Astrophysicists at the University of Toronto have found that a close encounter with Jupiter about four billion years ago may have resulted in another planet's ejection from the Solar System altogether.
The existence of a fifth giant gas planet at the time of the Solar System's formation -- in addition to Jupiter, Saturn, Uranus and Neptune that we know of today -- was first proposed in 2011. But if it did exist, how did it get pushed out?
For years, scientists have suspected the ouster was either Saturn or Jupiter.
"Our evidence points to Jupiter," said study's lead author Ryan Cloutier.
Planet ejection occurs as a result of a close planetary encounter in which one of the objects accelerates so much that it breaks free from the massive gravitational pull of the Sun.
Cloutier and his colleagues turned their attention to moons and orbits, developing computer simulations based on the modern-day trajectories of Callisto and Iapetus, the regular moons orbiting around Jupiter and Saturn respectively.
They then measured the likelihood of each one producing its current orbit in the event that its host planet was responsible for ejecting the hypothetical planet, an incident which would have caused significant disturbance to each moon's original orbit.
"Ultimately, we found that Jupiter is capable of ejecting the fifth giant planet while retaining a moon with the orbit of Callisto," Cloutier said.
"On the other hand, it would have been very difficult for Saturn to do so because Iapetus would have been excessively unsettled, resulting in an orbit that is difficult to reconcile with its current trajectory," he explained.
The study was published in The Astrophysical Journal.