Other worlds are a stone’s throw away in the TRAPPIST-1 system ESO/M. Kornmesser

The newly discovered planets of the TRAPPIST-1 system could be a playground for rock-riding microbes.

Three of the small, dim star’s seven planets orbit firmly within its habitable zone – the region with the right temperature to retain liquid water, thought to be a requisite for life. They keep close to each other, only a few times the distance between Earth and the moon, looming large in one another’s sky.

At such short distances, when a meteorite hits the surface of one of the planets, the resulting debris could make its way between them.


If bacteria or other forms of life stowed away on a piece of debris, they could hitch-hike between worlds in a process called panspermia. Some scientists believe life on Earth may have started this way, as microbial stowaways from Mars.

Now, Manasvi Lingam and Avi Loeb at Harvard University have determined that this sort of transfer is 1000 times more likely to occur between the TRAPPIST-1 planets than between Earth and Mars.

More than flinging rocks

Bringing life to another planet is more complicated than just flinging rocks. Any stowaways would have to survive the vacuum and harsh radiation in space, which few known organisms can do. But the quick commute between TRAPPIST-1’s habitable planets – about 100 times quicker than between Earth and Mars – should help.

“Because these distances are so close, a lot more different kinds of species, microbial or otherwise, could migrate from one planet to another,” says Lingam.

This means that if there is life on one of the TRAPPIST-1 planets, there is probably life on all three in the habitable zone. The team compared the TRAPPIST system to a series of islands, using mathematical methods from island ecology to describe migration and extinction between them.

“It would not be surprising to find the same forms of life on all three habitable planets near TRAPPIST-1,” Loeb says.

But some biologists reject this metaphor. “This work is interesting, but no, planets are not islands, even if they are close,” says Valeria Souza at the National Autonomous University of Mexico.

Even on Earth, she says, it is difficult for species to migrate between islands, and evolution would take them all down a different route once they arrived.

Three dice rolls

The idea of panspermia itself is also uncertain. If it doesn’t happen in our solar system, the fact that it is 1000 times more likely at TRAPPIST-1 may not mean much. “To quantify panspermia is an interesting idea – but whether it happens in the first place is something that we haven’t determined yet,” says Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell University in Ithaca, New York.

But panspermia might not have to transfer living organisms to help life grow. If molecular building blocks, like water or simple proteins, can travel between the planets, that could also improve life’s chances on the three neighboring worlds.

If there is life on any of TRAPPIST-1’s habitable planets, especially if it is on more than one, it would be an extraordinary laboratory to study how life begins and evolves.

“The fascinating story in science really would be that life evolved on all of these planets individually and you could see the diversity of what nature could come up with,” says Kaltenegger.

“We can roll the dice three times in the TRAPPIST-1 system and have a higher chance of success,” says Loeb.

Journal reference: arXiv, DOI: 1703.00878