Uranus’ strange tilt was caused by a missing moon

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Hundreds of millions of years ago, a large moon caused Uranus to tilt by more than 80 degrees with its gravitational pull, before disappearing, crashing into the icy giant and fixing its current position at 98 degrees. In this way, the entire Uranus system was turned to one side, in an arrangement that does not exist in the rest of the planets of the Solar System.

According to new international research, recently published in the journal Astronomy & Astrophysics, Uranus’s anomalous tilt, unique in the Solar System, could have originated when the planet rapidly captured a massive moon early in its history. The dimensions of the natural satellite, then disappeared when it finally collided with the planet, would have generated the gravitational influence enough for Uranus to acquire its strange inclination.

The “green dog” of the Solar System

All the planets in the Solar System have orbital inclinations of less than 30 degrees, with the sole exception of Uranus, which currently has a 98 degree tilt. Not only does the planet turn on its side, affecting its rotation, but also its rings and moons acquire anomalous positions, orbiting around the planet perpendicular to the movement of the star around the Sun.

The mystery increases if we take into account that the other icy giant in our cosmic neighborhood, its companion NeptuneIt has a normal inclination. However, everything indicates that Uranus and Neptune shared similar formation histories, within the framework of an outer zone of the Solar System plagued by constant collisions. How can it be explained, then, that Uranus has acquired such a pronounced inclination and Neptune has not?

Until now, the most successful hypothesis among astronomers indicated that one or more giant impacts during the formation of Uranus determined its inclination. Scientists thought that a well-timed crash could have delivered enough energy to push Uranus while it was still in its protoplanetary stage. Subsequently, the planet would never have been able to recover and thus developed its entire system.

an invisible rope

Now, a new study involving scientists from the Sorbonne Université, the University of Maryland College Park, the University of Pisa and the Université Côte d’Azur develops a different theory to explain Uranus’ strange tilt and its opposite spin. According to these specialists, an ancient and already disappeared moon It is the one that caused the mysterious phenomenon and left Uranus in its current situation.

according to a Article published on, Uranus once had a large enough moon that, from the beginning of its history and until a few hundred million years ago, it was able to tilt the planet to more than 80 degrees, thanks to a complex gravitational pull mechanism. To finish its job, the satellite crashed into Uranus, blocking the tilt of the planet and leaving it at its current value of 98 degrees.

In addition, the scientists indicated that although Uranus already exhibited a random tilt from the period of its formation, the huge missing moon fulfilled the role of a “invisible rope” attached to the top of the planet. Over the course of millions of years, the tilt of Uranus it began to go deeper and deeper, as the “gravitational rope” tightened and pulled in the opposite direction. As this happened, the moon’s orbit grew closer and closer to the planet, until it finally collided with the icy giant.


Tilting Uranus via the migration of an ancient satellite. M. Saillenfest, Z. Rogoszinski, G. Lari, K. Baillié, G. Boué, A. Crida, and V. Lainey. Astronomy & Astrophysics (2022). DOI: Preliminary version on arXiv:


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