An artist's conception of TRAPPIST-1. (Credit: ESO) If we detect alien life on a planet in the TRAPPIST solar system, there’s a chance they’ve already spread to one or more of the other six planets orbiting this ultra-cool, ultra-tiny star some 40 light-years away. In May 2016, scientists made headlines when they discovered three, Earth-size, rocky planets (in February scientists announced they found four more) orbiting a red dwarf star that’s roughly the size of Jupiter. Planets in this system huddle around their home star in tightly packed orbits—TRAPPIST-1b circles its star once per day. And since red dwarfs are cooler than our sun, it’s thought that several of these planets could be habitable, despite close proximity to their star. This tightly packed solar system begs a tantalizing question: Are these planets exchanging life via space rocks? There are theories that life on Earth may have come from Martian rocks. And if that’s the case, Harvard University researchers Mansavi Ligam and Abraham Loeb argue that the odds of the same thing occurring in the TRAPPIST-1 system are magnitudes higher.
The panspermia hypothesis, credited to Big Bang skeptic Fred Hoyle, underlies all of this life-exchanging talk. Basically, it states that asteroids, comets and meteorites laden with microbes crash into virgin planets, and the surviving microbes sow new life. A few years ago, scientist Steven Benner threw out a provocative idea during a geochemistry conference: Perhaps life on Earth began with organisms clinging to rocks from Mars. Billions of years ago, conditions on Earth were far too hellish for life to take root. But perhaps a major impact sent Martian rocks flying into space on a course toward Earth. The rest, as they say, is history. It’s still a theory, but there are organisms that can survive the ravages of space—ahem, tardigrades. In a study published Tuesday in Proceedings of the National Academy of Sciences, Ligam and Loeb built a mathematical model, borrowing from theoretical ecology, to see how the panspermia hypothesis might play out in the TRAPPIST-1 system. Based on their analysis, if Mars could send life to Earth, it’s far more likely that the same thing would happen at TRAPPIST-1. If you boil down the math, their results are intuitive: The distance between two planets in the TRAPPIST-1 system is tens of times less than the distance between Earth and Mars, which increases the efficiency of material exchanges. Thus, there’s a higher chance TRAPPIST planets are throwing life-seeds back and forth.
An Exciting Theory, But a Theory Nonetheless
Of course, there’s no telling what kind of organisms, if any, would be hitching a ride on TRAPPIST-1 rocks, so it isn’t possible to know if TRAPPIST organisms would survive the trip. Their model is also very simplified, because we just don’t know much about TRAPPIST-1. When did the planets settle into their orbits? Are there a sufficient number of meteorites slamming into planets to launch life-seeding rocks through space (a.k.a. cause spallation)? These are just a few Ligam and Loeb raise in their work. Looking forward, we first need to determine if life indeed exists in TRAPPIST-1. Telescopes that will be launched in the coming years will be able to probe planets’ atmospheres in the system to detect biosignatures associated with life as we know it on Earth. Scientists could also search for “red edge” spectral signals that hint at the presence of vegetation. If life exists, researchers could then theoretically verify the panspermia hypothesis, as well. If “red edge” signals are the same on various TRAPPIST-1 planets, that would indicate life sprung forth from a singular source. Life as we know it relies on “left-handed” amino acids and right-handed sugars, for example. If organisms on TRAPPIST-1 planets all adhere to the same chirality rules, that’s a good sign that life hopped from planet to planet. But if the rules were different on each planet, it would be an indication that life evolved in isolation. Extraterrestrial life will always capture even the most casual stargazer’s imagination, and these theories about TRAPPIST-1 are certainly titillating. Unfortunately, patience will be the mantra for years to come, as we’ll need to wait for new, technologically advanced telescopes to come online before putting hypotheses to the test.