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Journal Club: Potential life could have spread with relative ease amongst newly-discovered group of seven exoplanets

If the TRAPPIST-1 system harbors life, models suggest there's a high probability that it could hop between planets. Image credit: Shutterstock/Cylonphoto

If the TRAPPIST-1 system ever harbored life, models suggest a high probability that life could hop between its planets.
Image credit: Shutterstock/Cylonphoto

The odds of life spreading between the worlds of the newly-discovered seven-planet TRAPPIST-1 system are up to 1,000 times greater than in our own solar system. That’s the conclusion of a new analysis posted March 2 to the arXiv, an online repository of scientific papers.

The idea that simple organisms could accidentally travel between planets is known as panspermia. “Imagine one planet has life, and then you have a meteorite or asteroid impact that ejects some rock into space,” says mathematical physicist and lead author Manasvi Lingam of Harvard University in Cambridge, Massachusetts. “If these rocks are captured by a different planet, they could spawn life there.”

Panspermia has ancient origins, dating back to the Greek philosopher Anaxagoras in 500 B.C., who believed that cosmic “life seeds” brought organisms to Earth. The first detailed scientific treatment came in 1908, when Swedish chemist Svante Arrhenius wrote a book arguing that bacterial spores could have bounded from one planet to another. A modern-day version of the hypothesis briefly gained popularity in 1996 when a Martian meteorite found in Antarctica seemed to bear signs of microbial fossils—a claim ultimately rejected by most researchers

Announced last month, TRAPPIST-1 is the newest exoplanetary system to spur excitement among astronomers. Orbiting the cool red dwarf star are seven rocky worlds ranging in size from slightly larger than Mars to a bit bigger than Earth. Three of them are in the star’s habitable zone, meaning liquid water could theoretically exist on their surface—though because red dwarf stars produce more flares and harsh radiation than sun-like stars, some scientists think life is unlikely on the surrounding planets. Others disagree. (See our news feature for further debate about the habitable zone.)

Because the star TRAPPIST-1 is smaller and dimmer than our sun, its habitable zone encompasses a closer-in region—the three potentially habitable planets take only six, nine, and 12 days, respectively, to go around their parent star. The planets are much closer to each other as well. The two inner potentially habitable planets, for instance, are 30 times closer than Venus is to Earth, while the two farther ones are 65 times closer than Earth and Mars.

Applying models from the field of ecology, the researchers presuppose that the microbial transfer is similar to species jumping between islands. If the islands are closer together, a larger number of species could potentially travel between them. Hence, by analogy, it’s possible that many species might have made it from one planet in the TRAPPIST-1 system to another. “Of course, this is only an analogy and not an exact match,” says Lingam. “But we know that greater biodiversity in any ecosystem also implies greater stability. If you have more species being transferred to the new planet, there’s less of a chance that they’ll die.”

Many unknowns remain, including just how long microbes can survive in space, whether or not they would be viable after traversing an atmosphere and crash-landing on a surface, and if the habitat of one world is similar enough to its sibling planets to allow such organisms to flourish. The TRAPPIST-1 system is well-positioned to answer at least some of these; its exoplanets all eclipse in front of their parent star, meaning that telescopes here on Earth can capture starlight filtering through any potential planetary atmospheres. Such observations could reveal signs of vegetation—plants on Earth preferentially absorb red light, for example, giving our planet a “red edge” as seen from far away—or other biosignature molecules. If the TRAPPIST-1 worlds display similar signals, it could imply that panspermia occurred.

“What’s exciting to me more than anything is that this hypothesis is testable. This is no longer in the realm of theory,” says astronomer Rory Barnes of the University of Washington in Seattle, who was not involved in the work. Current and upcoming telescopes might be up the challenge, though it will likely take a large new space-based observatory such as the proposed Large UV/Optical/Infrared Surveyor (LUVOIR), which wouldn’t be launched until the 2030s, to answer such questions in detail.

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