How A Cataclysmic Crash Caused Primordial Uranus’ Tilt
By Judith E Braffman-Miller
Strange things occurred long ago in our primordial Solar System. At that ancient time, there were turbulent, violent, clashes and crashes between primeval objects that created a veritable “cosmic shooting gallery” during our Solar System’s tumultuous youth–when newborn worlds collided around our young Sun. For decades, astronomers considered the bluish-green ice-giant planet Uranus to be a dull and featureless world, almost entirely devoid of the colorful bands and whirling storms that characterize the three other gaseous giant planets inhabiting our Solar System’s outer kingdom. Nevertheless, Uranus might really be the most interesting planet in our Solar System’s outer limits. In July 2018, a team of astronomers announced that their new study of this mistakenly “dull” greenish-blue world reveals that Uranus was smashed into by a massive protoplanet approximately twice the size of Earth, that caused the tragic giant planet to tilt, and that could also explain its frigid temperatures.
Like many embarrassed adults, Uranus has tried to keep its tumultuous youth a secret. However, the team of scientific detectives refused to let Uranus hide what occurred during its flaming youth from their prying eyes. The astronomers investigated how Uranus came to be tilted on its side, and what consequences a giant impact would have had on the mysterious planet’s evolution.
The team ran the first high-resolution computer simulations showing a variety of massive smash-ups between the green ice-giant and another object when our Solar System was young. The astronomers did this in order to discover how this truly weird world became the oddball that it is today.
The new research confirms an earlier study that showed Uranus’ tilted position was caused by a collision with a massive body–probably a young protoplanet composed of rock and ice–during the ancient formation of our Solar System about 4.56 billion years ago.
The new simulations also demonstrate that debris from the unlucky impactor could create a thin shell near the edge of Uranus’ ice layer and thus imprison the heat flowing out from the planet’s core. The capture of this internal heat could in part help to answer why the outer atmosphere of Uranus is much colder than it should be, according to the researchers.
A Mysterious And Misunderstood World
In the dim, cold realm of the four gaseous giant planets, far from the heat and light of our Sun, Uranus reigns in splendor surrounded by a retinue of tiny frozen moons of sparkling ice, and encircled by an elegant system of slender dark rings. Some planetary scientists have even suggested that beneath Uranus’ thick gaseous atmosphere, it hides a hidden heart of diamond. In the frigid twilight of our Solar System’s outer regions, Uranus casts a haunting, eerie greenish shadow upon the chaotic, jumbled ice of its small, frozen, shattered moon, Miranda. This tiny moon may have been blasted into by a crashing object very long ago, only to be pulled back together again. This second time around, Miranda has been reborn as a sphere of bizarre, chaotic icy chunks, held together by the relentless pull of merciless gravity.
Uranus, the seventh planet from our Sun, is categorized as an ice giant planet, along with the dark blue, beautiful, and banded Neptune–the eighth major planet from our Sun. In our Star’s outer kingdom, the two gas-giants–Jupiter and Saturn–are much larger than the ice giants. This enormous duo are giant balls of gas, perhaps containing only very small solid cores beneath their extremely heavy and thick atmospheres. Some planetary scientists have even proposed that Jupiter and Saturn may have no solid surfaces at all, and that they are really all gaseous atmosphere! Uranus and Neptune, the smaller–though still enormous–ice giants, are thought to contain larger rock-ice cores than Jupiter and Saturn, and relatively thin atmospheres.
Uranus and Neptune are similar in composition, and both ice giants have a different bulk chemical make up than Jupiter and Saturn. Uranus’ atmosphere is similar to that of the two gas giants in its primary composition of hydrogen and helium, but it contains more “ices” such as ammonia, methane, and water ice, along with trace quantities of other hydrocarbons. Uranus also possesses the coldest planetary atmosphere in our Solar System, with a minimum temperature of -371 degrees Fahrenheit. It also has a complicated, layered cloud structure, with water composing the lowest layer of clouds, while methane makes up the uppermost cloud layer. The hidden interior of Uranus is primarily composed of ices and rock.
Like Jupiter, Saturn, and Neptune, Uranus sports a system of rings, a magnetosphere, and many moons. However, the Uranian system sports a unique configuration among those of the other planets because of its bizarre axis of rotation, that is actually tilted sideways, nearly into the plane of its orbit around our Star. Uranus’ north and south poles, as a result, are situated where most other planets have their equators. In 1986, images derived from the traveling Voyager 2 revealed Uranus to be an almost featureless world in visible light–bereft of the lovely bands of clouds or whirling storms associated with the three other giant planets. However, observations of Uranus obtained from Earth revealed seasonal changes and an increase in weather activity as Uranus approached its equinox in 2007. Ferocious wind speeds can roar at 560 miles per hour.
