“Astronomers have discovered a number of free-floating planets, wandering between the stars”
The early history of the Solar System is believed to have been unstable and tumultuous, with strong gravitational interactions between the freshly formed worlds causing the initial orbits of the giant planets to migrate into the architecture of our planetary neighbourhood we see today.
The model that best explains the current configuration of our Solar System is known as the ‘Nice model’, and neatly accounts for the observed orbits of the four giant planets, the structure of the Kuiper belt, Jupiter’s capture of its Trojan asteroids, and the triggering of the Late Heavy Bombardment that is believed to have occurred around four billion years ago. However, there are still a few inconsistencies with it. One proposed solution is that perhaps our Solar System today isn’t all that it once was: if in fact there used to be a fifth giant planet, an ice giant like Uranus, that suffered several close encounters with Jupiter and Saturn and was eventually flung into interstellar space, the fit to the observed architecture of the current Solar System is much better.
This possibility, where the gas giant planets push one of their siblings right out of the nest, is known as the ‘jumping Jupiter scenario’. And indeed, astronomers have discovered a number
is an astrobiologist at the University of Leicester and the author of The Knowledge: How to Rebuild our World from Scratch (www. the-knowledge.org) of free-floating planets, wandering between the stars in our Galaxy. However, any close encounters between the giant planets in the Solar System’s tumultuous early history would also have disrupted the orbits of those planet’s outer moons. So as Ryan Cloutier and his colleagues at the University of Toronto point out, considering the orbit of Callisto (Jupiter’s outermost Galilean moon) and Iapetus (around Saturn) would provide vital information on how closely their parent planets could have approached an extra ice giant.
If the Solar System ever did have a long-lost extra ice giant planet, its gravitational ejection into interstellar space must be consistent with the orbits of the moons we observe today around Jupiter and Saturn. Cloutier and his team therefore simulated various swing-by encounters between Jupiter or Saturn and a hypothetical ice giant planet to see how plausible it was that the ice giant could be ejected from the Solar System whilst still leaving the outermost large moon undisturbed in its orbit.
Some of these simulated close encounters resulted in the ice giant receiving a large velocity kick – just like our deliberate attempts at gravitational slingshots with space probes – and being flung on an escape trajectory. But how many of these left the gas giant planet with a recognisable system of moons? When modelling Saturn, Cloutier and his team found that the orbit of Iapetus is irreconcilably disrupted in 99 per cent of the ejection encounters, leading them to conclude that if we ever did have another ice giant planet in our solar family, it almost certainly wasn’t Saturn that banished it.
But for Jupiter, around half of the ejection encounters preserve Callisto in its orbit. Cloutier is quick to point out, though, that this doesn’t necessarily mean that there was an extra planet that became ejected, just that Callisto’s orbit turns out to not offer a very tight constraint on the plausibility of the fifth giant planet hypothesis.