The TRAPPIST-1 solar system, home to potentially habitable planets, wasn’t bombarded by rocks like early Earth

TRAPPIST-1 would be an ordinary star without the scientific interest generated by its seven planets.

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

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

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Around the star, seven Earth-sized exoplanets, designated by simple letters from TRAPPIST-1 b to h according to their distance from the star, orbit in a way that astronomers call “resonant”. The resonance means that although each planet takes a different time to complete an orbit, the pairs regularly end up 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 orbits, planet d 4 and planet e 2 orbits. And in the new research, scientists say this eerily regular orbital dance wouldn’t be possible if these planets were subjected to too much pounding by space rocks after they were born into the protoplanetary disk that surrounded the newly formed TRAPPIST-1 star there. about 7 billion years ago. since.

“We understood that after these planets formed, they were only bombarded by a very small amount of stuff,” said astrophysicist Sean Raymond of the University of Bordeaux in France and lead author of the study. in a press release. “That’s pretty cool. That’s interesting information when you think about other aspects of the planets in the system.”

The team of American and European researchers simulated the evolution of the TRAPPIST-1 system on a computer. Scientists were trying to find out how many “things” could hit these planets before their synchronized orbital dance was disrupted.

“We can’t say exactly how many things have hit one of these planets, but thanks to this special resonance pattern, we can put an upper limit on it,” Raymond said. “We can say, ‘It can’t have been more than that. And it turns out that upper limit is actually quite small.”

The model suggests that planets in the TRAPPIST-1 system must have formed very early and very quickly, about a tenth of the time 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 only a few million years after the formation of a new star. Computer models suggest that it’s the gravitational pull of this disk that propels the planets into orbital resonance, the researchers said. The impact of a large body, similar to the one that crossed young Earth some 4.5 billion years ago in a collision that formed the moon, would certainly have disrupted this synchronized orbital dance.

Scientists hope that understanding the intensity of bombardment by space rocks in the early stages of a planet’s life can help them understand the chemical makeup of the planet. In the case of Earth, many chemical elements – including life-giving water – would have been introduced by the impact of comets, asteroids and meteorites. The collision that created the moon alone would have delivered most of the planet’s current carbon and nitrogen, two essential prerequisites for life to exist.

Currently, scientists know very little about the chemical makeup of the TRAPPIST-1 worlds. Understanding the amount of space rock that hit them could improve those estimates.

“We now have certain constraints on the composition of these planets, such as how much water they can have,” said Andre Izidoro, an astrophysicist at Rice University in Houston and co-author of the paper, in the press release. “But we have very large error bars.”

But those 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 example, if one of these planets has a lot of water, say a mass fraction of 20%, the water must have been incorporated into the planets early, during the gas phase,” Izidoro said. “So you’ll have to figure out what kind of process could be bringing that water to this planet.”

Currently, scientists have limited tools to go much further. But new observatories like the James Webb Space Telescope, slated for launch in 2022 and the most powerful space telescope ever built, and the 2024 completion of the Extremely Large Telescope at the European Southern Observatory, could highlight place the different pieces of the puzzle. .

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

At the end of this puzzle, scientists will hopefully know if there could be life on one of these distant lands.

The research is described in an article published Thursday, November 25 in the journal Nature Astronomy.

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Arline J. Mercier