Uranus is the only major planet in our Sun’s family whose name is derived directly from a figure in ancient Greek mythology. The name Uranus is the Latinised version of the Greek god Ouranos. Like the other planets spotted in ancient times, Uranus is visible to the unaided eye. However, it was not recognized to be a planet by ancient sky watchers because of its slow orbit and faintness. The German-born British astronomer Sir William Herschel (1738-1822) announced its discovery as a planet on March 13, 1781, thus expanding the boundaries of our Solar System for the first time in history and making Uranus the first planet discovered with a telescope.
Many astronomers are almost certain that the ice giant duo, Uranus and Neptune, were not born where they are now seen–at 19 and 30 astronomical units (AU) from the Sun, respectively. One AU is equivalent to the mean distance between the Earth and Sun, which is about 93,000,000 miles. The accretionary processes, that formed full-sized planets in our new-born Solar System, worked much more slowly farther from our Star, where the two ice giants are now located. The whirling, swirling protoplanetary accretion disk, made up of gas and dust, was much too thin in this distant region to enable planets of this enormous size to form as quickly as they would in the much warmer and denser regions of the accretion disk closer to the heat and light emanating from our Star.
Therefore, astronomers have a hard time explaining how Uranus and Neptune could have grown to be giants if they had both been born in their current locations far from our Sun. This is because the protoplanetary accretion disk would have dissipated long before worlds of this enormous size had a chance to form in this distant, cold twilight region. Instead, many astronomers propose that the cores of Uranus and Neptune formed closer to the young Sun and later traveled to their current, more distant, locations.
The primordial Solar System was a violent place, where objects of all sizes, both rocky and icy, continually blasted into each other, shattering one another into fragments. However, on the brighter side, these ancient objects sometimes met up with each other gently enough to meld themselves together, thus creating larger, and larger, and larger bodies. These growing objects went from pebble-size, to mountain-size, to planet-size. Occasionally, in this “cosmic shooting gallery”, migrating planets hurled some other planets into other regions of our Solar System–where they wreaked havoc. For this reason, some astronomers propose it is possible that the cores of Uranus and Neptune formed in the same region, closer to our Sun, as the gas-giants Jupiter and Saturn. The duo of ice-giants then migrated outward to more distant orbits as a result of gravitational interactions.
Indeed, there are definite signs that some very weird things happened long ago in the outer Solar System. For example, Uranus displays that bizarre orbit which is tilted much more than any other planet in our Sun’s family.
Clearly, the formation history of our Solar System’s ice-giant duo is one of the most important unanswered mysteries in astronomy. It is well-known, however, that giant collisions happened frequently during the formation of these outermost giant planets, and this provides an important clue about their origins.
Dr. Jacob Kegerreis, lead author of the 2018 study, explained in a July 3, 2018 University of Durham Press Release that “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. 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 on its side and setting in process the events that helped create the planet we see today.” Dr. Kegerreis is an astronomer at Durham University’s Institute for Computational Cosmology.
But, how did Uranus manage to keep a hold on its atmosphere when a violent collision might have been expected to send it screaming off into interplanetary space?
According to the new simulations, this question may at last be answered. According to the new research the impact object struck a grazing blow on the young planet. The collision was devastating enough to affect Uranus’ tilt. However, the planet was still able to keep a grip on most of its atmosphere.
The new research could also help explain another mystery having to do with the formation of Uranus’ rings and moons. Scientists have long suspected that Uranus was knocked over on its side as a result of a catastrophic collision with a primordial protoplanet. However, the problem with this theory is that it cannot explain why Uranus’ many moons spin sideways, relative to their orbital planes, almost precisely matching their planet’s weird 98-degree tilt. In order to envision just how weird Uranus’ tilt is, by comparison, Earth’s spin axis is tilted 23 degrees; Jupiter’s 3 degrees; Saturn’s 26 degrees, and Neptune’s by 29 degrees. A single collision would have left any moons–then accreting out of a ring of materials orbiting Uranus–spinning in the opposite direction from they way they currently spin.
The supercomputer simulations of the 2018 study provide an answer to this intriguing mystery, suggesting that the impact could have shot rock and ice into orbit around Uranus. Afterwards, the rock and ice could then have melded together to create the planet’s inner moons, and perhaps even change the rotation of any pre-existing moons already in orbit around their ice-giant planet.
Uranus is similar to the most common type of exoplanets, which are alien planets that orbit a star beyond our own Sun. The team of astronomers hope that their findings will help shed new light on how these strange worlds evolved, and also contribute to our scientific understanding about their chemical composition.
Study co-author Dr. Luis Teodoro, of the BAER/NASA Ames Research Center in Moffett Field, California, commented in the July 3, 2018 University of Warwick Press Release that “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.”
The findings are published in the July 2, 2018 issue of The Astrophysical Journal, under the title Consequences of Giant Impacts on Early Uranus for Rotation, Internal Structure, Debris, and Atmospheric Erosion.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, magazines, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others some of the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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