Friday, November 20, 2015

Unknown Planet Was Ejected from Our Solar System

This image of Jupiter in true color is the most detailed color portrait of Jupiter ever made. It was created using multiple images from NASA's Cassini spacecraft, which flew past the planet in December 2000 and snapped photos from 6.2 million miles away. Image credit: NASA/JPL/Space Science Institute

Our solar system could have had nine planets once, but one of them was kicked out, a new research shows. Astrophysicists at the University of Toronto have found that Jupiter may have booted out a gas giant about four billion years ago. “Many astronomers would argue that it's impossible to 'know' about such an event given that we can't watch it happen but we do have evidence for the existence of such a planet,” Ryan Cloutier of the University of Toronto, told astrowatch.net.

Planet ejections occur 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. However, earlier studies which proposed that giant planets could possibly eject one another did not consider the effect such violent encounters would have on minor bodies, such as the known moons of the giant planets, and their orbits.

Cloutier’s team decided to look at the trajectories of Callisto and Iapetus – two of the regular moons orbiting Jupiter and Saturn respectively. Using computer models, they investigated the likelihood of the moons having the same orbit as they do today if they'd been involved in a mass planetary ejection 4 billion years ago.

“The theory is that in order to re-create the current solar system, starting from just after its formation, it is statistically easier to do so if one includes an extra giant planet that is able to carry away excess orbital energy. That energy would otherwise go into the giant planets that we have today and make their orbits more dynamically excited than what we observe,” Cloutier said. “The fact that we don't have more than four giant planets in the present-day solar system implies that any additional giant planet must have been ejected long ago and taken that excess energy with it.”

By running thousands of simulations of the dynamics of the early solar system with an extra giant planet, the researchers constrained the mass of that planet to be approximately equal to the mass of Uranus or Neptune. Given that the evidence for the planet's existence comes from dynamical arguments, the only planet parameter worth constraining is its mass because its mass governs its gravity. 

“So we don't know about its specific composition but one might speculate that it would have to be similar to Uranus or Neptune seeing that it would have formed in the same location as those planets and has a similar mass. Its orbit is not very meaningful because it becomes unbound following an ejection. This means that you don't have an orbit any more, the planet instead becomes just a free-floating body,” Cloutier noted.

The study confirms that Jupiter is likely to have been responsible for the ejection. The computer model showed that there's about a 42 percent chance that Callisto would have its current orbit around Jupiter if it had been involved in the planetary ejection. Jupiter is more massive than Saturn meaning that a gravitational kick that Jupiter could provide should be stronger than what Saturn could provide. 

“We confirm this by using the current moon of Jupiter called Callisto. We say that if Jupiter had ejected the fifth giant planet, then this must be possible to do without excessively messing up the orbit of Callisto because we see Callisto today with a relatively unexcited orbit. One can do the same analysis for Saturn using its moon Iapetus. We found through dynamical simulations that it is possible for Jupiter to eject the fifth giant planet while maintaining a moon on an orbit that is reconcilable with the current orbit of Callisto. Meanwhile, it is difficult for Saturn to eject the fifth giant planet while maintaining a moon on an orbit that is reconcilable with Iapetus,” Cloutier said.

The hypothesis of the fifth giant planet being ejected by Jupiter is consistent with the orbit of Callisto. This is a promising test that the theory has passed, supporting the fifth giant planet's existence.

The existence of a fifth giant gas planet at the time of the solar system’s formation, was first proposed in 2011.

The new findings are reported in a paper titled "Could Jupiter or Saturn have ejected a fifth giant planet?" published in the Nov. 1 issue of The Astrophysical Journal.

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