TRAPPIST-1 solar system that's home to potentially habitable planets was not bombarded by rocks like early Earth

TRAPPIST-1 would be an unremarkable star if not for the scientific interest generated by its seven planets. 

Astronomers first spotted the new worlds, at least three of which might be habitable, in 2016. Now, a new study suggests that the way the TRAPPIST-1 planets orbit might reveal clues about their evolution and how frequently space rocks smashed into them in their formative years. 

Located some 40 light-years away from the sun in the constellation Aquarius, TRAPPIST-1 is a cool dim star called a red dwarf, the type that is the most common in our galaxy, the Milky Way. 

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Around the star, seven exoplanets about the size of Earth, referred to by simple letters from TRAPPIST-1 b to h based on their distance from the star, orbit in a way that astronomers call 'resonant.' Resonance means that although each planet takes a different amount of time to complete one orbit, pairs regularly meet again at the same starting point. 

For example, for every 8 orbits completed by planet TRAPPIST-1 b, which is closest to the star, planet c makes 5 laps, planet d 4 and planet e 2 orbits. And in the new research, scientists argue that this strangely regular orbital dance would not be possible if those planets were subjected to too much hammering by space rocks after their birth in the protoplanetary disk that surrounded the newly formed TRAPPIST-1 star some 7 billion years ago. 

"We figured out that after these planets formed, they weren't bombarded by more than a very small amount of stuff," astrophysicist Sean Raymond of the University of Bordeaux in France and the lead author of the study said in a statement. "That's kind of cool. It's interesting information when we're thinking about other aspects of the planets in the system."

The team of U.S. and European researchers simulated the evolution of the TRAPPIST-1 system on a computer. The scientists were trying to find out how much 'stuff' could hit those planets before their synchronized orbital dance would get disrupted.

"We can't say exactly how much stuff bashed into any of these planets, but because of this special resonant configuration, we can put an upper limit on it," Raymond said. "We can say, 'It can't have been more than this.' And it turns out that that upper limit is actually fairly small."

The model suggests that planets in the TRAPPIST-1 system must have formed very early and very fast, in about one-tenth of the time that it took our Earth to form, the scientists said in the statement. 

By the time the protoplanetary disk around TRAPPIST-1 disappeared, these planets were already orbiting close to their parent star. Scientists believe that protoplanetary disks, filled with gas and dust, exist for only a few millions of years after the formation of a new star. The computer models suggest that it is the gravitational force of this disk that kicks the planets into the orbital resonance, the researchers said. An impact of a large body, similar to the one that crossed paths with the young Earth some 4.5 billion years ago in a collision that formed the moon, would definitely have disrupted this synchronized orbital dance. 

Scientists hope that understanding the intensity of the bombardment by space rocks in the early stages of a planet's life may help them understand the planet's chemical composition. In the case of Earth, many chemical elements — including life-giving water — are believed to have been introduced by impacting comets, asteroids and meteorites. On its own, the collision that created the moon is believed to have delivered the bulk of the planet's current carbon and nitrogen, both essential prerequisites for the existence of life.

Currently, scientists know very little about the chemical composition of the TRAPPIST-1 worlds. Understanding the amount of space rocks that smashed into them might improve these estimates.

"We have some constraints today on the composition of these planets, like how much water they can have," Andre Izidoro, an astrophysicist at Rice University in Houston and a co-author of the paper, said in the statement. "But we have very big error bars."

But these planets may have already formed from matter containing more hydrogen and naturally have more water than Earth, even without all those incoming comets and space rocks. 

"For instance, if one of these planets has a lot of water, let's say 20% mass fraction, the water must have been incorporated into the planets early, during the gaseous phase," Izidoro said. "So you will have to understand what kind of process could bring this water to this planet."

Right now, scientists have limited tools to get much farther. But new observatories like the James Webb Space Telescope, scheduled to begin operations in 2022 and the most powerful space telescope ever built, and the 2024 completion of the Extremely Large Telescope at the European Southern Observatory, may bring the various pieces of the puzzle into place. 

"For the TRAPPIST-1 system, we have these Earth-mass planets that formed early," Rajdeep Dasgupta, a planetary scientist at Rice University and another co-author of the paper, said the statement. "So one potential difference, compared to the Earth's formation, is that they could have, from the beginning, some hydrogen atmosphere and have never experienced a late giant impact. And this might change a lot of the evolution in terms of the interior of the planet, outgassing, volatile loss and other things that have implications for habitability."

At the end of this puzzle, scientists will hopefully know whether there might be life on any of those distant Earths. 

The research is described in a paper published on Thursday (Nov. 25) in the journal Nature Astronomy.

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