Jupiter Possibly Expelled a Rival Gas Giant

Keenan Thungtrakul/Staff Reporter

Scientists believe that there may have been a fifth gas giant planet that existed when the Solar System was still forming. If this planet actually existed, where is it now? A gravitational interplay between that planet and Jupiter could be the reason why it isn’t among the current gas giants orbiting the Sun. Astronomer David Nesvorny of the Southwest Research Institute in Boulder, Colorado, originally proposed the theory in 2011. In he suggests that a fifth gas giant planet existed during the formation of the Solar System. He points to a peculiar group of asteroids in the Kuiper Belt as evidence. The asteroids seem to always stick together and orbit the sun like a planet would. This so-called “kernel” is thought to be the remnant of some extra planet, an orphaned baby in a cold sea of rocks beyond Pluto. The kernel may have been thrown out of the solar system as a result of planetary ejection, where one planet’s gravity accelerates another planet to the point that it escapes the gravitational pull of the parent star.
Nesvorny used a computer simulation to go back 4 billion years to the formation of the planets. In the model, the bodies that made up the kernel were caught in Neptune’s gravity as it moved away from the sun to settle in its current orbit.

According to Nesvorny’s calculations, when it was 2.6 billion miles from the sun, approaching its current orbit, Neptune jumped 4.7 million miles farther from the sun, freeing the kernel to fly off into the orbital position it occupies today.

In explaining why Neptune would make such a drastic jump in its migration to its current orbit, Nesvorny says that only a gravitational field as big as that of a planet would be enough to pull Neptune further away from the sun. He adds that none of the other gas giants were responsible since their orbits never interacted with Neptune’s in a fashion as seen in the model.

In a new study, astrophysicists at the University of Toronto point to Jupiter in explaining the ejection of the unknown planet that would have had a strong enough gravitational field to force Neptune further out during its migration. Previous investigations into the possibility of giant planets ejecting each other did not consider the effects of such violent encounters with smaller bodies, such as the moons of the giant planets. Ryan Cloutier, lead author of the study concerning Jupiter, worked with his colleagues to create computer simulations of two particular moons, Jupiter’s moon Callisto and Saturn’s moon Iapetus. The simulations show the current trajectory of each moon, which was used to measure the likelihood that each moon produced its current orbit as a result of the host planet’s ejecting of the hypothetical fifth planet.

Such an ejection would have caused a significant disturbance in the moon’s orbit. The results of the simulations show that Saturn would not have been able to eject the planet since the disturbance would cause too abrupt of a shift in the orbit of Iapetus, a shift that is difficult to reconcile with its current trajectory. Jupiter, on the other hand, is capable of ejecting the fifth planet while retaining a moon with the orbit of Callisto. Therefore, scientists have reached the conclusion that Jupiter likely expelled another planet from the Solar System.