In the world of planetary formation, it feels like every week brings new unknowns as often as solved mysteries.

Let’s lay out some of the basics. In your mind’s eye, picture a newly formed star with a disk of gas and dust orbiting around it. Soon, that disk begins to have gaps, as small protoplanets begin to clump together, clearing paths as they orbit. Then the increasing gravity of those clumps pulls in more and more dust and gas until a planet actually exists in a lovely cleared orbit.

Yes, that’s the basic idea. Basics, however, never seem to satisfy scientists, who want to understand every detail possible and how changes in the steps affect the entire process. So research led by the University of Arizona Lunar and Planetary Laboratory set out to understand just how the conventional view of planetary accretion, where those tiny baby planets get bigger and bigger, exactly works. And, of course, it probably doesn’t work quite the way we thought. Quel surprise.

Rather than gently rounding up material as they rotate and revolve, it now seems that growing planets are more like billiard balls, running amok in their orbits and colliding again and again with each other until their momentum is slowed down enough for a collision to stick.

Computer models are amazing, y’all.

Lead author Erik Ashpaug explains: 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. 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.

Okay, so not only is our concept of a gently-ish building planet wrong but so is our current hypothesis on the formation of the Moon? That’s partially true. Yes, Earth likely got hit by a Mars-sized body we refer to as Theia, but instead of kicking out a ton of Earth’s material on the first pass, Theia skimmed the newly forming planet instead, losing momentum. About a million years later, Theia returned, hit the Earth again, and this time was slow enough to partially stick and partially remove a bit of Earth to create a satellite with the same isotopic chemistry as the parent body. Asphaug notes: The double impact mixes things up much more than a single event, 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.

On top of this new hypothesis, one of the two papers published in The Planetary Science Journal proposes that Earth provided a bit of a vanguard for Venus, which means that when objects hit Earth, they lost energy and continued closer to the Sun. They would then, at this lower energy level, collide with Venus and stick. That’s a result of the Sun’s gravity well, which is strongest closer to the Sun, so things that move in and slow down aren’t going to come back out of that well. They no longer have the energy to escape, and they’re low-energy enough that a collision means Venus gets to grow.

But what about Mercury and Mars? They are probably what’s left of that earliest population of baby planets that just never got hit all that often because Earth pulled the other lumps in and slowed them down, and then Venus made them stick. No lumps for you, Mercury!

Of course, as always, this research has led to more questions. How did Earth grow as big as Venus? Why is Earth’s magnetic field so much stronger than Venus? Why doesn’t Venus have a moon if it was hit in a similar fashion to Earth?

Planetary formation. Causing me headaches for years now. We’ll keep bringing you these stories, but for now, it seems that the mystery of planetary formation got a few more unknowns.

More Information

The University of Arizona press release

“Collision Chains among the Terrestrial Planets. II. An Asymmetry between Earth and Venus,” Alexandre Emsenhuber et al., 2021 September 23, The Planetary Science Journal