Introducing Synestia – A “New” Planetary Object That Looks Like a Donut
published during a waning crescent moon.

By now, we’re all familiar with the celestial staples: we know planets—or dwarf planets, as the case may be, stars, comets, asteroids, black holes. But we might have to update our astronomy textbooks soon to include another type of celestial body: a synestia.

Researchers from Harvard University and the University of California Davis published a study in the Journal of Geophysical Research: Planets in which they describe and name the new planetary object. The study examined how planets form, particularly the collisions that cause rotating bodies to get so hot that solids and liquids turn into vapor before they cool down and form the typical sphere shape.


Planetary collision illustration. Credit: NASA/JPL

Our solar system demonstrates how much planets’ size can vary, but it turns out planets not only differ widely in size but also in shape—at least for a while. When the researchers accounted for different planetary rotation rates and thermal energy types and temperatures, they noticed something interesting: a donut.

Sometimes collisions between forming planets affect their temperatures and angular momentum,  or the force of their rotations. If temperatures and angular momentums are high enough, the impact sends some of the planet’s substance into orbit. The planet’s matter changes—vaporizes and/or melts—at different rates. This means the planet can’t rotate as one uniform object, and instead of forming a round shape, the planet becomes disc-shaped (with the center filled in), or the shape of a red blood cell.


Synestia illustration. Credit: Simon Lock, Harvard University

The inner part of a synestia—the filled-in center of the donut—rotates at one rate while the bigger, outer part rotates at another, as its exposure to gravity and the pull of other orbital objects is greater. Study co-author Sarah Stewart believes Earth was a synestia for a short period of time—perhaps about one hundred years—until it cooled down enough to coalesce into the shape it is now. The researchers believe many if not most planets experienced the type of collision that could form a synestia. Bigger planets or stars involved in such collisions could produce long-lasting synestias, and now scientists will start looking for them. Perhaps they can catch one in the act.

The existence of synestias raises some new questions about the moon’s formation. Most scientists believe the moon resulted from a (possibly incredibly violent) collision between Earth and another planetary body often referred to as Theia. This study advances the possibility that the collision turned Earth into a synestia, and that perhaps both our planet and the moon formed when the synestia began cooling and shrinking.