Earth and Venus, planets that grew up like rioters

Planets such as Earth or Venus, which reside inside solar systems, were likely born from repeated ‘hit and run’ collisions, defying conventional models of formation.

Planet formation, the process by which distinct, round, neat planets form out of a swirling, turbulent cloud of rough asteroids and mini-planets, was probably even more messy and complicated than most scientists would like to admit. , according to new research led by researchers at the Lunar and Planetary Laboratory (LPL) at the University of Arizona.

The findings challenge conventional wisdom, in which collisions between smaller building blocks cause them to stick together and, over time, repeated collisions accumulate new material on the growing baby planet.

Instead, the authors propose and demonstrate evidence for a novel “hit and run” scenario, in which preplanetary bodies spent a good portion of their journey through the inner solar system colliding and bouncing off each other, before meeting again. at a later time. Having been slowed down by their first collision, they would be more likely to stick together the next encounter.

The research is published in two studies appearing in The Planetary Science Journal, one focusing on Venus and Earth and the other on Earth’s moon. A central element of both publications, according to the team of authors, which was led by professor of planetary sciences and LPL Erik Asphaug, is the largely unrecognized point that giant impacts are not the efficient mergers that scientists believed they were. were.

“We found that most giant impacts, even relatively ‘slow’ ones, are hit-and-run. This means that for two planets to merge, you usually first have to slow them down in a hit-and-run collision,” he said. Asphaug in a statement. “Thinking of giant impacts, for example, the formation of the moon, as a singular event is probably wrong. Most likely, it took two collisions in a row.”

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One implication is that Venus and Earth would have had very different experiences growing as planets, despite being immediate neighbors in the inner solar system. In this article, led by Alexandre Emsenhuber, who did this work during a postdoctoral fellowship in Asphaug’s laboratory and is now at the Ludwig Maximilian University in Munich, the young Earth would have served to slow down the interrelated planetary bodies, ultimately making them more prone to to collide with Venus and join him.

“We believe that during the formation of the solar system, the early Earth acted as a vanguard for Venus,” Emsenhuber said.

The solar system is what scientists call a gravity well, the concept behind a popular attraction at science exhibits. Visitors toss a coin into a funnel-shaped gravity well, then watch their money complete several orbits before falling into the central hole. The closer a planet is to the sun, the stronger the gravitation experienced by the planets. That’s why the inner planets of the solar system that these studies focused on – Mercury, Venus, Earth, and Mars – orbit the Sun faster than, say, Jupiter, Saturn, and Neptune. As a result, the closer an object gets to the sun, the more likely it is to stay there.

So when an intruder planet hit Earth, it was less likely to stick to Earth and instead more likely to end up on Venus, Asphaug explained.

“Earth acts as a shield, providing a first stop against these shocking planets,” he said. “Most likely a planet bouncing off Earth will hit Venus and merge with it.”

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Emsenhuber uses the analogy of a ball bouncing down a staircase to illustrate the idea of ​​what drives the avant-garde effect: a body coming from the outer solar system is like a ball bouncing down stairs, and each bounce represents a collision with another body.

“Along the way, the ball loses energy and you will find that it always bounces down, never up,” he said. “So the body can’t get out of the inner solar system anymore. Usually it just goes down the stairs, towards Venus, and an impactor that collides with Venus is pretty happy to stay in the inner solar system, so somewhere moment will strike Venus again. “

Earth has no such cutting edge to slow down its interrelated planets. This leads to a difference between the two similarly sized planets that conventional theories cannot explain, the authors argue.

“The prevailing idea has been that it doesn’t really matter if the planets collide and not immediately merge, because they will meet again at some point and then merge,” Emsenhuber said. “But that’s not what we found. We found that they end up becoming part of Venus more often, rather than returning to Earth. It’s easier to go from Earth to Venus than the other way around.”


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