Orbital harmony of TRAPPIST-1 planets could only survive limited early bombardment

This artist’s concept depicts the seven rocky exoplanets of the TRAPPIST-1 system, located 40 light years from Earth. Credit: NASA and JPL / Caltech

The fragile orbits of seven exoplanets limit the late arrival of water.

Seven Earth-sized planets orbit the star TRAPPIST-1 in near perfect harmony, and American and European researchers used this harmony to determine how well the planets might have withstood physical abuse as children.

“Once the rocky planets are formed, things collide with them,” said astrophysicist Sean Raymond of the University of Bordeaux in France. “It’s called bombardment, or late accretion, and we care, in part, because these impacts can be an important source of water and life-promoting volatiles.”

In a study available online today (25 November 2021) in Nature astronomy, Raymond and his colleagues at Rice University ">NasaThe project-funded CLEVER Planets project and seven other institutions used a computer model of the bombardment phase of planetary formation in TRAPPIST-1 to explore the impacts its planets could have endured without being unbalanced.

Deciphering the history of the impact of planets is difficult in our solar system and can seem like a hopeless task in systems light years away, Raymond said.

“On Earth, we can measure certain types of elements and compare them with meteorites,” Raymond said. “This is what we are doing to try to figure out how much has sunk into the Earth after much of it was formed.”

But these tools do not exist to study the bombardment on exoplanets.

TRAPPIST-1 System - Artist Concept

An illustration showing what the TRAPPIST-1 system might look like from a point of view near the planet TRAPPIST-1f (right). Credit: NASA / JPL-Caltech

“We will never get stones from them,” he said. “We’re never going to see craters on them. So what can we do? This is where the special orbital configuration of TRAPPIST-1 comes in. It is a kind of lever that we can use to put limits on that.

TRAPPIST-1, about 40 light years away, is much smaller and cooler than our sun. Its planets are named in alphabetical order from b to h according to their distance from the star. The time required to orbit the star – equivalent to one year on Earth – is 1.5 days on planet b and 19 days on planet h. Remarkably, their orbital periods form almost perfect ratios, a resonant arrangement reminiscent of harmonious musical notes. For example, every eight “years” on planet b, five pass on planet c, three on planet d, two on planet e, and so on.

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

“We understood that after the formation of these planets, they had not been bombarded with more than a very small amount of things,” he said. “It’s pretty cool. This is interesting information when we think about other aspects of the planets in the system.

Comparison of TRAPPIST-1 with the solar system and the Jovian moons

The planets of TRAPPIST-1 compared to the moons and planets of Jupiter in the solar system. Credit: NASA / JPL-Caltech

The planets thrive in protoplanetary disks of gas and dust around newly formed stars. These disks only last a few million years, and Raymond said that previous research has shown that resonant chains of planets like the one in TRAPPIST-1 form when young planets migrate closer to their star before the disk does. disappears. Computer models have shown that disks can cause planets to resonate. Raymond said that it is believed that resonant chains like those in TRAPPIST-1 need to be set before their disks disappear.

The result is that the planets in TRAPPIST-1 formed rapidly, in about a tenth of the time it took Earth to form, said Andre Izidoro, Rice Study co-author, astrophysicist and fellow. postdoctoral fellowship of CLEVER Planets.

Jean Raymond

Sean Raymond. Credit: Rice University

CLEVER Planets, led by study co-author Rajdeep Dasgupta, Maurice Ewing Professor of Earth Systems Science at Rice, explores ways in which planets might acquire the elements necessary to support life. In previous studies, Dasgupta and his colleagues at CLEVER Planets have shown that a significant portion of the the volatile elements came from the impact who formed the moon.

“If a planet forms early and is too small, like the mass of the moon or ">March, it can’t build up a lot of gas from the disk, ”Dasgupta said. “Such a planet also has much less opportunity to acquire life-sustaining volatiles through late bombardment.”

Izidoro said this would have been the case for Earth, which gained most of its mass relatively late, including about 1% of impacts after the moon-forming collision.

“We know that Earth had at least one giant impact after the gas (in the protoplanetary disk) disappeared,” he said. “It was the moon formation event.

“For the TRAPPIST-1 system, we have these land mass planets that formed early,” he said. “So a potential difference, compared to how the Earth was formed, is that they could have a hydrogen atmosphere from the start and never experienced a late giant impact. And that could change the evolution a lot in terms of the interior of the planet, outgassing, volatile loss and other things that have implications for habitability. “

Raymond said this week’s study has implications not only for the study of other resonant planetary systems, but for exoplanet systems that were believed to have started as resonant systems.

André Izidoro

André Izidoro. Credit: Rice University

“Super-Earths and sub-Neptunes are very abundant around other stars, and the prevailing idea is that they migrated inward during this gaseous disk phase, then possibly had a late phase. collisions, ”Raymond said. “But during that first phase, where they were migrating inward, we think they pretty much – universally maybe – had a phase where they were resonant chain structures like TRAPPIST-1. They just didn’t survive. They ended up becoming unstable later.

Izidoro said one of the study’s major contributions could come years from now, after NASA’s James Webb Space Telescope, the European Southern Observatory’s Extremely Large Telescope and other instruments allow astronomers to ” directly observe the atmospheres of exoplanets.

“Today we have constraints on the makeup of these planets, such as how much water they can have,” Izidoro said of planets forming in a phase of resonant migration. “But we have really big error bars.”

In the future, observations will better limit the interior composition of exoplanets, and knowing the history of the late bombardment of resonant planets could be extremely useful.

“For example, if one of these planets has a lot of water, say 20% mass fraction, the water must have been incorporated into the planets early, during the gas phase,” he said. “So you will have to understand what kind of process could bring this water to this planet.”

Reference: “An upper limit on late accretion and water delivery in the exoplanet Trappist-1 system” November 25, 2021, Nature astronomy.
DOI: 10.1038 / s41550-021-01518-6

Additional study co-authors include Emeline Bolmont and Martin Turbet from the University of Geneva, Caroline Dorn from the University of Zurich, Franck Selsis from the University of Bordeaux, Eric Agol from Washington University, Patrick Barth from the University of St. Andrews, Ludmila Carone from the Max Planck Institute for Astronomy in Heidelberg, Germany, Michael Gillon from the University of Liège and Simon Grimm from University of Bern.

The research was supported by NASA (80NSSC18K0828), the Brazilian Federal Agency for the Support and Evaluation of Higher Education (88887.310463 / 2018-00), the Brazilian National Council for Scientific and Technological Development (313998 / 2018-3), the University of St Andrews, the German Research Foundation (SP1833-1795 / 3), the Horizon 2020 program of the European Union (832738 / ESCAPE), the Swiss National Science Foundation ( PZ00P2 174028), the National Program of Planetology of the National Center for Scientific Research, the National Program of Computer Center for Higher Education (A0080110391) and the Gruber Foundation.

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