There is a lot of interesting things on Uranus. Its season lasts as long as its day, it is the second least dense planet, and it has a collection of 27 moons.
But perhaps the most puzzling fact about Uranus is that it’s the only planet that borders its side — relative to its orbital plane, at least.
The most common suggestion for why the planet is tilted 98 degrees on its axis is that it was hit by a series of large impacts early in the formation of the solar system.
However, new studies from a team at the Sorbonne point to a potential alternative explanation – Uranus used to have another larger moon that pulled it sideways and then impacted the planet itself.
The flaws of the model
A flurry of research into the orbital mechanics of gas giants has been underway lately. This burst helped point out a series of flaws in our current model of what happened to Uranus. The most obvious flaw is evidenced by a similarity between Uranus and one of its neighbors – Neptune.
Neptune’s axial tilt is only 30 degrees (still a lot by planetary standards) but far from the level of Uranus. However, the two planets have similar rotation speeds. Impacts large enough to flip an entire planet would also have a fairly large impact on its rotational speed.
It is therefore unlikely that random minor impacts would have caused one planet to be nearly 70 degrees warmer than the other without affecting their rotational velocities.
Similar rotational speeds indicate much “smoother” processes that took place over eons, but which significantly affected the properties of the two gas giants.
The real reason for the tilt?
Scientists have previously hypothesized the “great migration” of the outer planets through the inner solar system and its cloud of early planetesimals, which was likely the cause of events such as the late heavy bombardment, which is responsible for many visible craters on our planet. Moon.
This got the researchers thinking – the axial tilt of gas giants is usually established early in its formative years. The relatively small axial tilts of Jupiter, Saturn, and Neptune can all be explained by one-time phenomena that occurred after the planets were fully formed. However, Uranus’ higher tilt is different.
Scientists posit that this could have happened in two stages. At first, a relatively large moon, about 0.03% the size of the planet itself, got stuck in a resonance loop with its host planet and slowly pulled the planet aside.
Due to the complex orbital mechanics, which is well explained in a recent post the authors posted on preprint server arXiv, a large enough moon could interact with its parent planet’s gravitational pull in a way that would cause it to be pulled sideways.
However, the second phase is the more intriguing of the two, at least from the perspective of astronomical catastrophe. Currently, Uranus has 27 known satellites, none of which fit the description required to create such orbital resonances. So where did he go? Most likely, it crashed into the planet itself after orbital dissonance caused it to destabilize its orbit.
There are still some problems with this two-step theory – for example, would Uranus’ current crop of moons have survived a gigantic slithering towards its ultimate destruction?
But based on the models the Sorbonne team ran, it appears there is a plausible physical scenario where Uranus finds itself tilted with the correct orbit and rotation rates, and an impact on the planet destroys the satellite that caused this tilt.
It’s a fascinating idea worth exploring. Unfortunately, it will probably be a long time before we find other physical evidence that could indicate the validity of the theory or not.
But at the very least, it’s an interesting thought exercise to imagine Uranus slowly being pulled by one of its moons for billions of years, only to see that moon crash into it. Such a dramatic story would make the planet even more interesting.