A team of international scientists centered at Durham University, UK, has done research suggesting that a giant collision by an object about twice the size of Earth caused Uranus to tilt on its side and might also explain why the planet is so cold.
These astronomers did the first high-resolution computer simulations showing varying massive collisions with the ice giant planet to try to figure out how the planet might have tipped onto its side. Via these simulations, they found that a massive object, probably a protoplanet made of rock and ice, smashed into Uranus about 4 billion years ago during the solar system's formation, and likely the caused the planet's tilt.
“Uranus spins on its side, with its axis pointing almost at right angles to those of all the other planets in the solar system. 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," says Jacob Kegerreis, a PhD researcher at Durham University's Institute for Computational Cosmology. “We ran more than 50 different impact scenarios using a high-powered super computer to see if we could recreate the conditions that shaped the planet’s evolution. Our findings confirm that the most likely outcome was that the young Uranus was involved in a cataclysmic collision with an object twice the mass of Earth, if not larger, knocking it onto its side and setting in process the events that helped create the planet we see today.”
But if such a violent collision occurred, how did the planet manage to keep its atmosphere? In other occurrences, such a collision would have caused the planet's atmosphere to be spewed off into space. The simulations explain this by showing that an impact object striking at an angle, a glancing blow, could have enough strength to change Uranus's tilt, but the planet would still maintain it's atmosphere.
Such a collision could also explain the formation of Uranus's rings and moons. When the big smash took place, rocky and icy matter would have been jettisoned from both the bodies, and this matter would have been captured by gravity into orbit around Uranus. Eventually this matter would coalesce into moons, and may have altered the rotations of any pre-existing moons Uranus had.
Uranus isn't just tilted onto its side. Its magnetic field is off-center, as well, and if the collision caused lumps of rock to form lopsided within the planet, it provides a reasonable explanation for this.
How can a collision explain Uranus's extremely cold temperatures? If debris from the impactor formed a thin shell near the edge of the planet's ice layer, it could trap the heat emanating from the planet's core. Because the heat is prevented from escaping, the outer atmosphere would not be affected by it, and this can explain the super cold -357 degree Fahrenheit temperature of the outer atmosphere (as a comparison, Neptune is further from the sun than Uranus is, and its lower temperature is -360 degrees Fahrenheit, with it's average temperature actually being warmer than Uranus's at -353 degrees [via Cal Tech/Cool Cosmos]).
Luis Teodoro from the BAER/NASA Ames Research Center, who co-authored the research paper, says, "All the evidence points to giant impacts being frequent during planet formation, and with this kind of research we are now gaining more insight into their effect on potentially habitable exoplanets.”
All the findings of the Durham team's research can be found in paper "Consequences of Giant Impacts on Early Uranus for Rotation, Internal Structure, Debris, and Atmospheric Erosion," in The Astrophysical Journal.
Melanie R. Meadors writes about science, magic, superheroes, and other nerdy things. She was part of the dual degree program for physics and astronomy at Northern Arizona University until she discovered that her evil super-genius powers were useful for writing fiction as well. She is a blogger and general b*tch monkey at The Once and Future Podcast. You can learn more at melaniermeadors.com