The findings contradict the widely held belief that collisions between smaller building blocks cause them to stick together and, over time, repeated collisions add new material to the growing baby planet.

According to new research led by researchers from the University of Arizona Lunar and Planetary Laboratory, planet formation – the process by which neat, round, distinct planets form from a roiling, swirling cloud of rugged asteroids and mini planets – was likely messier and more complicated than most scientists would care to admit.

The findings cast doubt on the conventional view, which holds that collisions between smaller building blocks cause them to stick together and that repeated collisions add new material to the growing baby planet over time.

Rather than that, the authors suggest and show scientific proof for a novel “hit-and-run-return” scenario in which pre-planetary bodies spend a significant portion of their journey through the inner solar system colliding and ricocheting off of one another, only to collide and ricochet off of one another again at a later time. They would be more likely to stick together the next time, having been slowed down by their first collision. Consider a game of billiards, with the balls coming to rest, rather than pelting a snowman with snowballs.

Two reports on the research appear in the Sept. 23 issue of The Planetary Science Journal, one on Venus and Earth and the other on Earth’s moon. According to the author team, which was led by planetary sciences and LPL professor Erik Asphaug, the central point of both publications is the largely unrecognized fact that giant impacts are not the efficient mergers previously believed.

“We find that most giant impacts, even relatively ‘slow’ ones, are hit-and-runs. This means that for two planets to merge, you usually first have to slow them down in a hit-and-run collision,” Asphaug said.

“To think of giant impacts, for instance the formation of the moon, as a singular event is probably wrong. More likely it took two collisions in a row.”

One implication is that, despite being close neighbours in the inner solar system, Venus and Earth would have had very different experiences as planets. The young Earth would have served to slow down interloping planetary bodies, making them more likely to collide with and stick to Venus, according to this paper led by Alexandre Emsenhuber, who did this work during a postdoctoral fellowship in Asphaug’s lab and is now at Ludwig Maximilian University in Munich.

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

The solar system is a gravity well, which is the concept behind a popular attraction at science museums. Visitors toss a coin into a funnel-shaped gravity well, then watch as their money completes several orbits before falling into the centre hole. The stronger the gravitational pull of planets, the closer they are to the sun. That is why the inner planets of the solar system studied – Mercury, Venus, Earth, and Mars – orbit the sun faster than Jupiter, Saturn, and Neptune. As a result, the closer an object approaches the sun, the more likely it is to remain there.

As a result, Asphaug explained, when an interloping planet collided with Earth, it was less likely to stick to Earth and was more likely to end up at Venus.

“The Earth acts as a shield, providing a first stop against these impacting planets,” he said. “More likely than not, a planet that bounces off of Earth is going to hit Venus and merge with it.”

Emsenhuber illustrates the vanguard effect with the analogy of a ball bouncing down a set of stairs: A body coming in from the outer solar system is like a ball bouncing down a set of stairs, with each bounce representing a collision with another body.

“Along the way, the ball loses energy, and you’ll find it will always bounce downstairs, never upstairs,” he said. “Because of that, the body cannot leave the inner solar system anymore. You generally only go downstairs, toward Venus, and an impactor that collides with Venus is pretty happy staying in the inner solar system, so at some point it is going to hit Venus again.”

Earth lacks a vanguard to slow down intruding planets. According to the authors, this results in a difference between the two similar-sized planets that conventional theories cannot explain.

“The prevailing idea has been that it doesn’t really matter if planets collide and don’t merge right away, because they are going to run into each other again at some point and merge then,” Emsenhuber said. “But that is not what we find. We find they end up more frequently becoming part of Venus, instead of returning back to Earth. It’s easier to go from Earth to Venus than the other way around.”

The team used machine learning to obtain predictive models from 3D simulations of giant impacts in order to track all of these planetary orbits and collisions, and eventually their mergers. The team then used these data to rapidly compute orbital evolution, including hit-and-run and merging collisions, to simulate terrestrial planet formation over a 100-million-year time span.

The authors suggest and illustrate their hit-and-run-return scenario for the moon’s formation in the second paper, recognizing the primary problems with the standard giant impact model.

“The standard model for the moon requires a very slow collision, relatively speaking,” Asphaug said, “and it creates a moon that is composed mostly of the impacting planet, not the proto-Earth, which is a major problem since the moon has an isotopic chemistry almost identical to Earth.”

In the team’s new scenario, a protoplanet roughly the size of Mars collides with Earth, as in the standard model, but is a little faster, allowing it to keep going. It returns in about 1 million years with a massive impact that resembles the standard model.

“The double impact mixes things up much more than a single event,” Asphaug said, “which could explain the isotopic similarity of Earth and moon, and also how the second, slow, merging collision would have happened in the first place.”

The researchers believe that the resulting asymmetry in how the planets were assembled paves the way for future research into the diversity of terrestrial planets. For example, we don’t know how Earth got its much stronger magnetic field than Venus, or why Venus doesn’t have a moon.

According to Asphaug, their research reveals systematic differences in dynamics and composition.

“In our view, Earth would have accreted most of its material from collisions that were head-on hits, or else slower than those experienced by Venus,” he said. “Collisions into the Earth that were more oblique and higher velocity would have preferentially ended up on Venus.”

This would result in a bias in which, for example, protoplanets from the outer solar system would have preferentially accreted to Venus rather than Earth. In short, Venus could be made of materials that are more difficult to obtain on Earth.

“You would think that Earth is made up more of material from the outer system because it is closer to the outer solar system than Venus. But actually, with Earth in this vanguard role, it makes it actually more likely for Venus to accrete outer solar system material,” Asphaug said.

Saverio Cambioni and Stephen R. Schwartz of the Lunar and Planetary Laboratory, as well as Travis S. J. Gabriel of Arizona State University in Tempe, Arizona, are co-authors on the two papers.

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