Ancient Uranus may have been struck by a protoplanet with twice the mass of Earth
According to the results of a new study, Uranus may have survived a dramatic collision with a protoplanet twice or even three times the size of Earth, roughly four billion years ago. The cataclysmic event would explain a number of mysteries surrounding Uranus, including the reason for the dramatic tilt of the gas giant and the formation of its moons.
Uranus is arguably one of the most underrated planets in our solar system, and for good reason. It's probably the most boring to look at, and, because of its unfortunate name, has been made the butt (pun intended) of countless terrible jokes. The last point wasn't helped when, recently, it was confirmed via spectroscopic analysis that the gas giant smells like rotten eggs.
But look past its outwardly dull visage and Uranus really has a lot going for it. For example, Uranus has its own ring system, and plays host to an extended family of fascinating moons, many of which are named for Shakespearean characters.
It also has the weirdness factor. Case in point – Uranus is the only planet in our solar system that spins on its side. Astronomers believe that the cause of Uranus' unusual orientation is most likely rooted in the formative period of our solar system.
The solar system coalesced some four billion years ago in a period of incredible chaos and violence. During this time, the evolutionary paths of entire planets are thought to have been reshaped by cataclysmic collisions with huge impactors.
These collisions can fundamentally alter the characteristics of a planet. For example, the prevailing theory on the creation of Earth's Moon involves a dramatic rendezvous between our Blue Marble and a Mars-sized body known as Theia. The resulting debris from this collision slowly coalesced into the satellite that we know today, which continues to affect our planet, by, for example, slowing its spin and creating tides with its gravitational influence.
The newly released study sought to use cutting edge computer simulations to work out the consequences that a massive collision would have on the evolution of Uranus, and led to the quirky planet that astronomers observe today. Some of the more specific goals of the research were to shed light on why Uranus spins on its side, why its atmosphere is so surprisingly cold, and why its many moons orbit on the gas giant's unusual tilted plane.
"Uranus spins on its side, with its axis pointing almost at right angles to those of all the other planets in the solar system," says lead author Jacob Kegerreis, PhD researcher in Durham University's Institute for Computational Cosmology. "This was almost certainly caused by a giant impact, but we know very little about how this actually happened and how else such a violent event affected the planet."
The team used a powerful supercomputer to run over 50 different simulations involving a proto-Uranus being struck by a series of three impactors that were one, two, and three times the mass of the Earth, respectively.
The high-resolution simulations revealed that Uranus was likely struck by a young protoplanet that was at least twice the mass of the Earth, and comprised largely of rock and ice, similar in some respects to the rocky core of the gas giant itself.
The force of this collision would have been sufficient to knock the planet onto its side. The results of the simulations also suggest that if the impactor had struck Uranus with a glancing blow, the gas giant would have been able to hold onto the majority of its atmosphere – in excess of 90 percent in some scenarios – rather than having it blown off into space.
The study could also help solve a long-standing mystery regarding the extremely low temperature, -216 °C (-357 °F), of Uranus' atmosphere. It is possible that debris from the impactor could have formed a thin shell near the planet's ice layer, which would have the effect of preventing heat from the planet's core from transferring to the upper atmosphere.
The computer collisions also showed that the impact between a youthful Uranus and a protoplanet could have thrown significant quantities of rock and ice into orbit, which would have settled around the tilted plane. From this debris, the gas giant's moons could have coalesced, or the influence of the material could have altered the orbit of existing moons, bringing them into line.
Debris from the impact could also explain the unusual nature of Uranus' off-axis magnetic field, which, according to the study, could be distorted by unevenly distributed lumps of molten ice and lumps of rock deposited inside the planet during the chaotic rendezvous.
The research is not only useful in expanding astronomer's understanding of our own solar system. Uranus shares many characteristics with the alien worlds frequently discovered by orbital observatories such as NASA's Kepler Space Telescope. Astronomers can use the insights gleaned from the new study, and apply them to better understand the atmospheres and histories of those distant worlds.
So next time you're looking for a planet to take a cheap shot at, maybe pass on Uranus, its been through a lot … plus Jupiter could do with getting taken down a peg or two.
The findings are detailed in a new paper published in The Astrophysical Journal and one of the computer simulations conducted by the team can be seen below.
Source: Durham University