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Hot-Donut Earth? Planet May Once Have Taken on Odd 'Synestia' Form
The structure of a planet, a planet with a disk and a synestia, all of the same mass.
Credit: Simon Lock and Sarah Stewart

Earth probably became a "synestia" for a brief period about 4.5 billion years ago, according to a new study.

That's the term for a newly proposed cosmic object: a huge, hot, donut-shaped mass of vaporized rock that results from the collision of two planet-size bodies

Astronomers think Earth endured such an impact shortly after the planet's birth, slamming into a Mars-size object known as Theia. (This smashup — or series of smashups, as one recent study posits — also resulted in the creation of Earth's moon, scientists believe.) 

"We looked at the statistics of giant impacts, and we found that they can form a completely new structure," study co-author Sarah Stewart, a planetary scientist at the University of California, Davis, said in a statement.

Stewart and lead author Simon Lock, a graduate student at Harvard University, modeled what happens when rocky worlds about the size of Earth crash into other large bodies that are moving, and spinning, relatively fast. The researchers' simulations suggest that the most violent encounters would produce synestias, putative structures that look like gigantic red blood cells.

Previous studies have posited that such impacts might produce a debris ring around a planet. But a synestia is a far bigger and weirder object; it is made mostly of vaporized rock and has no solid or liquid surface, Lock and Stewart said. (The name synestia, incidentally, comes from "syn," Greek for "together," and "Estia," the Greek goddess of structures and architecture.)

Synestias — if they exist — are short-lived objects. Earth likely stayed in the synestia phase for just a century or so after the collision with Theia, according to the new study. The synestia-Earth then lost enough heat to condense back into a solid object, the thinking goes.  

At the moment, synestias are hypothetical objects; nobody has ever seen one. But astronomers may be able to spot them in alien solar systems, Stewart said. 

The new study was published online May 22 in the Journal of Geophysical Research: Planets.

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