Astronomers believe they have discovered the reason why Uranus must have fallen over at some point in the past.
The peculiar manner Uranus spins on its side is one of the strangest events in our solar system. All the other planets revolve with their faces upward, so that is puzzling. What may have happened to separate Uranus from its neighbour Neptune, which originated at around the same time and under similar conditions, and to make Uranus so unique?
According to traditional wisdom, Uranus was turned on its side by a series of collisions with some of the countless planetesimals that passed through the region shortly after the solar system originated.
This argument has a flaw in that Neptune was unharmed by the same circumstances. This suggests that Uranus’ peculiar behaviour was caused by some other mechanism. What, though, might it be?
Now, owing to the research of Melaine Saillenfest and colleagues at the Paris Observatory in France, we may have an answer. They believe that Uranus could have become turned on its side in another way. They assert that the tilting is explicable if Uranus formerly had a sizable, long-ago satellite whose orbit gravitationally interacted with the planet’s rotation in a way that gradually flipped the planet on its side.
First, some context. The gravitational interaction between planets and their satellites can be intricate and persistent, as astronomers have long known. Small satellites’ periodic gravitational nudges as they orbit can, in fact, have a major effect on their larger hosts.
The impacts can be greatly amplified when the nudges happen at a frequency that resonates with a feature of the host planet, especially when the satellite is gradually drifting away from its host.
The Moon is progressively moving away from Earth, eclipsing the planet by 1.5 inches (3.8 centimetres) every year, according to astronomers. The satellites that orbit Jupiter and Saturn are also moving, according to new measurements.
This gave Saillenfest and his associates the idea that perhaps Uranus had a similar event. Under these circumstances, gravitational nudges from a sizable satellite may have echoed with the precession of Uranus’ spin axis, causing the planet to slowly tip onto its side.
In order to ascertain the circumstances under which this would have happened, the scientists used Uranus to mimic the process.
As it turns out, a satellite that was only a thousandth of Uranus’ mass might have tilted the planet as it moved away from it over a distance of around ten times Uranus’ radius. The mean drift rate of the satellite must be equivalent to the Moon’s current orbital expansion, according to Saillenfest and colleagues, in order to achieve the tilting in less time than the age of the solar system.
According to the team’s calculations, the satellite’s orbit and the planet’s behaviour both become chaotic and unpredictable after the planet’s tilt exceeds 80 degrees, putting it in danger of colliding with Uranus.
When this occurs, however, Uranus’ behaviour stabilises and its spin locks at this incredibly odd, slanted angle. The fact that Uranus now lacks a large satellite, in contrast to Neptune, Saturn, and Jupiter, which all have large satellites like as Ganymede and Titan, is telling.
Spin made of fossils
That theory is intriguing and has a lot going for it. According to Saillenfest and colleagues, this is the first time that a single mechanism has been able to tilt Uranus and fossilise its spin axis in their final states without including a massive impact or other extraterrestrial occurrences.
It is not, however, a sure thing. The astronomers state that while the tilting’s necessary parameters “seem broadly conceivable,” it is still unclear whether Uranus could have supported a large primordial satellite susceptible to significant tidal migration.
A greater comprehension of the current motion and other characteristics of Uranus’ satellites could help shed additional light on this scenario. Much of this information regarding Saturn and Jupiter had to wait until the arrival of several orbiting spacecraft, including Galileo, Juno, and Cassini.
There has only been one spacecraft that has travelled alone to Uranus. In January 1986, Voyager 2 sped by as it left the Solar System. Additionally, no mission has been authorised despite preparations by several space agencies to deploy an orbiter.
Astronomers will have to settle for the ever-detailed observations from Earth and from the JWST until they are. According to Saillenfest and co., those data should be useful: “Our findings can provide a solid foundation for subsequent research.”