Planets’ chaotic orbits shook up asteroids like a snow globe

An artist's conception of a dense asteroid belt. The one in our Solar System is very sparse compared to this.

NASA/JPL-Caltech

Francesca DeMeo wanted to know what the asteroid belt looked like. “I thought, everyone else must know this, but not me,” says the Harvard planetary scientist.

As she searched through the literature, she found that an overview of space rocks didn't exist—not one made within the last few decades, at least. So she set out to make a map herself. The result, published in Nature last Thursday, reveals a trend: rogue asteroids—those appearing at a spot different from where they formed—are quite common. Something must have displaced them, which suggests that the Solar System must have once been volatile.

The most recent map DeMeo could find was made in the early 1980s. Composed of just over 1,000 objects, it illustrates a neat arrangement of space rocks within the asteroid belt. Those in the inner part of the main belt, closer to the Sun, are asteroids that, in compositional measurements, appear red. These formed at warm temperatures, where it is too hot for some molecules (like water) to condense and become part of the asteroid. Farther away, closer to Jupiter, were blue-appearing asteroids created in the cold. Here, water can condense to ice. In between the two extremes was a neat gradient. Asteroids were born and stayed put, the story went, and this was repeated for years to come. The Solar System was always pretty calm.