The tell-tale clue to how meteorites were made, at the birth of the solar system

April 26, 1803 was an unusual day in the small town of L’Aigle in Normandy, France – it rained rocks.

Over 3,000 of them fell out of the sky. Fortunately no one was injured. The French Academy of Sciences investigated and proclaimed, based on many eyewitness stories and the unusual look of the rocks, that they had come from space.

The Earth is pummeled with rocks incessantly as it orbits the Sun, adding around 50 tons to our planet’s mass every day. Meteorites, as these rocks are called, are easy to find in deserts and on the ice plains of Antarctica, where they stick out like a sore thumb. They can even land in backyards, treasures hidden among ordinary terrestrial rocks. Amateurs and professionals collect meteorites, and the more interesting ones make it to museums and laboratories around the world for display and study. They are also bought and sold on eBay.

Despite decades of intense study by thousands of scientists, there is no general consensus on how most meteorites formed. As an astronomer and a geologist, we have recently developed a new theory of what happened during the formation of the solar system to create these valuable relics of our past. Since planets form out of collisions of these first rocks, this is an important part of the history of the Earth.

This meteor crater in Arizona was created 50,000 years ago when an iron meteorite struck the Earth. It is about one mile across. W. Herbst, CC BY-SA

The mysterious chondrules

Drew Barringer (left), owner of Arizona meteor crater, his wife, Clare Schneider, and author William Herbst in the Van Vleck Observatory Library of Wesleyan University, where an iron meteorite from the crater is on display. W. Herbst
About 10% of meteorites are pure iron. These form through a multi-step process in which a large molten asteroid has enough gravity to cause iron to sink to its center. This builds an iron core just like the Earth’s. After this asteroid solidifies, it can be shattered into meteorites by collisions with other objects. Iron meteorites are as old as the solar system itself, proving that large asteroids formed quickly and fully molten ones were once abundant.

The other 90% of meteorites are called “chondrites” because they are full of mysterious, tiny spheres of rock known as “chondrules.” No terrestrial rock has anything like a chondrule inside it. It is clear that chondrules formed in space during a brief period of intense heating when temperatures reached the melting point of rock, around 3,000 degrees Fahrenheit, for less than an hour. What could possibly account for that?

A closeup of the Semarkona meteorite showing dozens of chondrules. Kenichi Abe
Researchers have come up with many hypotheses through the last 40 years. But no consensus has been reached on how this brief flash of heating happened.

The chondrule problem is so famously difficult and contentious that when we announced to colleagues a few years ago that we were working on it, their reaction was to smile, shake their heads and offer their condolences. Now that we have proposed a solution we are preparing for a more critical response, which is fine, because that’s the way science advances.

The flyby model

Our idea is quite simple. Radioactive dating of hundreds of chondrules shows that they formed between 1.8 and 4 million years after the beginning of the solar system – some 4.6 billion years ago. During this time, fully molten asteroids, the parent bodies of the iron meteorites, were abundant. Volcanic eruptions on these asteroids released tremendous amounts of heat into the space around them. Any smaller objects passing by during an eruption would experience a short, intense blast of heat.