Jupiter May Have Kicked A Gas Giant Planet Out Of The Solar System Billions Of Years Ago

A view of Jupiter from the Galileo spacecraft. NASA.

Like in a high-stakes cosmic pool game, four billion years ago Jupiter may have been responsible for ejecting a giant planet from the primordial solar system. A team of astronomers from the University of Toronto simulated how the orbits of Jupiter and Saturn's moons would look now if their planet had an interaction with another giant body and based on that, Jupiter is likely to be responsible for the ejection of this alleged extra planet. 

The beginning of the Solar System is still full of mysteries that are difficult to unravel, and scientists have relied on simulations to understand the finer details of planetary formation. Models can predict the current orbits of the gas giant planets, the type of satellites that formed, and structures in the Kuiper belt (where Pluto resides) with a moderate degree of accuracy.

But although they can predict many features, existing models fail to account for the eccentricities of the outer planets (the smaller the eccentricity, the more circular the orbit). A correction proposed in 2011 showed that if a fifth gas giant was present in the formation of the Solar System and was later ejected, then simulations predict the current Solar System with a higher likelihood.

According to the 2011 model, the lost planet was an ice giant, made of elements such as carbon and oxygen, and similar in mass and composition to Uranus and Neptune, the ice giants in the Solar System. Jupiter and Saturn are made of lighter elements, mostly hydrogen and helium and they are called gas giants.

In the five-giant-planets model, either Saturn or Jupiter could have been responsible for ousting the hypothetical planet. To test this claim, astronomers in the latest study simulated the effect that a planetary interaction of that magnitude would have on the moons of Jupiter and Saturn. Even a single encounter could significantly alter the orbits of the satellites.

The moons studied were Jupiter's Callisto and Saturn's Iapetus because they are currently on wide orbits and are expected to have formed in a disk of gas and dust around their host planet, similar to how we think planets form around stars, so they were present during the instability phase of the Solar System. They assumed that Iapetus was formed in the same disk as Saturn's other satellites. This assumption is plausible but not certain: Iapetus' formation is still somewhat of a mystery.

According to the research, which is published in the November 1 issue of The Astrophysical Journal, Jupiter is significantly more likely to have ejected the ice planet than Saturn, as the orbit of Callisto in the simulation resembles the actual orbit of the satellite. The probability that Saturn is the culprit is only 1%. 

Ryan Cloutier, the lead author of the study, told IFLScience: “I think numerical simulation studies such as ours are important for developing an understanding of the evolution of the early Solar System. We know about so many objects in the Solar System today and they provide us with constraints on what the Solar System may have looked like billions of years ago. The more studies like ours that are performed and published, the closer we get to achieving a complete understanding of how our solar system formed and got to be the way it is today.”

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