Meteorites Tighten Timeline for Giant Planets’ Movement through the Solar System

For billions of years, our solar system has existed in a state of relative stability. But scientists believe that its early years were far more dynamic. Shortly after the planets formed, a process known as giant planet instability reshuffled the orbits of Jupiter and Saturn, moving them from their originally more packed and circular orbits to their current wider configuration. The process scattered smaller chunks, or planetesimals, some toward the inner solar system, others toward the Kuiper belt and beyond..

For a long time this migration has been thought to occur about 600 million years after the solar system’s birth. The number of lunar craters originating at that time, during a period known as the late heavy bombardment, increased significantly, suggesting a sudden influx of impactors.

But other evidence has suggested that the planetary dance might have occurred much earlier. Now, new research published April 16th in Science, significantly tightens the timeframe for the giant planets’ migration, placing it between 60 and 100 million years after the solar system’s birth.

The Athor Family Connection

Like in a detective story, a team of researchers led by Chrysa Avdellidou (University of Leicester, UK) followed a loose end to get to the culprit of the crime. In this case, the loose end is the Athor family of asteroids, a group of objects in the inner asteroid belt. Using ground-based observations, Avdellidou and her colleagues realized that the Athor family matches a type of meteorite called low-iron enstatite chondrites — a finding they published back in 2019.

“This is an important discovery because this source [the Athor asteroids] is the only one that we have found so far in the whole asteroid belt, and the enstatite meteorites are the ones that have the closest composition to our planet,” says Avdellidou, who presented their findings at the European Geosciences Union General Assembly in Vienna.

Enstatite chondrites are believed to have formed very close to Earth in the early solar system. In fact, many researchers believe that they are leftovers of our planet’s original building blocks. The chemical link between the Athor family and enstatite chondrites reveals that the family’s forefather was likely a planetesimal that formed very close to Earth.

By carefully dating the radioactive decay within enstatite chondrite meteorites on Earth, previous research had found that the original parent body, which was about 400 kilometers (250 miles) in diameter, broke up into smaller pieces no earlier than 60 million years after the formation of the solar system. One of those pieces eventually made it into the asteroid belt, where it further broke down to form the Athor family. But the timing of the first break-up marks  the moment when the Athor  forefather still orbited near Earth.

By learning how this piece could have made it from near Earth’s orbit all the way to the asteroid belt, the team had to explore what was going on in the solar system at the time. Trying different scenarios via computer simulations, they concluded that the only mechanism that worked was the influence of the migrating giant planets. This, combined with previous constraints, suggests that the giant planet instability likely occurred between 60 and 100 million years after the solar system's birth.

“So you form the body, you have to wait 60 million years, then you can break it, and then a fragment is implanted [in the asteroid belt] by the giant planet instability,” Avdellidou says.

“I think this is a super cool new look at this question,” says cosmochemist Graham Edwards (Dartmouth College). “It's a really interesting idea and a cool way of using this relationship to try and get the most out of our combined knowledge of the meteorites we have on Earth and the observations we can make of asteroids in the asteroid belt.”

Yes, But…

Not everybody in the field agrees with the new conclusion, sparking scientific debate. Even before the Science paper became publicly available, another group of researchers had already put together a rebuttal, arguing that the implantation of the Athor progenitor is not as tightly constrained as the new study proposes.

Yet another paper, led by Edwards and currently under review by Nature Astronomy, points to an even earlier instability, a mere 11 million years after the solar system's formation. They base this conclusion on the impact history recorded in meteorites already noted in previous studies, which suggest a surge in collisions around this time period.

The urgency in refuting the new claims stems from the importance of the giant planet migration in the history of the early solar system. An earlier instability, soon after the Sun's gaseous disk dispersed, would have significantly impacted the growth of the terrestrial planets. Its timing also influences the evolution of the asteroid belt, the Kuiper Belt, and other small bodies. Therefore, accurately determining when this instability occurred has the potential to validate or disprove various other scenarios, including the timing of the giant impact that led to the formation of Earth’s Moon.

“There have been some other studies in the recent years that have showed that this can happen, so the giant planet instability could trigger the Moon-forming event about 10, 20, or even 30 million years after its occurrence” Avdellidou notes. “Our study coincides very well.”

A Golden Age for Solar System Studies

As the debate continues, scientists will soon have new ways to dig deeper into the solar system’s past. New telescopes such as the Vera C. Rubin Observatory promise to unveil a wealth of new asteroids, potentially offering additional clues about the timing and nature of the giant planet instability.

“I’m really excited to be working in this field right now because there's a lot of excitement and a lot of new ideas moving around,” Edwards says. “This is science working, science doing what it's supposed to do, where folks are throwing a lot of ideas out there and hopefully converging on the right answer.”