spaceSpace and Physics

New Type Of Planetary Object May Have Formed Earth And The Moon


Jonathan O'Callaghan

Senior Staff Writer

We think Earth was hit by a Mars-sized object 4.5 billion years ago. NASA/JPL-Caltech

Astronomers have proposed a whole new type of planetary object called a synestia, a large donut of vaporized rock that may form early in a planet’s life. That might apply to Earth, too.

The idea was put forward in a study in the Journal of Geophysical Research: Planets by Simon Lock from Harvard University and Sarah Stewart from the University of California, Davis.


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

The researchers suggest that when two large bodies collide, they can create a huge disk that looks sort of like a red blood cell or donut, with a “filled-in” center. Around this would be a ring of vaporized rock with no surface.

You can think of it like a small core in the middle of a large planet-sized ring of material. Over time, this material gradually comes together to form a more solid planet.

A diagram of what a synestia might look like. Simon Lock and Sarah Stewart

Most planets are thought to experience a large impact at some point in their life. According to this study, if the body is large enough, then the collision is so violent that the two bodies melt and partially vaporize.


They then create this sort of swirling mass of debris, spinning around a central core. Some of the material moves so fast that it escapes the pull of the original planet, expanding the size of the ring, which is key to the formation of the synestia. It would be much larger than a solid planet and a disk like Saturn.

According to the researchers, Earth may have gone through this process, albeit for only 100 years. We think that Earth was hit by a Mars-sized object 4.5 billion years ago, which led to the formation of the Moon – known as the Giant Impact Hypothesis.

This theory predicted the Moon formed from a ring of debris that resulted from this impact. However, a synestia provides a possible new explanation for the Moon’s origin, with both Earth and the Moon condensing from this donut-shaped object.

Given the rate we are finding exoplanets, the researchers think it might be possible to find a synestia somewhere in our galaxy. Their brief lifetimes might make it difficult, but finding one would be hugely fascinating.


“The large population of exoplanets, with larger rocky bodies and higher stellar flux compared to our solar system, offers an opportunity to apply our new understanding of hot, rotating rocky planetary structures,” the researchers wrote in their paper.


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