Did fifth giant planet get the boot?

A violent tug of war between the outer planets may have resulted in the ejection of a fifth giant planet from the early Solar System.

Impression of a giant planet
Impression of a giant planet (NASA)

Our Sun’s family of worlds houses four rocky planets and four giant planets. But what if there were once five giant planets? This is a likely situation according to computer simulations performed by David Nesvorny of the Southwest Research Institute in Colorado.

He explored thousands of possible scenarios of the dynamics of the early Solar System and found that the present configuration of planets is much more likely to be reproduced by the simulations if there were initially five giant planets instead of four.

The fifth giant planet most likely formed between Saturn and Uranus. “Given that this planet had mass comparable to that of Uranus and Neptune, and was originally located beyond the orbit of Saturn, it is more likely that it looked just like Uranus or Neptune,” Nesvorny told Skymania News.

After planets form, they are not content to remain where they were born. They might migrate closer to or further away from their parent star. Migration occurs because of interactions between the young planets and the disc of gas and dust that engulfs them. Planets will migrate until they become locked in what is known as a “mean motion resonance.”

This means that there will be a ratio between the orbits of two planets. For example, for every 2 orbits that Pluto completes of the Sun, Neptune will do 3. Thus there is a resonance of 2 to 3 for this duo.

The gas and dust in a planet forming disc is short-lived. When the young star bursts into life, the stellar wind blows away the gas and dust in the disc that hasn’t been used up in planet formation. This leaves the planets which were previously in a comfortable resonance vulnerable and they can become unstable enough to start scattering off each other, wildly changing their orbits. Planets can then be evicted from the system or be being flung into a different orbit around the star.

All the computer simulations of the early Solar System were started assuming that planets were in resonances with each other. “If the non-resonant planets are too close to each other, they immediately start scattering each other,” explains Nesvorny. It is believed that Jupiter and Saturn were originally in a 3 to 2 resonance, where Jupiter did three orbits of the Sun for every two orbits of Saturn.

Prior to the fifth giant planet embarking on its journey into the unknown, it played a crucial role in the formation of our Solar System. “Before it gets ejected from the Solar System, the fifth planet has encounters with Jupiter and makes Jupiter jump,” says Nesvorny.

“This is believed to help the terrestrial planets. If Jupiter’s orbit evolved more smoothly, resonances would sweep through the inner Solar System, and the orbits of the terrestrial planets could become more eccentric, and be destabilized.” Thus if the fifth giant planet hadn’t been evicted then one of the terrestrial planets, including Earth, could have taken its place in outer space.

The recent discovery of free floating planets gives weight to this theory of a giant planet being ejected from the Solar System. Is one of these lone wanderers a former planetary neighbour of ours?


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By Amanda Doyle

I am an astrophysics postdoctoral research assistant at the University of Warwick. I obtained my PhD from Keele University in 2014 and my thesis title was "Spectral analyses of solar-like stars". My research involves refining stellar parameters with the aim of improving our understanding of both stars and planets. I completed my masters in astronomy at Swinburne University of Technology via the Swinburne Astronomy Online programme in 2010, and I obtained my degree in physics with astronomy from Dublin City University in 2008. When I'm not doing research, I like to write about all aspects of astronomy. I am a freelance science writer and I contribute to Astronomy Now, NASA's Astrobiology Magazine, BBC Sky at Night magazine, Skymania News, and Sen. I am also the editor of Popular Astronomy magazine.

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