There’s Something New In The Heavens Named “Synestia”
By Judith E Braffman-Miller
Scientific detectives now have something new to look for in the mysterious heavens–a doughnut shaped body composed of vaporized molten rock that has been named synestia by its discoverers. This new and strange beast inhabiting the celestial zoo is believed to form when planet-sized objects smash catastrophically into one another with high energy and angular momentum, and their discovery opens up a new window of fascinating ways for astronomers to think about how Earth’s Moon was born. Furthermore, these objects may also eventually be detected in other planetary systems circling stars beyond our own Sun. These primordial collisions were so violent that the resulting bodies melted and partially vaporized–ultimately cooling and solidifying to create newborn, (nearly) spherical planets, such as those that exist in our Solar System today. Planetary scientists described their recent study concerning these celestial “doughnuts” in a paper published in the May 22, 2017 issue of the Journal of Geophysical Research: Planets–also suggesting that at one point in its early history, our own Earth was probably a synestia, that was born as the result of a series of giant impacts in the chaotic “cosmic shooting gallery” that was our ancient Solar System.
Simon Lock, who is a graduate student at Harvard University in Cambridge, Massachusetts, and Dr. Sarah Stewart, who is a professor in the Department of Earth and Planetary Sciences at of the University of California, Davis, propose that hot, vaporized rock forms as planet-sized objects crash violently into each other.
Current theories of planet formation suggest that rocky planets such as Earth, Mars, and Venus formed early in the existence of our Solar System, as the result of catastrophic collisions of smaller objects into one another.
The Era Of Planet Formation
How did our Sun and its retinue of planets form? For centuries, scientists and philosophers alike have pondered this question. As telescopes became increasingly sophisticated, and as space missions were dispatched to explore regions that had never been seen before by human eyes, scientists have learned more and more about our Sun and its family of planets. This scientific knowledge has provided new and intriguing clues to how it might have occurred long ago.
For centuries, humanity could only observe the end result of planet birth, not the process itself. Scientists had no other examples to observe and study. Even with the knowledge acquired about our own Solar System, astronomers could only wonder–from a distance–if there existed other solar systems beyond our Sun that may have formed like our own.
According to current scientific understanding, a star is born–along with its retinue of planets, moons and smaller objects–when a dense blob, composed of gas and dust, collapses under the pull of its own gravity. This dense blob is embedded within the undulating, billowing folds of one of the many beautiful, dark, and cold giant molecular clouds that haunt our Milky Way Galaxy. As gravity tugs material together in the collapsing cloud, pulling it ever closer, and closer, and closer together, the center of the cloud gradually grows more compressed. As a result, it grows hotter, and hotter, and hotter. This extremely hot, dense core eventually becomes the birthplace of a new, sparkling baby star.
In the meantime, movement within the collapsing cloud, causes it to stir itself up. As the cloud becomes extremely compressed, a large portion of it starts to rotate in the same direction. The rotating cloud finally flattens out into a pancake-like disk that grows ever thinner as it continues to spin. Imagine the way that a spinning blob of dough flattens out as a chef shapes it into a pizza. These circumstellar or protoplanetary accretion disks are destined to become the strange birthplace of baby planets.
Protoplanetary accretion disks contain large amounts of nutritious gas and dust that feed growing, newborn protoplanets. As the disk whirls, the material within it travels around the young star. By the time the fiery stellar baby has reached what is termed the T Tauri phase of its development, the extremely hot, massive surrounding disk has become much thinner. A T Tauri is a stellar toddler–an extremely active variable star of the tender age of only 10 million years, or so. These stellar tots sport large diameters that are several times greater than that of our middle-aged Sun today. However, the stellar youngster is still in the process of shrinking. Sun-like stars, unlike human toddlers, shrink as they grow up. By the time the fiery tot has reached the active T Tauri stage of its development, less volatile materials have begun to condense near the center of the encircling dusty disk, forming very fine and sticky dust motes. The delicate, fragile particles of dust contain crystalline silicates.
The sticky little dust motes bump into one another and then “glue” themselves to one another within the dense environment of the protoplanetary accretion disk. As these tiny clumps of dust orbit within the disk, they sweep up surrounding material, growing bigger and bigger. In this way, increasingly larger and larger objects form–from pebble-size, to boulder-size, to mountain-size, to moon-size, to planet-size. The gravity of boulder-sized and larger chunks starts to pull in yet more dust and other clumps. The larger these conglomerates become, the more material they pull in with their gravitational grip, and the bigger they get. Ultimately, these growing chunks evolve into what are termed planetesimals–the building blocks of planets.
