SCIENCE NEWS: Orbital harmony limits the late arrival of water upon TRAPPIST-1 planets

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A diagram showing how the TRAPPIST-1 system could look from a vantage point close to planet TRAPPIST-1f (right). Credit: NASA/JPL-Caltech

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

“After rocky planets form, things bash into them,” said astrophysicist Sean Raymond of the University of Bordeaux in France. “It’s called bombardment, or late accretion, and we care about it, in part, because these impacts can be an important source of water and volatile elements that foster 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 impacts its planets could have withstood without getting knocked out of harmony.

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

“We can measure elements on Earth and compare them to meteorites,” Raymond stated. Raymond said, “That’s why we do that to find out how much stuff was blasted into the Earth after it formed. “

But these tools aren’t available for studying bombardment of exoplanets.

” We won’t get rocks from them,” said he. They won’t let us see any craters. What can we do? Here is where TRAPPIST-1’s special orbital configuration comes in. This is a lever that we can pull to limit this. “

TRAPPIST-1, about 40 light-years away, is far smaller and cooler than our sun. The planets are named alphabetically, starting with b and ending with h depending on their distance from the star. The time needed to complete one 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 near-perfect ratios, a resonant arrangement reminiscent of harmonious musical notes. For example, eight years on planet B are followed by five on planets c and d respectively, while three pass on planets e and f.

” We can’t tell exactly how much stuff was blasted into these planets but because of the special resonant configuration we can put an upper limit,” Raymond stated. Raymond said, “It couldn’t have been greater than this.” It turns out, that this upper limit is quite small.

” We discovered that these planets were not bombarded with more than a small amount of stuff after they formed,” he stated. That’s pretty cool. This information is very interesting when we think about other planets in our system. “

Planets are formed within protoplanetary disks made of gas and dust that surround newly formed stars. These disks are only good for a few million of years. Raymond stated that previous research has shown that protoplanetary disks of gas and dust around newly formed stars can form resonant chains, such as TRAPPIST-1. This happens when young planets move closer to their star. Computer models have shown that disks can help planets achieve resonance. Raymond stated that it is believed that disks can be used to create resonant chains such as TRAPPIST-1 before they disappear.

The bottom line is that TRAPPIST-1’s moons formed quickly, in approximately one-tenth of the time it took Earth for them to form, according to Andre Izidoro (a Rice study coauthor and CLEVER Planets postdoctoral fellow), an astrophysicist.

CLEVER planets is led by Rajdeep Dasgupta (the Maurice Ewing Professor Earth Systems Science at Rice), who studies how planets might acquire the elements necessary to sustain life. In previous studies, Dasgupta and colleagues at CLEVER Planets have shown a significant portion of Earth’s volatile elements came from the impact that formed the moon.

” If a planet is formed early, but it is too small like the mass of Mars or the moon, it can’t accrete much gas from the disk,” Dasgupta stated. Late bombardments are less likely to give rise to life-essential volatile components. “

Izidoro stated that this would have been the case with Earth, which gained its majority of mass relatively late after the collision of the moon and Earth.

” We know that Earth experienced at least one major impact after the gas (in protoplanetary disk) had gone,” he stated. That was the moon-forming event.

” For the TRAPPIST-1, we have these Earth mass planets that were formed early,” he stated. He said that one difference to Earth’s formation is that these planets could have had some hydrogen atmosphere from the beginning and never have experienced a giant impact. This could have implications for the planet’s interior, outgassing, volatile losses, and other aspects that affect habitability. “

Raymond stated that this week’s research has implications for not only the study of other resonance planetary systems but also for common exoplanet system which were thought to have started as resonant.

“Super-Earths and sub-Neptunes are very abundant around other stars, and the predominant idea is that they migrated inward during that gas-disk phase and then possibly had a late phase of collisions,” Raymond said. We believe that during the early phase of their migration inward, they had a phase when they were resonant chains structures such as TRAPPIST-1. They didn’t make it. They became 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 exoplanet atmospheres.

” We have constraints today on how these planets will be composed, such as how much water they can have.” Izidoro spoke of planets formed in a resonant or migration phase. We have large error margins. “

In the future, observations of exoplanets will be more precise in determining their interior composition. Knowing the bombardment history of resonant bodies could prove extremely valuable.

“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,” he said. “So, you’ll need to know what process might bring this water to the planet. “

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

More information:
Sean Raymond, An upper limit on late accretion and water delivery in the TRAPPIST-1 exoplanet system, Nature Astronomy (2021). DOI: 10.1038/s41550-021-01518-6.

Orbital harmony limits late arrival of water on TRAPPIST-1 planets (2021, November 25)
retrieved 25 November 2021

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