The Solar System is one of the comforting constants of human existence.
No matter what else we have going on, the planets will continue to whirl about the Sun, making predictable patterns in the sky, lined up in a neat row along the system's orbital plane.
But it may not always have been quite so orderly.
According to one of the more popular models of the early Solar System, the Nice model, our corner of the galaxy may once have been two planets richer, boasting four ice giants instead of just the two hanging around today – Uranus and Neptune.
However, a new test of this idea has uncovered a potentially serious problem.
By simulating the turbulent period when these hypothetical worlds would have been ejected from the Solar System, astronomers have found that this model implies a far more violent history for Uranus's moons than their present appearance suggests.
It's a finding that suggests the story of how the Solar System evolved into the clockwork precision we know today is still very much a work in progress.
"These results have three potential implications," the team, led by astrophysicist Matthew Clement of Johns Hopkins University, writes in their paper.
One, the Uranian satellites were destabilized to the point of collisions a few times during this turbulent time.
Two, the current version of the Nice model needs revising; or three, "the Solar System is the result of fairly unlikely instability evolution that entailed almost no deep encounters between Uranus and the other giant planets," the team writes.
The nascent Solar System is thought to have been a far messier place than it is today. As the giant planets migrated through a disk of debris, their gravitational interactions may have destabilized the entire outer Solar System.
To try to explain the gravitational mayhem that created a wild pinball machine of the Solar System, scientists devised the Nice model, published in 2005.
In its modern forms, the Nice model proposes that the Solar System may have started with one or even two extra ice giants, and all six of the giant planets then migrated away from their starting positions, stirring up instability as they went.
The four giant planets still in our Solar System eventually settled into their current orbits, while the other two skedaddled off who-knows-where.
For the period known as the Late Heavy Bombardment, the large-scale architecture of the Solar System, and Jupiter's extensive collection of Trojans, the Nice model works splendidly.
But Clement and his colleagues wanted to know: What does it do to the Solar System on a more granular level?
They set up a simulation playground to test what happens to the moons of Jupiter and Uranus under two Nice model variants: one with an extra ice giant, and another scenario with two.
The team simulated encounters based on the "full spectrum" of proposed outer Solar System initial conditions, producing more pathways than had been considered before.
And here's where it gets interesting: Most of the simulations destabilized Uranus' moons to the point of collisions, ejections, and major orbital reshuffles.
Jupiter's moons fared significantly better, but it turns out that it's surprisingly difficult to leave both planets' moon systems intact.
The researchers found only one scenario in which the moons of Jupiter and Uranus all consistently survive the same instability.
If the Nice model is correct, we're left with one of two leading options.
Either the Solar System took a very unlikely evolutionary path that left Jupiter and Uranus alone, or Uranus's current system of moons was born of collisions and gravitational pandemonium.
The latter would suggest that Uranus was messed with at least twice – the first being the event that tipped it sideways, and the second being the Nice model instability that disordered its moons.
Of course, the third option is that the Nice model is incomplete, which would not really be a huge surprise, since it's trying to reconstruct something that took place 4 billion years ago.
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"It is highly likely that none of the modeled instabilities in the literature contain the precise sequences of encounters necessary to exactly reproduce all aspects of the Solar System," the researchers write.
"While it is certainly possible that all four primordial regular satellite systems in the outer Solar System were unaffected by planetary encounters, our results strongly suggest that this is not the case."
The findings should motivate future work that continues to investigate the consequences of various collision courses on our Solar System.
The research has been published in Icarus.