Meet The Synestia, A Possible New Type of Planetary Object

The UC Davis researchers published their study in a May 2017 edition of the Journal of Geophysical Research: Planets. In it, they describe what a synestia would look like: a "huge, spinning, donut-shaped mass of hot, vaporized rock, formed as planet-sized objects smash into each other," a press release on the paper explains.

A synestia would be much larger than a solid planet, and would last for only a short amount of time—from a couple hundred to a couple thousand years. They explain that synestias are essentially part of a late-adolescent phase in planet formation, before the disks condense and form planets. 

The findings could shed light on how planets in our own solar system formed, including Earth. While synestias likely don't form during every planet's development, researchers told Nature that they wouldn't be surprised if synestias turned out to be fairly common.

In fact, Earth was probably a synestia for a brief period—think about 100 years—more than 4.5 billion years ago. As the impacts lessened and the synestia began to cool, it would shrink and condense, with leftover material coming together to form orbiting structures like the moon.

Of course, scientists haven't yet directly observed a synestia, which isn't surprising given how brief the phase is in a planet's development. But that doesn't mean they aren't looking: researchers hope that once astronomers start searching for these proto-planets, they'll find them alongside fully formed planets and stars.