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Planet-sized collisions could form planetary body known as ‘synestia’


Three different planet structures: (a) planet, (b) planet with a disk, and (c) a synestia – all of the same mass. Image Credit: Simon Lock & Sarah Stewart

Giant impacts between rapidly spinning celestial objects can produce a new type of doughnut-shaped planetary body known as a “synestia”, according to a new study in which researchers modeled planet formation.

Synestias are huge masses of hot, vaporized rock created when planet-sized objects impact one another violently, melting and partially vaporizing both of the objects. Eventually, the bodies cool and return to spherical or nearly spherical shapes.

Sarah Stewart of the University of California at Davis and co-author of a paper on the study published in the American Geophysical Union‘s Journal of Geophysical Research: Planets, believes Earth was once a synestia.

She and the study’s lead author, Simon Lock of Harvard University, noted that spinning objects, much like skaters on ice, have angular momentum, which is conserved when they impact one another.

They compared the collision of rapidly spinning Earth-sized rocky planets to two rotating skaters grabbing hold of one another.

In both cases, the angular momentum of both bodies adds together, so there is no change to their total angular momentum. High-energy impacts between objects with high angular momenta produce synestias, indented disks shaped like doughnuts but with filled-in centers.

Consisting mostly of vaporized rock, synestias have neither solid nor liquid surfaces. Some of the colliding material can become molten or gaseous in the wake of the impact, causing its volume to expand. If this material grows large enough and is moving very fast, a doughnut-shaped synestia will form once its velocity exceeds that required to hold an orbiting satellite.

Stewart and Lock’s concept differs from traditional models of planet or moon formation, which envision such high-impact collisions between Earth-sized objects to produce a disk of either molten or solid material around the remaining planet. The disk eventually coalesces, forming a second object. Earth’s Moon is thought to have formed via the latter scenario.

According to Stewart and Lock, the collision that formed Earth’s Moon, which ejected material into Earth’s orbit, may have formed a synestia rather than a ring of debris around the Earth.

An Earth-sized synestia would not last much longer than a century before cooling and re-condensing into a spherical world. However, larger, hotter objects, such as colliding gas giants, could create a much longer-lasting synestia, the researchers surmised. If both the Earth and Moon condensed from a synestia, that would explain why both objects have such similar compositions.

The researchers, who together coined the term “synestia”, acknowledge that none has yet been observed but hope they will eventually be found during searches for planets orbiting other stars.



Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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