One of our planets may be missing, and it could explain why the solar system looks the way it does

By Paul Arnold, Phys.org

Edited by Sadie Harley, 

Reviewed by Robert Egan

Our solar system has two ice giants, Uranus and Neptune, but there may have been a third. According to a new study published in the journal Icarus, this extra world might have triggered a violent planetary shuffling billions of years ago that could have disrupted some of Jupiter's and Uranus's moons and possibly led to the formation of others.

Near misses

Shortly after the planets formed between 4 billion and 4.5 billion years ago, the outer solar system experienced a period of extreme chaos known as the Nice Model instability. During this era, the orbits of Jupiter, Saturn, Uranus, and Neptune shifted dramatically, becoming highly unstable.

Close encounters were common, with the giant planets coming incredibly near one another and pulling each other with powerful gravitational forces. This chaotic movement eventually resulted in the planets settling into their current positions. But scientists have long wondered how their regular moons survived this violent shakeup.

To get a handle on what was happening, particularly to the moons of gas giant Jupiter and ice giant Uranus, a team of researchers analyzed 122 computer simulations of the early solar system. They were chosen out of thousands of options because they reproduced key features of the modern outer solar system.

The scientists used software that tracked the complex gravitational interactions between the planets, moons, the sun, and passing space rocks over millions of years. They tested versions of history that started with five or six giant planets. That's because the current version of the Nice Model includes scenarios with one or two additional giants that were later ejected from the solar system.
Cosmic hit and runs

The computer models showed that the survival rate of the moon systems of both Jupiter and Uranus was low. As the researchers note in their paper, "We find that the survival probability for the Jovian and Uranian moon systems are both less than 15%." Of all the scenarios tested, only one had the original moons and planets survive together.

When the planets got too close to Uranus, the immense gravity almost guaranteed the destruction of its moons. But instead of flying off into space, they smashed into each other at high speed. This resulted in a massive field of ice debris that, over time, clumped back together. The research team believes this could explain how Uranus's moon Miranda was formed.

But this wasn't the only time the simulations revealed a violent history for its moons. "Our results indicate that Uranus's moons were likely perturbed to the point of collisions at least twice: as a result of both the impact that tilted the planet and the giant planet instability."

While the paper paints a fascinating picture of the early solar system, the researchers acknowledge that simulations cannot capture every detail and that more modeling will be needed to determine the fate of individual moons.


Written for you by our author Paul Arnold, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.