Exoplanets dodge bombardment in their infancy

In 2017, NASA announced the discovery of seven rocky, Earth-sized planets orbiting in the habitable zone of a star called TRAPPIST-1. Now, new research has used their harmonious orbits to determine just how much bombardment the planets could have withstood in their infancy.

“After rocky planets form, things bash into them,” says 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.”

This happened during the formation of our own solar system. The planets coalesced out of the disk of gas and dust leftover from the formation of the Sun, and all the extra bits of debris continued careering around, smashing into the planets and moons.

Here on Earth, it’s possible for astronomers to look back in time to study the impacts that occurred billions of years ago by measuring certain types of elements in rocks and comparing them with meteorites.

But how can they study this for planets across nearly 40 light-years of space?

“We’ll never get rocks from them,” says Raymond. “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’s a kind of a lever we can pull on to put limits on this.”

Seven planets of varying sizes side by side
A size comparison of the planets of the TRAPPIST-1 system, lined up in order of increasing distance from their host star. The planetary surfaces are portrayed with an artist’s impression of their potential surface features, including water, ice, and atmospheres. Credit: NASA/R. Hurt/T. Pyle

The seven TRAPPIST-1 planets are in orbital ‘resonance’ with each other, which means they have orbital periods that form ratios of whole numbers, circling their parent star in a mathematically predictable pattern. For example, for every eight orbits completed by the innermost planet, the next one completes five, the next three, the next two, and so on.

Researchers think that resonant chains like these form as young planets migrate towards their parent star, shepherded into place by the gravitational influence of all the leftover matter from the formation of the solar system – the stuff that also crashes into them.

In the study, led by Raymond and published in Nature Astronomy, researchers created a computer model to see just how much early asteroids and impactors could have crashed into the planets without knocking them out of their harmonious orbits.

“We can’t say exactly how much stuff bashed into any of these planets, but because of this special resonant configuration, we can put an upper limit on it,” says Raymond.

And that limit was pretty low.

Illustration showing that the trappist-1 system is incredibly small compared to earth
All seven planets discovered in orbit around the red dwarf star TRAPPIST-1 could easily fit inside the orbit of Mercury, the innermost planet of our solar system. In fact, they would have room to spare. Credit: NASA/JPL-Caltech

“We figured out that after these planets formed, they weren’t bombarded by more than a very small amount of stuff.”

The study was co-authored by researchers from the CLEVER Planets project at Rice University in the US, which explores the ways planets could acquire the necessary elements to support life. In a previous study, they showed that a large proportion of Earth’s volatile elements came from the massive impact that formed the Moon.

“If a planet forms early and it is too small, like the mass of the Moon or Mars, it cannot accrete a lot of gas from the disk,” says co-author Rajdeep Dasgupta from CLEVER Planets. “Such a planet also has much less opportunity to gain life-essential volatile elements through late bombardments.”

The researchers think that these Earth-sized planets in the TRAPPIST-1 system formed fairly early, and could have had a hydrogen atmosphere. They also may never have experienced a massive impact late in the game – like the one that formed our Moon.

“This might change a lot of the evolution in terms of the interior of the planet, outgassing, volatile loss and other things that have implications for habitability,” says co-author Andre Izidoro from Rice University.

These kinds of computational studies about the bombardment, he adds, will be useful to future telescopes such as NASA’s James Webb Space Telescopes or the Extremely Large Telescope, which will be able to directly peer into the atmospheres of exoplanets.

“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,” Izidoro explains. “So you will have to understand what kind of process could bring this water to this planet.”

In the meantime, researchers will continue to listen to the harmonious melodies of the planets.

The orbital periods of the TRAPPIST-1 planets form near-perfect ratios, in a resonant arrangement reminiscent of musical notes. Credit: SYSTEM sounds

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