Astronomers have a catastrophic new theory to explain Uranus’ tilt

There are many of interesting things about Uranus. Its season lasts as long as its day, it is the second-least dense planet, and has a cluster of 27 moons.

But perhaps the most puzzling fact about Uranus is that it’s the only planet that lies on its side — at least relative to its orbital plane.

The most common suggestion for why the planet is tilted 98 degrees on its axis is that it was struck by a series of large impacts early in the formation of the solar system.

However, new studies by a team at the Sorbonne point to a possible alternative explanation – Uranus used to have another, larger moon that pulled it on its side and then struck the planet itself.

error in the model

Recently, there has been a flurry of research into the orbital mechanics of gas giants. This excitement helped expose a number of flaws in our current model of what happened to Uranus. The most obvious flaw is highlighted by a resemblance between Uranus and one of its neighbors – Neptune.

Neptune’s axial tilt is only 30 degrees (still a lot by planetary standards), but nowhere near the level of Uranus. However, both planets have similar spin rates. Impacts large enough to throw an entire planet on its side would also have a fairly significant impact on its spin rate.

Therefore, it is not very likely that accidental minor impacts caused one planet to be nearly 70 degrees more than the other without affecting their spin rates.

Similar spin rates indicate much “smoother” processes that took place over eons but dramatically affected the properties of the two gas giants.

The real reason for the inclination?

Scientists have already conjectured the “Great Migration” of the outer planets through the inner Solar System and its cloud of early planetessimals, which was likely the cause of events such as the late heavy bombardment responsible for many of the craters visible on our moon.

That got researchers thinking — axial tilt in gas giants is typically detected early in their formative years. The comparatively small axial tilts of Jupiter, Saturn, and Neptune can all be explained by unique phenomena that occurred after the planets were fully formed. However, the higher tilt of Uranus is different.

The scientists assume that it could have happened in two phases. First, a relatively large moon, about 0.03 percent the size of the planet itself, got stuck in a resonant loop with its host planet, slowly pulling the planet onto its side.

Because of complex orbital mechanics, well explained in a recent article published by the authors on preprint server arXiv, a large enough moon could interact with its parent planet’s gravitational pull in a way that would result in it being attracted to its side .

The second phase, however, is the more intriguing of the two, at least from the standpoint of astronomical catastrophe. Currently, Uranus has 27 known satellites, none of which fit the description required to produce such orbital resonances. So where did it go? Most likely, it crashed into the planet itself after an orbital dissonance caused it to destabilize its orbit.

There are still some problems inherent in this two-stage theory – for example, would the current harvest of Uranus moons have survived a gigantic moon making its way towards its eventual destruction?

But based on models run by the Sorbonne team, there appears to be a plausible physical scenario in which Uranus is tilted at the correct orbit and spin rates, and an impact with the planet destroys the companion satellite that caused the tilt.

It’s an intriguing idea that warrants further study. Unfortunately, it will likely be a long time before we find any other physical evidence that may or may not indicate the theory’s validity.

But at least it’s an interesting thought exercise to imagine Uranus being slowly pulled away from one of its moons over billions of years, only to eventually hit that moon. Such a dramatic story would make the planet even more interesting.

This article was originally published on universe today by Andy Tomaswick. Read the original article here.