Liz Epsilon Eridani
More info i
 Credit: NASA/JPL-Caltech 

Where is all the missing planetary junk in the solar system?

Contributed by
Sep 8, 2021, 4:51 PM EDT

The asteroid belt might as well be the solar system’s junkyard. Asteroids, comets and other objects floating around in there are mostly trash from eons ago.

So if that much garbage exists in the asteroid belt, then where is the impact debris from violent collisions that happened when the solar system was still in its temperamental youth? There is nothing on the record. This was the same question researchers Dr. Travis Gabriel, staff scientist at a federal research lab and Harrison Allen-Sutte, a Ph.D. student at Arizona State University, had until they ran a series of simluations. They recently published that study in The Astrophysical Journal Letters. 

Gabriel and Allen-Sutter used an existing hydrodynamics code that simulates what rocks undergo when exposed to extreme conditions, such as the overkill gravity that occurs when they smack into each other, which has enough power to make those rocks behave more like a fluid. Turns out the debris really did vanish into thin air.

“Large planets collide can collide at supersonic speeds," Gabriel told SYF WIRE. "This is similar to fighter jets breaking the sound barrier in Earth's atmosphere (over 760 mph) causing the well known 'sonic boom', except the collisions that formed our planets occurred at over 30 times that speed, exceeding even the sound speed of rocks and causing tremendous shocks.”

Billions of years before Earth ever saw a dinosaur, the collisions that were thought to have created the missing debris actually vaporized it before it ever had a chance to drift over to the asteroid belt. This is the "Missing Mantle Paradox" or the "Great Dunite Shortage" that baffled scientists because rocky planet chunks should have been in the asteroid belt, but weren't. Forming planets means that protoplanets have to smash into each other. Gas giants and other huge planets affected impact velocities, instigating supersonic collisions between protoplanets that were at least the size of Mars, powerless in the face of a monster’s gravity. Some of these protoplanets were knocked out and ended up in the asteroid belt.

It was the intense heat generated by such huge collisions that ended up turning rock straight to vapor. This was more likely to happen in planets of Martian size or larger (which might explain why smaller protoplanets from that era are still found in the asteroid belt) that experienced extra gravitational pushing from gas giants or several immense impacts. Earth also took its share of hits. The gaseous debris easily left the solar system, taking evidence of the impacts with it, and the effects of solar radiation made whatever was left disappear.

A protoplanetary disc, in which planets form. Credit: Universal History Archive/Universal Images Group via Getty Images

"Our impact simulations showed that certain planetary collisions tend to eject almost entirely vapor," Gabriel said. "The debris was mostly vaporized, leaving minimal record of this violent past. Vaporized debris tends to efficiently be removed from the solar system by condensing down to droplets and being 'blown out' by the Sun's radiation forces over time."

Embryonic planets would form pretty fast—1 to 10 million years is rather fast in cosmic terms—starting from pebbles that accreted into larger and larger bodies. Impacts were something that was unavoidable in the early solar system. When protoplanets grew large enough, whether they were future rocky planets or gas giants, so did their gravity, which became the perpetrators behind gravitational disturbances. They kept colliding at higher velocities as they continued growing. Meaning, impact velocity was proportional to their mass, but it gets worse.

Collisions between larger planets meant that shock heating caused escape velocity of debris that was released to be faster than the speed of sound. Post-impact heating, or heat generated by the impact itself depending on how fast and hard one object crashed into another, made sure to vaporize the debris that was already falling apart at such high velocities. Wandering gas giants that were still figuring out their orbits would pass by smaller objects and boost the escape velocities of their debris, which is why accretion wasn’t exactly that efficient. That, and Gabriel believes what he find out could mean planet formation might have previously been misunderstood. 

"We believe our result is a key piece in the debate between competing planet formation models," he said. "Classical planet formation suggests planets formed through gentle collisions and a recent model called 'pebble accretion' suggests there was only one to a few giant impacts in our Solar System's history. In both of these scenarios, we predict mostly vaporized debris."

It is possible the evidence hasn’t been completely obliterated. There are pristine or nearly pristine moons and asteroids out there that have remained undisturbed and might still have traces of vaporized debris on their surfaces. Another possibility is the Moon. The research can help us better understand the formation of the Moon, since it is thought to have been born in the aftermath of a collision that released this debris. Still, there are some things missing.

“The Moon-forming giant impact would have ejected plenty of material into the inner solar system, bombarding the planets and polluting the asteroid belt," said Gabriel. "By comparing Moon various formation models, we may be able to see which is more in line with the nature of our asteroid belt that we observe today."

This breakthrough could still help us better understand otehr star systems with impace debris recently discovered by astornomers.. Similar phenomena probably happened among exoplanets, and there could be an Earth among them.