Orbital harmony limits the late arrival of water on TRAPPIST-1 planets

Orbital harmony limits the late arrival of water on TRAPPIST-1 planets

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

Seven Earth-sized planets orbit the star TRAPPIST-1 in near-perfect harmony, and American and European scientists have used that harmony to determine how much physical abuse the planets could have withstood in their infancy.

“After rocky planets are formed, things bump into them,” said astrophysicist Sean Raymond of the University of Bordeaux in France. “It’s called bombardment or late growth, and we worry about it in part because these influences can be a major source of water and volatile elements that promote life.”

In a study available online today in Nature astronomy, Raymond and colleagues from Rice University’s NASA-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 influences its planets could have withstood without being knocked out of harmony.

Deciphering the impact history of the 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 to meteorites,” Raymond said. “That’s what we do to try to figure out how much matter crashed into the Earth after it was mostly formed.”

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

“We never get stones from them,” he said. “We’ll never see craters on them. So what can we do? This is where TRAPPIST – 1’s special orbital configuration comes in. It’s a kind of handle we can pull on to set limits on this.”

TRAPPIST-1, about 40 light-years away, is far smaller and cooler than our sun. Its planets are named alphabetically from b to hi in order of their distance from the star. The time required to complete an orbit around 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 conditions, a resonant arrangement , reminiscent of harmonic musical tones. For example, for every eighth “year” on planet b, five pass on planet c, three on planet d, two on planet e and so on.

“We can not say exactly how much things hit any of these planets, but because of this particular resonant configuration, we can set an upper limit for it,” Raymond said. “We can say, ‘It could not have been more than this.’ And it turns out that the upper limit is actually quite small.

“We found out that after these planets were formed, they were not bombarded by more than a very small amount of stuff,” he said. “It’s a little cool. It’s interesting information when we think about other aspects of the planets in the system.”

Planets grow within protoplanetary disks of gas and dust around newly formed stars. These disks last only a few million years, and Raymond said previous research has shown that resonant chains of planets like TRAPPIST-1’s form when young planets migrate closer to their star before the disk disappears. Computer models have shown that disks can shepherd planets to resonate. Raymond said it is believed that resonant chains such as TRAPPIST-1s must be tuned before their disks disappear.

The result is that TRAPPIST-1’s planets formed quickly, in about a tenth of the time it took the Earth to form, said Rice study co-author Andre Izidoro, an astrophysicist and CLEVER Planet’s postdoc.

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

“If a planet is formed early and it is too small, like the mass of the moon or Mars, it cannot collect a lot of gas from the disk,” Dasgupta said. “Such a planet also has much less chance of getting vital volatile elements through late bombings.”

Izidoro said that would have been the case for Earth, which got most of its mass relatively late, including about 1% from impact after the month-long collision.

“We know that Earth had at least one huge impact after the gas (in the protoplanetary disk) was gone,” he said. “It was the lunar event.

“For the TRAPPIST-1 system, we have these early-formed masses of planets,” he said. “So a potential difference, compared to the formation of the Earth, is that from the beginning they could have a certain hydrogen atmosphere and have never experienced a late gigantic impact. And this can change a lot on the development in terms of the interior of the planet, degassing, volatile losses and other things that have consequences for habitability. “

Raymond said this week’s study is not only relevant to the study of other resonant planetary systems, but to far more common exoplanet systems that were thought to have begun as resonant systems.

“Super-Earths and sub-Neptunes are very abundant around other stars, and the prevailing idea is that they migrated inward during that gas disk phase and then possibly had a late phase of collisions,” Raymond said. “But in the early phase, when they migrated inward, we think they pretty much – universally perhaps – had a phase where they were resonant chain structures like TRAPPIST-1. They just did not survive. They ended up becoming unstable later on. “

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

“We have some limitations today on the composition of these planets, such as how much water they can have,” Izidoro said of planets forming in a resonant, migration phase. “But we have very large error bars.”

In the future, observations will better limit the intrinsic composition of exoplanets, and it may be extremely useful to know the late bombardment history of the resonant planets.

“For example, 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 in the gaseous phase,” he said. “So you have to understand what kind of process could bring this water to this planet.”

Additional 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 the University of Washington, 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 Li├Ęge and Simon Grimm from the University of Bern.

The orbital flatness of planetary systems

More information:
Sean Raymond, An upper limit for late growth and water supply in the TRAPPIST-1 exoplanet system, Nature astronomy (2021). DOI: 10.1038 / s41550-021-01518-6. www.nature.com/articles/s41550-021-01518-6

Provided by Rice University

Citation: Orbital Harmony Limits Late Arrival of Water on TRAPPIST-1 Planets (2021, November 25) Retrieved November 25, 2021 from https://phys.org/news/2021-11-orbital-harmony-limits-late-trappist- .html

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