What Happens as a Planet Forms?

A Synestia!

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Petersen, Carolyn Collins. "What Happens as a Planet Forms?" ThoughtCo, Jun. 26, 2017, thoughtco.com/synesta-definition-4143307. Petersen, Carolyn Collins. (2017, June 26). What Happens as a Planet Forms? Retrieved from https://www.thoughtco.com/synesta-definition-4143307 Petersen, Carolyn Collins. "What Happens as a Planet Forms?" ThoughtCo. https://www.thoughtco.com/synesta-definition-4143307 (accessed October 23, 2017).
A computer model of a synestia, the intermediate step in Earth's formation when it was a molten, spinning glob. Simon Lock and Sarah Stewart.

A long time ago, in a nebula that no longer exists, our newborn planet was hit with a giant impact so energetic that it melted part of the planet and the impactor and created a spinning molten glob. That whirling disk of hot melted rock was turning so fast that from the outside it would have been difficult to tell the difference between the planet and the disk. This object is called a "synestia" and understanding how it formed may lead to new insights into the process of planetary formation.

The synestia phase of a planet's birth sounds like something out of weird science fiction movie, but it may be a natural step in the formation of worlds. It very likely happened several times during the birth process for most of the planets in our solar system, particularly the rocky worlds of Mercury, Venus, Earth, and Mars. It's all part of a process called "accretion", where smaller chunks of rock in a planetary birth créche called a protoplanetary disk slammed together to make bigger objects called planetesimals. The planetesimals crashed together to make planets. The impacts release huge amounts of energy, which translates into enough heat to melt rocks. As the worlds got larger, their gravity helped hold them together and eventually played a role in "rounding" their shapes. Smaller worlds (such as moons) can also form the same way.

Earth and its Synestia Phases

The process of accretion in planetary formation is not a new idea, but the idea that our planets and their moons went through the spinning molten glob phase, probably more than once, is a new wrinkle.

Planetary formation takes millions of years to accomplish, depending on many factors, including the size of the planet and how much material there is in the birth cloud. Earth probably took at least 10 million years to form. Its birth cloud process was, like most births, messy and busy. The birth cloud was filled with rocks and planesimals continually colliding with each other like a huge game of billiards played with rocky bodies.

One collision would set off others, sending material careening through space.

Large impacts were so violent that each of the bodies that collided would melt and vaporize. Since these globs were spinning, some of their material would create a spinning disk (like a ring) around each impactor. The result would look something like a donut with a filling in the middle instead of a hole. The central area would be the impactor, surrounded by molten material. That "intermediate" planetary object, the synestia, was a phase. It's very likely that infant Earth spent some time as one of these spinning, molten objects.

It turns out that many planets could have gone through this process as they formed. How long they stay that way depends on their masses, but eventually, the planet and its molten glob of material cool and settle back into a single, rounded planet. Earth probably spent a hundred years in the synestia phase before cooling.

The infant solar system didn't quiet down after the baby Earth formed. It's possible that Earth went through several synestias before the final form of our planet appeared. The entire solar system went through periods of bombardmenet that left craters on the rocky worlds and moons.

If Earth were hit several times by large impactors, multiple synestias would happen.

Lunar Implications

The idea of a synestia comes from scientists working on modeling and understanding the formation of the planets. It may explain another step in planetary formation and could also solve some interesting questions about the Moon and how it formed. Early in solar system history, a Mars-sized object called Theia crashed into the infant Earth. The materials of the two worlds mingled, although the crash did not destroy Earth. The debris kicked up from the collision eventually coalesced to create the Moon. That explains why the Moon and Earth are closely related in their composition. However, it's also possible that after the collision, a synestia formed and our planet and its satellite both coalesced separately as the materials in the synestia donut cooled.

The synestia is really a new class of object. Although astronomers haven't observed one yet, the computer models of this intermediate step in planet and moon formation will give them idea of what to look for as they study planetary systems currently forming in our galaxy. In the meantime, the search for newborn planets continues.