It would appear that the so-called "great instability" event that wreaked chaos among the planets, sending the gas giants careening through space until they settled into the orbits we know today, occurred between 60 and 100 million years after the birth of the solar system. This is the conclusion of some careful scientific detective work that has connected a type of meteorite to an asteroid that was once pushed around by those marauding planets.
What's more, scientists believe the migrating planets — primarily Jupiter — could have led to the formation of Earth's moon by destabilizing the orbit of a Mars-size protoplanet called Theia. This destabilization may have instigated a collision with Earth that sent debris into space. It is this debris, scientists believe, that may have formed the moon.
Related: What happened when the moon 'turned itself inside out' billions of years ago?
Thanks to studies of the compositions and locations of various types of asteroids and comets, scientists know the aforementioned carnage occurred early in the history of the solar system. Still, there are some puzzles yet to be solved when it comes to how exactly everything went down.
For instance, scientists are aware that the objects in the solar system we see today, including Earth, formed around the sun from a disk of gas and dust. However, some of those objects, namely asteroids and comets, appear to consist of material that was not present in the disk — at least, the material shouldn't have been present in the locations those objects currently find themselves in. Instead, it'd make more sense for these items to have formed closer to the sun before being scattered farther afield. If Jupiter and the other giant planets migrated from where they formed, maybe asteroids and comets could've as well.
In the young solar system, the four gas giant planets — Jupiter, Saturn, Uranus and Neptune — were nestled closer together. Over time, gravitational interactions with planetesimals beyond Neptune led to Saturn, Uranus and Neptune migrating outward. Meanwhile, Jupiter migrated inward, where scientists think it was, in turn, able to destabilize bodies in the inner solar system.
"The idea of this orbital instability is now well established in the planetary community, however the time at which this instability occurred is still a matter of debate," planetary scientist Chrysa Avdellidou of the University of Leicester told Space.com.
Scientists call the theory behind this orbital instability the "Nice Model," after the French city that houses the Côte d’Azur Observatory, where scientists originally developed the idea. Initially, those scientists thought this instability occurred between 500 and 800 million years after the birth of the solar system. If true, that would have coincided with an event known as the Late Heavy Bombardment, in which the inner planets would have been peppered by comets dislodged from their orbits thanks to the migrating gas giants. However, evidence has turned against the concept of the Late Heavy Bombardment, and scientists now think that the instability occurred no later than 100 million years after the solar system formed, based on when Jupiter could have accrued its trojan asteroids at its L4 and L5 Lagrange points.
"People seem to agree that the Nice Model-like instability probably happened less than 100 million years after the start of the solar system, but a few different camps are emerging," Kevin Walsh of the South-west Research Institute in Boulder, Colorado, told Space.com. One camp posits that the instability would have occurred very quickly, within four million years of the solar system's birth. The other camp thinks it took place later, after about 60 million years.
So, Avdellidou, aided by Walsh and other planetary scientists, set about finding an answer.
The team focused on a kind of meteorite called an EL enstatite chondrite, which has a low iron abundance and is very similar in composition and isotopic ratio to the material that formed Earth. This tells scientists that Earth and EL chondrites likely condensed out of the same part of the planet-forming disk.
However, the EL-chondrite parent body no longer appears to be near Earth. In fact, astronomical observations from ground-based telescopes have connected these meteorites to the Athor family of asteroids, which is found pretty far away in the asteroid belt between Mars and Jupiter. For context, the Athor family and the EL chondrites were once part of one big asteroid that was smashed apart in a collision some 3 billion years ago, an event unrelated to the great instability.
Something should have scattered the progenitor of the Athor family into the asteroid belt, and that "something," the team says, must have been the instability that led Jupiter to go wandering. EL chondrites thus make the perfect chronometers for this event because they should contain a clear record of what must have happened.
"Specifically, the EL meteorites thermal history tells a rich story, constraining both the size of the original parent body and the time it must have taken to cool before being broken," said Walsh.
Using dynamical simulations, Avdellidou's team was able to model the different scenarios involving a migrating Jupiter, and concluded that Jupiter could have scattered the Athor progenitor into the asteroid as early as 60 million years after the birth of the solar system. Coupled with data about Jupiter's Trojan asteroids, scientists can now say that the great instability took place between 60 million and 100 million years.
"Avdellidou specifically finds that the Nice Model itself — the giant planet's orbits going wild for a short 10 or 20 million year period — is the best and maybe only time to send asteroids into the region of this specific Athor asteroid family," said Walsh.
And, intriguingly, the collision between Earth and Theia that formed the moon occurred around this period of time. "We understand that there was a giant collision on proto-Earth by Theia, which had a very similar composition," said Avdellidou. "From studies of samples [from the Moon] there are age estimates, while other colleagues have shown that this collision could have been a result from the giant planet instability."
Although there's no way to prove it. "'Proof' is a strong statement and something difficult when we deal with events 4.5 billion years ago," said Avdellidou, though the scientist admits the collision that formed Earth's moon does seem to coincide with the great instability.
"Our study placed these events in a nice, tight timeframe," said Avdellidou. While it might not be possible to conclusively prove that Jupiter had a hand in the Moon’s formation, the evidence is certainly suggestive.
So, the next time that you look up at the silvery face of the moon in our night sky, think of it as a legacy from the early solar system when Jupiter was bullying all around it.
The findings were published on April 16 in the journal Science, and presented at the European Geological Union General Assembly in Vienna.