The inner portion of the accretion disk, at this stage, contains mostly rocky material. This is because much of the original gas has probably been devoured and cleared out by the hungry Sun-like star. This results in the formation of smaller, rocky planetesimals close to the stellar tot. In contrast, within the outer region of the disk, more gas lingers, as well as ices, that have not yet been vaporized by the hot, active, growing star. This additional material enables the planetesimals that form farther from their star to collect more material and evolve into gas and ice giant planets. The banded behemoth Jupiter, and the somewhat smaller, beautiful ringed planet Saturn, are the two gas giant planets inhabiting our own Solar System. The ice-giant denizens of our Sun’s family, Uranus and Neptune, are smaller than the gas giants–but, nevertheless, enormous when compared to the smaller planets of the inner Solar System. Uranus and Neptune have larger cores and thinner gaseous envelopes than the gas-giants, Jupiter and Saturn.
As each planetesimal in the young planetary system grows larger, it starts to clear out the material that is in its path, gobbling up nearby, lazily-moving rubble and gas while gravitationally hurling other material out of its way. Ultimately, the debris in its path is consumed, and as a result thins out. At this point, the planetesimal has a relatively clear lane as it travels around its young parent-star.
Planetesimals represent an abundant population inhabiting young solar systems. Literally, hundreds of these planetary building blocks are forming simultaneously–and, of course, they eventually bump into one another. If their paths converge at just the right time and they are traveling fast enough relative to one another, they blast into each other in a violent head-on smash-up. The collision hurls debris here, there, and everywhere. However, if the planetesimals wander towards each other more slowly, gravity can behave much more gently, tenderly drawing them together. The two colliding planetesimals do not smash each other to pieces. Instead they join together to create a larger object. If the duo of colliding participants are farther apart, they might not physically influence one another, but their gravitational encounter can pull on each of them. These wandering, wayward objects then begin to enter into other lanes of planetesimal traffic. This creates a brand new opportunity for additional catastrophic encounters of the worst kind.
After the passage of millions of years, a multitude of encounters between these traveling planetesimals has successfully cleared out most of the disk’s debris. Much larger, and considerably fewer, objects now inhabit the region. A new planetary system has formed, and is now ready to grow up and reach maturity.
There’s Something New In The Heavens Named “Synestia”
Sarah Stewart and Simon Lock are especially interested in smash-ups between spinning objects. A rotating object has angular momentum, which must be conserved in a collision. Imagine a ballerina doing a pirouette. She extends her arms, and as she does so, she slows down her rate of spin. If she wishes to spin faster, she holds her arms close to her side–but her angular momentum remains constant.
Now consider a duo of ballerinas spinning around on the stage. If they catch hold of one another, the angular momentum of each adds together. This means that their total angular momentum must be the same.
Lock and Stewart created a model of what would occur when the “ballerinas” are Earth-sized rocky planets bumping into other large objects with both high angular momentum and high energy.
“We looked at the statistics of giant impacts, and we found that they can form a completely new structure,” noted Dr. Stewart in a May 22, 2017 University of California at Davis (UCD) Press Release.
Lock and Stewart found that over a range of high temperatures and high angular momenta, planet-sized bodies could form a new and much larger structure. This new structure would be an indented disk that resembled a red blood cell or a doughnut with the center filled in. The newly emerged object would be composed primarily of vaporized rock, with no solid or liquid surface. This new object, the synestia, received its name from “syn”, “together”, and “Estia,” the ancient Greek goddess of architecture and structures.
The key to synestia construction is that some of the structure’s material goes into orbit. In a spinning, solid sphere, every point from the core to the surface is rotating at the same rate. However, in the case of a giant impact, the material of the planet can become molten or gaseous and expands in volume. If it grows large enough and is traveling fast enough, portions of the object exceed the velocity needed to hold onto a satellite in orbit–and at this point it forms an immense, disk-shaped synestia, according to the study.
Earlier theories had proposed that giant impacts could cause planets to create a disk of solid or molten material encircling the planet. But for the same mass of planet, a synestia would be much larger than a solid planet sporting a disk.
It is generally thought that most planets probably experience collisions that could create a synestia at some stage of their evolution, according to Dr. Stewart. For an object like our own planet, the synestia would be short-lived–it would only hang around for about a hundred years before it lost enough heat to condense back into a solid object. However, synestia formed from hotter or larger objects, such as gas giant planets or stars, could potentially linger longer, Dr. Stewart added in the UCD Press Release.
The synestia structure also suggests new ways to consider how Earth’s Moon may have been born, Dr. Stewart went on to explain. Earth’s Moon is remarkably similar to Earth in composition, and because of this great similarity most current theories explaining how our Moon formed involve a giant impact of the primordial Earth with a doomed, Mars-sized protoplanet named Theia. This ancient collision would hurl material into orbit around our still-forming planet. However, such an impact could have instead created a synestia from which our Earth and its Moon both condensed.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, 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 the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.