When NASA announced in February that not one, but seven Earth-like planets had been discovered in a solar system called Trappist-1, 39 light years away, the eyes of astronomers (and amateur space enthusiasts) lit up at the new possibility of finding life outside Earth.
All of the Trappist-1 system are rocky (like our Earth) and that they all revolve around an “ultracool” star. Although Trappist-1’s central star gives off lower temperatures than the sun, the planets in this new solar system are much closer to the star than Earth is to its star. Scientists suspect that at least three of these seven planets can therefore harbor life with a temperate climate and water. In addition, because the Trappist-1 planets are all relatively close to each other, the system has ideal conditions for panspermia—when life is displaced from one planet and starts thriving on the next.
Panspermia hasn’t actually been observed. But in theory, if a planet with life on it is hit by an asteroid, the asteroid would displace some rocks and send them hurling into space—potentially carrying bacteria on them, explains Avi Loeb, an astronomer at the Harvard-Smithsonian Center for Astrophysics. If these rocks with stow-away bacteria eventually made it to another planet where the conditions were right (warm enough, with water and gases like oxygen or methane), the bacteria could “immigrate” and colonize the new planet, says Loeb.
Some astronomers think (paywall) that panspermia happened on Earth. Their theory is that rock bits from Mars traveled to Earth with the smallest, most basic units of life hitching a ride. After millions of years of evolutionary fine-tuning on Earth, the seed of life from Mars finally flourished into life as we know it today. This would explain why even though life’s building blocks—chemicals that make up proteins—are relatively easy to assemble, scientists have never been able to get them to spontaneously build themselves here on Earth. Perhaps, then, there’s some vital ingredient missing from our planet that exists elsewhere in the universe, and life simply found itself here.
Mars is our closest neighbor, and it’s 33.9 million miles (54.6 million kilometers) away.”The journey [between] Earth and Mars could be millions of years,” says Loeb. Bacteria would have had to survive all that time in the freezing vacuum of space. Possible, but not probable.
Loeb and his colleague Manasvi Lingam took a look at the Trappist-1 planets and calculated the likelihood of panspermia occurring there. They took into account the average time it would take to get between planets—roughly 100 times less than the time it’d take to get from Earth to Mars—and the fact life could live on three planets instead of one and concluded (pdf) that planets spitting life back and forth on meteorites is 2,000 times more likely on Trappist-1 than it is in our own solar system.
Their paper, published on the open-access site ArXiv, has not yet been peer-reviewed, although Lingam and Loeb have submitted it for publication. “In a single planetary system, like Trappist-1, the interchange of bacterial life is almost inevitable,” Chandra Wickramasinghe, an astrobiologist at the University of Buckingham, told National Geographic. Wickramasinghe was one of the first astronomers to postulate the idea of panspermia in the 1970s.
“The way to think of it is like islands that are more closely spaced, like the Hawaiian [or] Galapagos [islands],” compared to the continents that are oceans apart, says Loeb. On Earth, animal species can make their way across waters to reach different islands within a system much better than they can move between separate continents. Trappist-1’s planets are like islands, whereas our solar system is made of planetary continents.
Of course, there’s still no evidence that life even exists on any of the Trappist-1 planets (and some scientists are skeptical). Even if there was life on one of the planets, getting from that to another would not have been an easy journey: “The poor organism would be fried twice and would be radiated by ultraviolet photons,” Brice-Olivier Demory, an astronomer at the University of Bern in Switzerland, told National Geographic.