Researchers at Harvard and the University of California, Davis (UCD) have come up with a new type of planetary object they've called a "synestia". The proposed object would take the form of a giant, donut-shaped mass of hot, vaporized rock spinning around a molten mass left over from a planetary collision. It's not only a new word to remember, but may provide insights into the formation of the Earth and Moon.

Conceived by Simon Lock at Harvard University and Sarah Stewart at UCD, a synestia is what you get when two planet-sized objects bash into one another. This may not seem like the sort of thing we have to worry about today, but five billion years ago it was a painfully common occurrence as the Solar System formed from the giant disc of dust, gas, and debris that orbited the young Sun.


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According to current ideas about planetary formation, the planets and moons of our system were created as the debris disc about the Sun coalesced into larger and larger objects. At first, this was basically clumping, but as these objects became big enough, they started to collide with one another to shatter and reform into new bodies.

The basic idea is that if the colliding body was small enough, the bits left over from the impact would rain down on the protoplanet like meteorites do on present day Earth. If the object was big enough, it would shatter and form a disc that would orbit for a time around the planet like the rings of Saturn, before spiraling in and increasing the planet's mass.

However, if the two planetary objects are more or less the same size, the collision would be cataclysmic, with both destroyed to form a new molten core surrounded by a huge, thick disc of molten and even vaporized rock and debris – the synestia. The name is a portmanteau of "syn", meaning "together", and "Hestia", for the Greek goddess of architecture and structures.

To find out how a synestia would form, Lock and Stewart looked at the role conservation of angular momentum plays in the collision. This is a basic law of physics that states that the energy that rotates an object remains the same regardless of how an object is altered. In a classic analogy, the scientists compare this to a figure skater spinning in place with her arms extended. As she pulls her arms in, she spins faster, and as she extends them, her spin slows, thanks to this law. Taking this a step further, if two spinning skaters grab hold of one another, their respective angular momenta are combined.

The same thing goes with planets. If they expand, the rotate slower, if they contract, they rotate faster, and if they collide, the angular momenta of the two planets is combined. Therefore, high-impact collisions between planet-sized objects with high angular momentum would result in what Stewart refers to as a completely new structure.

The team says that in these high-energy impacts, some of the gas and debris would fly off so fast as to go into orbit. There, the expansion of the spinning mass would form a thick indented lozenge-like mass a bit like a red blood cell or a donut because each point in the mass would be spinning at the same rate around the core.

According to Stewart, the early Earth is likely to have suffered such an impact and formed a synestia that lasted about a hundred years, though such bodies would be longer lived around larger planets. If this did occur, then it could explain how the Moon formed and why it is so similar in composition to the Earth. Perhaps at some point in the distant past, two bodies collided to form a synestia out of which came the Earth from the molten core and the Moon from the orbiting mass.

The research was published in the Journal of Geophysical Research: Planets.

Source: UC Davis