Earlier this year, NASA announced the discovery of an ultracool dwarf star surrounded by seven Earth-size planets. The star, named Trappist-1, and its planetary system quickly became famous among people dedicated to searching for life—microbial, plant, intelligent—elsewhere in the universe. Now, a new mathematical model by researchers at Harvard University hints at just how likely there is life in that system.
Astronomers have counted thousands of previously unknown exoplanets—that is, planets beyond our solar system—in recent years, made possible most recently by the Kepler Space Telescope. Trappist-1, named for The Transiting Planets and Planetesimals Small Telescope, in Chile, is just one-eighth the size of our own sun, about 40 light years away from us and located inside the constellation of Aquarius. The seven planets orbiting Trappist-1 are relatively Earth-sized and relatively close to one another. And three of these planets orbit in what is called the "habitable zone," a distance from their star that would allow liquid water to pool on their surface.
For all these reasons, the Trappist-1 solar system has generated a great deal of excitement when it comes to the prospect of finding life outside our solar system. Aside from Proxima Centauri b, the exoplanet nearest us—a mere four light years away—Trappist-1 tops the list of likeliest spots for a so-called second genesis, an arising of life elsewhere. Scientists from the University of Chicago theorize that a life-spurring hit from a comet or asteroid could feasibly lead to transfer of that life among the system's planets.
The mathematical model developed by astrophysicists Abraham Loeb and Manasvi Langam, both of the Harvard-Smithsonian Center for Astrophysics, raises the stakes further. They created a calculation for estimating "panspermia," the spread of life among the planets orbiting Trappist-1. Because the planets are so close together, the odds that any life, be it microbial or mammalian, could transfer from one globe to another are high. Panspermia in a system like Trappist-1, says Langam, "has the potential advantage of seeding multiple planets in the same system with life." That this mechanism could increase prospects for life in the universe is a "motivating factor" for the search for life, he says.
Their model shows that the likelihood of life transferring among these rocky planets ("lithopanspermia") is "orders of magnitude higher" than the likelihood of the same event occurring between Earth and Mars, the authors write. They also conclude that the chances of life spontaneously arising in the Trappist-1 system are higher than in our solar system. Their findings further led them to assert that more exoplanetary systems with multiple planets huddled close together within a habitable zone will be discovered.
The possibility of finding life elsewhere is still rife with unknowns, including the very existence of microbes and their likelihood of survival, says Langam. The present model accounts for only "factors that could be addressed with some certainty," he says. Finally, the possibility of panspermia depends entirely on life originating elsewhere. Whether such an event has occurred "is a question that we cannot hope to address at this stage," says Langam. The model shows how favorable the Trappist-1 system is to the spread of life but not to its arising in the first place.
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