An illustration shows what the TRAPPIST-1 system might look like from a perspective point. Credit: Caltech.
Seven Earth-sized planets are in a perfect harmony with the star TRAPPIST-1, and researchers have used that to figure out how much physical abuse the planets could have had.
Sean Raymond of the University of Bordeaux in France said that after rocky planets form things bash into them. We care about bombardment because it can be an important source of water and volatile elements that foster life.
Raymond and colleagues from Rice University 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.
Raymond said thatciphering the impact history of planets is difficult in our solar system and might be impossible in systems light-years away.
Raymond said that on Earth, we can measure elements and compare them to meteorites. We try to figure out how much was bashed into the Earth after it was formed.
There are no tools for studying bombardment on exoplanets.
He said that they would never get rocks from them. We are never going to see craters on them. What can we do? This is where the special orbital configuration of TRAPPIST-1 comes in. It's a lever that we can use to put limits on this.
TRAPPIST-1 is 40 light-years away and cooler than our sun. The planets are named in order of their distance from the star. The time it takes to complete a one year around the star is 1.5 days on planet b and 19 days on planet h. Their orbital periods form near perfect ratios, reminiscent of harmonious musical notes. For every eight years on planet b, five pass on planet c, three on planet d, two on planet e and so on.
Raymond said that they can put an upper limit on how much stuff was bashed into the planets. We can say that it can't have been more than this. The upper limit is actually quite small.
He said that after the planets formed, they weren't bombarded with a lot of stuff. That's cool. It's interesting when we're thinking about other aspects of the system.
Planets grow in disks of gas and dust around stars. The disks only last a few million years, and Raymond said that previous research has shown that planets like TRAPPIST-1's form when they migrate closer to their star before the disk disappears. Disks can shepherd planets into resonance. Raymond said it's believed that the disks must be set before they disappear.
TRAPPIST-1's planets formed in about one-tenth of the time it took Earth to form, according to Rice study co-author, physicist and CLEVER Planets fellow, Andre Izidoro.
The Maurice Ewing Professor of Earth Systems Science at Rice, Rajdeep Dasgupta, is one of the study co-authors of Curled Planets. The impact that formed the moon has been shown to have created a significant portion of Earth's volatile elements.
"If a planet is too small, like the mass of the moon or Mars, it can't accrete a lot of gas from the disk," he said. The planet has less opportunity to gain life-essential volatile elements through late bombardments.
Izidoro said that Earth gained most of its mass relatively late, including about 1% from impacts after the moon-forming collision.
He said that Earth had at least one giant impact after the gas in the disk was gone. That was the beginning of the moon.
He said that the TRAPPIST-1 system has Earth-mass planets that formed early. They could have a hydrogen atmosphere from the beginning and have never experienced a giant impact. This could change a lot of the evolution of the planet in terms of outgassing, volatile loss and other things.
Raymond said that this week's study has implications for the study of more common exoplanet systems that were believed to have begun as resonant systems.
The idea is that the super-earths and sub-Neptunes migrated inward during the gas-disk phase and then had a late phase of collision. They had a phase where they were resonant chain structures like TRAPPIST-1 during the early phase. They didn't survive. Later on, they ended up going unstable.
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 astronomy to directly observe exoplanet atmospheres.
Izidoro said that there are constraints on the composition of planets that form in a migration phase. We have a lot of error bars.
Knowing the late bombardment history of planets could be very useful in the future.
If one of these planets has a lot of water, it must have been incorporated into the planets early in the gaseous phase. You will have to understand what kind of process could bring this water to this planet.
Other study co-authors include Emeline Bolmont and Martin Turbet of the University of Geneva, as well as Eric Agol of the University of Washington, Patrick Barth of the University of St. Andrew's, and Franck Selsis of the University of Bordeaux.
Sean Raymond has an upper limit on late accretion and water delivery in the TRAPPIST-1 exoplanet system. www.nature.com/articles/s41550-021-01518-5
Nature Astronomy journal has information.
There is a limit to the late arrival of water on TRAPPIST-1 planets.
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