Huge impact could have smashed early Earth into a doughnut shape

Potential portrait of really early Earth

For a brief time during its infancy, Earth was not a planet. It was a hot, doughnut-shaped blob called a synestia. Rocky worlds can be pulverised by collisions with each other, mushrooming into synestias before cooling off and becoming more familiar-looking celestial spheres, a new study says.

Worlds across the universe come in all shapes and sizes, from planetesimals to dwarf planets to giants with rings, but researchers don’t fully understand the ways they can change shape and size throughout their lifetimes, says Simon Lock, a graduate student at Harvard University.

In the solar system’s early days, huge impacts would have frequently occurred as small bodies smashed into each other, broke apart, re-formed and smashed again. Previous studies have suggested such impacts could pulverise a section of a planet, leaving behind debris that would coalesce into a moon or a ring like the ones surrounding Saturn.

But sometimes, the most violent collisions can vaporize entire worlds, heating them up and sending them spinning. Unlike a solid body in which all parts rotate at the same angular velocity, these gas blobs spin so fast that the outer edges spin at a higher rate than the inner material.

Once they reach a certain point, called the co-rotation limit, the planet takes on a new structure with an inner region rotating at a steady rate, loosely connected to a bulbous disc that rotates around it. The disc is not disconnected from the central region like a planet and its rings, but it sits at the outer limit of the planet’s gravitational pull.

“What happens is, eventually its radius goes far enough out that the edge of the body has the same velocity that it would if it was in orbit,” Lock says. “For Earth, the edge of where its satellites orbit would be the co-rotation limit.”

It resembles a puffy red blood cell, or a doughnut with a dented middle.

Molten for a moment

This structure is so much larger and more diffuse than a typical planet or a disc that it deserved a new name, Lock says. He and Sarah Stewart of the University of California-Davis dubbed it a synestia, using the Greek prefix syn-, which means together, and the goddess Hestia, who represents the home, hearth and architecture. Taken together, the word signifies “connected structure,” Stewart says.

A synestia lacks a surface, but instead has an exterior region marked by clouds of molten rock and dust, all at a scorching 2000°C or hotter. These conditions only last for a cosmic blink. Earth would have stayed in synestia form for just a century before cooling off enough to condense into a solid object again, Lock says.

Most planets and even some stars might form synestias at some point in their lives, according to Stewart. She plans to look for evidence of them around young star systems.

“One place to look is at planets very close in to their stars, where the outer part of the planet is very hot. That keeps it poofy, and if it is rotating very fast, it could be a synestia,” she says. “There are some really odd-looking exoplanets that are quite close, and we are going to look at them closely.”

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