easily be ejected from the solar system by gravitational interactions with planets due to their low gravitational binding energies and planet-crossing orbits. This makes them ideal, in principle, for both picking up life from Earth and exporting it out of from the solar system."
We have no way of knowing yet whether this has happened yet. An Israeli moon probe that crashed earlier this year was carrying tardigrades, a nearly indestructible creature that some believe could survive on the inhospitable lunar surface.
The two astronomers crunched the numbers, and estimate life has been exported from Earth at least once, up to possibly 50 times.
That number dramatically increases to 100,000 if there are microbes living 100km above us - the generally accepted border between atmosphere and space.
"The discovery of life above 100 km in the atmosphere would be a very encouraging sign for the feasibility of interstellar panspermia."
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At the same speed as NASA's New Horizons spacecraft, which took nine years to reach Pluto, it would take Earth microbes 78,000 years to reach the closest star to ours, Alpha Centauri. Life has existed on Earth for billions of years - plenty of time.
The full paper, yet to be peer-reviewed, can be read online.
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