Planet Earth May Have Once Been Donut-Shaped: Study

A pair of planetary scientists from two prestigious universities has come up with a fascinating new theory about how planets are formed.

Simon Lock, of Harvard University, and Sarah Stewart, of the University of California, Davis, studied the structure of celestial bodies and the way a series of giant impacts can give rise to planetary objects.

Their research led them to the idea that the universe may host a never-before-seen type of planetary object, which the scientists called “synestia” — a name derived from “syn-“, meaning “together,” and “Hestia,” the Greek goddess of architecture.

In a young solar system, rocky planets, such as the Earth, Mars and Venus, usually emerge as a result of violent collisions between smaller celestial bodies. If these smaller objects are also spinning, their collision typically conserves their angular momentum, which all rotating objects possess.

In their study, Lock and Stewart created a model of what would happen if Earth-sized rocky planets collided with other large objects, crashing into each other with high energy and high angular momentum.

“We looked at the statistics of giant impacts, and we found that they can form a completely new structure,” Stewart said in a UC Davis news release.

What Is A Synestia?

The team uncovered that when planet-sized objects smash into one other in these conditions (of high temperatures and high angular momentum), they can lead to the birth of a much larger structure.

“During the end stage of planet formation, planets collide together and produce bodies that are partially vaporized and rapidly rotating,” explain the scientists in their study, featured May 22 in the Journal of Geophysical Research: Planets.

Lock and Stewart calculated the shape and internal pressures of hot, rotating, Earth-like planets, and discovered that, in certain conditions of thermal energy and rotation rate, “a planet cannot rotate as if it were a solid body.”

Moreover, such planets also have an inner region which is spinning at a single rate, and is connected to a disk-like outer region that rotates at orbital velocities.

“The dynamics of this extended structure are significantly different than a normal planet, so we gave the extended structure a name: a synestia,” said the researchers.

They also point out that rocky planets are vaporized multiple times during their formation and are likely to form synestias.

“By analyzing calculations of giant impacts and models of planet formation, we show that typical rocky planets are substantially vaporized multiple times during accretion,” Lock and Stewart wrote in their paper.

All these considered, a synestia would end up being oddly shaped and would look a lot like an indented disk (similar in shape to a red blood cell) or “a donut with the center filled in.”

Earth Could Have Started Out As A Synestia

The UC Davis news release describes a synestia as “a huge, spinning, donut-shaped mass of hot, vaporized rock,” with no solid or liquid surface. In the Earth’s past, sometime in its early history, our planet may have looked just the same.

According to Lock and Stewart, the Earth itself was likely a synestia in the beginning, at least for a brief period about 4.5 billion years ago.

The two base their theory on the fact that, shortly after our planet was formed, the Earth endured a giant impact — much like the ones that can produce synestias — when it collided with Mars-size object known as Theia, resulting in the formation of the Moon.

The researchers explain our planet could have taken the shape of a synestia for about a century or so after the collision with Theia, before the Earth cooled down, contracted and condensed back into a solid object, becoming sphere-shaped.

In addition, Stewart indicated the synestia structure may shed new light into how the moon came to be.

Because our lunar tenant is remarkably similar to Earth in composition, both the moon and our planet could have condensed from the same synestia created by the original giant impact.

For the moment, synestias remain hypothetical, as no one has ever seen one. Yet Stewart believes astronomers may have the chance to spot one in alien solar systems, in the vicinity of rocky planets and gas giants.

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