🔭 A giant planet now vanished may have saved the moons of Jupiter and Uranus

Computer simulations have reconstructed 122 possible scenarios for the evolution of the solar system, 3 to 4 billion years ago. Researchers examined how the orbits of the giant planets shifted, and especially how these changes affected the moons of Jupiter and Uranus. These moons are indeed true archives of the solar system's history, as they have remained relatively stable around their planets.


In less than 15% of the simulations, Jupiter's moons survived the era of planetary migration. For Uranus's moons, the rate drops to 9%. Worse still, scenarios favorable to one set were often unfavorable to the other. The probability that both moon systems survive together is only 1%. Yet they are still here today, indicating a rare event.

Researchers identified two scenarios where the moons of both planets survive. Both of these scenarios include an extra planet at the beginning: a fifth ice giant. This now-vanished world would have altered the migrations of the other planets just enough to avoid overly violent disturbances. Jupiter would have crossed paths with this ice giant at about 7 million kilometers (4.3 million miles), enough to disturb its moons, but not to destroy them.

This ice giant, now expelled from the solar system, likely wanders somewhere in interstellar space. Its initial presence changed the course of events, making the migration period shorter and less brutal for Uranus. Uranus experienced at least two major upheavals: an impact that knocked it on its side, and then the migration of the giants. Yet its moons held on, despite some collisions.

Of course, these simulations are not perfect. The researchers note that they involve an element of randomness, and none exactly reproduces what happened. But they offer a solid overall picture: the presence of another world, now lost, likely saved the moons we observe today.