With a tilt of 98 degrees, Uranus has the highest inclination in the solar system and spins almost exactly perpendicular to its orbit. Uranus was thought to have been turned on its side by a series of massive impacts during the planet's early development, but recent evidence points to a much less violent cause: a wandering satellite.

Except for Uranus, all of the planets in the solar system have orbital tilts that are less than 30 degrees. The entire Uranus system is turned on its side, which has an impact on the planet's rotation as well as the orbits of its moons and rings, which travel in an arc perpendicular to the planet's spin around the sun.

Since Neptune, a nearby ice giant, has a typical tilt despite the two likely having comparable formation histories, which makes Uranus' peculiar tilt all the more perplexing. How come Uranus has problems?

Long-standing theories among astronomers suggest that Uranus formed as a result of at least one massive impact. It's simple to picture: Uranus, then still in its protoplanetary stage, would have been knocked over by the proper collision at the right time, and the planet would never have been able to right itself before developing its system of planets and moons.

And this conclusion is supported by certain scientific findings. There are many big rocks around to cause mayhem because the solar system was a really tumultuous place when it first formed.

Astronomers have long hypothesised that Uranus formed as a result of at least one massive impact. It's not difficult to picture how Uranus may have been knocked down while it was still in the protoplanetary stage by the correct collision at the right time, and the planet could not have stood back up before developing its system of planets and moons.

Scientists have some supporting data for this theory. There are plenty of big rocks nearby to cause destruction because the solar system was a very violent place when it was young. Further evidence that the two planets once experienced different conditions is provided by Neptune, which exhibits minor distinctions including a different temperature and a collection of moons with unique traits (such Neptune's being much larger).

The solar system of the early universe was very different from the solar system of today. Particularly the huge planets presumably developed substantially closer to the sun and much closer to one another. Over time, interactions between them and with roving planetesimals caused Uranus and Neptune to move outward the most. (In fact, some solar system models suggest that a fifth, large planet may have been ejected during all of this movement and may exist.)

Moons were there when the massive planets formed, but as the planets moved, the moons were rearranged. Some planets lost moons while others gained them as a result of the complex gravitational dynamics at play.

So Uranus might have been born with a large moon or swiftly captured one. Additionally, if the moon had been large enough, it might have begun tampering with the planet's spin.

Most likely, Uranus began with a tiny, haphazard tilt. That tilt will eventually precess, as scientists refer to it, causing the direction of the planet's rotation to sway like a huge top. (Earth behaves similarly.) A moon typically doesn't give a damn about the tilt precession of its planet. However, it is feasible for a moon to become locked into a resonance pattern in which the time required for precession coincides with a large number of lunar orbits.

The precession is strengthened by this resonance, which enables the moon's gravitational pull to be felt softly on the planet. Over the span of millions of years, that tilt worsens, like there's an invisible string tied to the top of the world. The moon would gradually approach the planet as this continued.

The scientists discovered that if Uranus had formerly had a moon large enough, it would have been able to pull the planet's tilt over 80 degrees within a few hundred million years. The satellite would subsequently complete the task by colliding with Uranus, which would fix the planet's tilt at its current angle.