Jupiter Pushed out Another Planet Long Ago, Says Study

In a seeming case of interplanetary chess, Jupiter bounced another planet from the Solar System about four billion years ago, say astrophysicists at the University of Toronto (U of T).

The idea that the Solar System might have had a fifth huge gassy planet in the beginning -- in addition to the known large planets of Jupiter, Saturn, Uranus and Neptune -- was proposed in 2011. Scientists have suspected for some time that either Saturn or Jupiter had pushed out the fifth planet.

But that has changed. The study findings on this were published recently in The Astrophysical Journal.

A planet ejection often works like this: One object speeds up enough to launch free of the Sun's gravitational pull. In the past, studies have concluded that this could not happen without having a violent effect on minor bodies, including the giant planets' moons and their orbits, according to a release.

In the recent study, lead author Ryan Cloutier, a Ph.D. candidate at U of T, and colleagues looked at moons and orbits. They conducted computer simulations like those of the current trajectories of Callisto and Iapetus, which orbit Jupiter and Saturn respectively. Then they did measurements of the likelihood of each of them having its current orbit if the host planet had ejected a hypothetical planet, confirmed the release.

"Ultimately, we found that Jupiter is capable of ejecting the fifth giant planet while retaining a moon with the orbit of Callisto," said Cloutier, in a release. "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."

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