Back in February, NASA announced the discovery of seven Earth-sized exoplanets orbiting the nearby red dwarf star, TRAPPIST-1. With three of those planets orbiting within the star's Habitable Zone (HZ), the system is one of our best bets for finding life beyond Earth, and new research from the University of Chicago suggests that if it is there, life could jump between the tightly-packed planets in a matter of decades.
At a distance of 40 light-years away, the TRAPPIST-1 system's planets aren't our closest possible homes-away-from-home – that honor goes to Proxima b, a galactic stone's throw away at just four light-years. But what makes TRAPPIST-1 such an attractive prospect for extraterrestrial life is the fact that all seven planets are in extremely close proximity to each other, so if life arises on one planet, it could spread to the others relatively quickly.
"Frequent material exchange between adjacent planets in the tightly packed TRAPPIST-1 system appears likely," says Sebastiaan Krijt, lead author of the study. "If any of those materials contained life, it's possible they could inoculate another planet with life."
While Earth is the only planet we know for sure is home to life, it didn't necessarily start here. The seeds of life, in the form of microscopic organisms, may have been brought to our planet by asteroids or comets – a hypothesis known as panspermia. On the other hand there's lithopanspermia, the idea that chunks of rock carrying tiny organisms could be thrown into space by these cosmic collisions, spreading that life to other planets.
But for this to work, a few factors need to be considered. The pieces of rock ejected into space would need to be large enough to protect the organisms from cosmic radiation, and even then, the journey to their new home would need to be relatively short, to keep the space travellers alive. And the speed at which the rocks are flung into space also needs to be just right: too fast and the lifeforms wouldn't survive the trip, too slow and the rocks would fall back to the planet's surface.
Running a series of simulations on these events, the researchers set out to determine how likely the scenario could be in the TRAPPIST system. The team found that rocks large enough to protect any hitchhiking organisms during space travel and re-entry would also tend to leave their home planet at a speed just above the minimum required to break free. Given how close together the system's planets are, the researchers concluded that this process could take place pretty quickly, seeding life from one world to another in as little as 10 years.
"Given that tightly packed planetary systems are being detected more frequently, this research will make us rethink what we expect to find in terms of habitable planets and the transfer of life — not only in the TRAPPIST-1 system, but elsewhere," says Fred Ciesla, co-author of the study. "We should be thinking in terms of systems of planets as a whole, and how they interact, rather than in terms of individual planets."
The research was published in Astrophysical Journal Letters.
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