Orbital Harmony in the TRAPPIST-1 System Governs Planet Development

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

“Once the rocky planets form, things bump into them,” said astrophysicist Sean Raymond of the University of Bordeaux in France. “It’s called bombing, or late addition, and we’re concerned about that, in part, because these impacts can be an important source of water and volatile elements that support life.”

In a study available online today in Nature Astronomy, Raymond and colleagues from the NASA-funded project CLEVER Planets at Rice University and seven other institutions used a computer model of the bombardment phase of planetary formation at TRAPPIST-1 to explore impacts that their planets could have resisted without losing their harmony.

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

“On Earth, we can measure certain types of elements and compare them to meteorites,” said Raymond. “This is what we do to try to find out how much stuff crashed into the Earth after it was mostly formed.”

But these tools don’t exist for studying exoplanet bombing.

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

TRAPPIST-1, about 40 light years away, is much smaller and cooler than our sun. Your planets are named alphabetically from bah 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 nearly perfect proportions, a resonant arrangement reminiscent of harmonious musical notes. For example, for 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 much material crashed into any of these planets, but because of this special resonant configuration, we can put an upper limit on it,” said Raymond. “We can say, ‘It couldn’t have been more than that.’ And it turns out that this upper limit is actually quite small.

“We found that after these planets formed, they weren’t bombed by more than a small amount of things,” he said. “That’s great. It’s interesting information when we think about other aspects of the planets in the system.”

Planets grow in protoplanetary disks of gas and dust around newly formed stars. These disks only last a few million years, and Raymond said previous research has shown that resonant chains from planets like TRAPPIST-1 form when young planets migrate closer to their star before the disk disappears. Computer models have shown that disks can drive planets to resonance. Raymond said that it is believed that resonant strings like TRAPPIST-1 must be set up before their disks disappear.

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

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

“If a planet forms early and is very small, like the mass of the moon or Mars, it can’t add much gas from the disk,” Dasgupta said. “This planet also has much less opportunity to obtain volatile elements essential to life through late bombing.”

Izidoro said that would have been the case for Earth, which gained much of its mass relatively late, including about 1% impacts after colliding with the moon.

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

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

Raymond said this week’s study has implications not just for the study of other resonant planetary systems, but for much more common exoplanet systems believed to have started out as resonant systems.

“Super-Earths and sub-Neptune are very abundant around other stars, and the prevailing idea is that they migrated inward during the gas disk phase and then possibly had a late collision phase,” said Raymond. “But during the initial phase, where they were migrating inward, we thought they pretty much – universally perhaps – had a phase where they were resonant chain structures like TRAPPIST-1. They just didn’t survive. They ended up getting unstable later on.”

Izidoro said one of the study’s biggest contributions could come a few 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 some restrictions on the composition of these planets, such as how much water they can have,” Izidoro said of planets that form in a resonant phase of migration. “But we have very large error bars.”

In the future, observations will further narrow down the internal composition of exoplanets, and knowing the history of the last bombing of resonant planets can be extremely helpful.

“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 on, during the gas phase,” he said. “Therefore, you will have to understand what kind of process could bring this water to this planet.”

Other co-authors of the study 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 from the University of Liège and Simon Grimm from the University of Bern.