The circumstances that formed Earth’s moon might be different than what scientists have thought. After a Mars-sized object crashed into Earth about 4.5 billion years ago, it’s possible that the collision of planetary matter swelled so much that it created a bizarre, intermediate variety of celestial object.
While previous theories hold that the impact created a ring around Earth that eventually coalesced into the moon, recent research suggests that the planet temporarily turned into a large mass of vaporized rock, with outer layers of the vaporized planet rotating in orbit around the rest of the body, forming an indented donut-shaped disk.
Two planetary scientists have proposed a new classification for this theoretical object — a transitory planetary state that has never been observed in space — which they call a “synestia.” The word the prefix “syn-” (united; together) with a reference to Hestia, the Greek goddess of architecture.
“We looked at the statistics of giant impacts, and we found that they can form a completely new structure,” said Sarah Stewart, co-author of the research and professor at the University of California, Davis, in a press statement.
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The research, which was led by Simon Lock, a graduate student in planetary sciences and geophysics at Harvard University, was published in the Journal of Geophysical Research: Planets. It was supported by NASA and the US Department of Energy.
Today’s theories posit that planets in our solar system were formed billions of years ago when bits of rock and gas were attracted to each other because of gravity. As these smaller objects crashed into each other, the resulting bodies melted and eventually solidified into the rocky planets we are familiar with now: Mercury, Venus, Earth, and Mars.
When considering objects that spin, however, the formation scenario shifts. Imagine a spinning skater on ice. To slow her rate of spin, she extends her arms. To increase her rate of spin, she holds her arms close. But her angular momentum, or rotational inertia, remains the same in both scenarios.
If there are two spinning skaters on the ice, their angular momentum also remains the same if they reach out and grab each other’s hands. This is a similar scenario to when a spinning, Earth-sized rocky planet collides with high energy into another spinning object, the researchers argue. And these are the conditions under which synestias may occur.
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The proposed structure of a synestia suggests that after a small body collides with a spinning planet, some of the material will go into orbit. That’s because the collision would make the planet become molten, in turn causing it to expand in volume. If the planet grows big enough and has a fast enough angular momentum, it would expand into a massive disk of rotating material.
In Earth’s case, if it formed a synestia after the big collision, it likely would have lasted that way for only a hundred years or so before heat escaped and it condensed. Synestias may persist longer if they form from larger or hotter objects such as gas giant planets or moons, however.
Whether the new classification takes hold will likely depend on whether observatories catch sight of a synestia directly, but Lock and Stewart’s work is expected to inspire astronomers to hunt for the phenomenon as they scan distant solar systems.
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