Found: The Oldest Known Family of Asteroids in the Solar System

Our solar system contains a very large main asteroid belt between Mars and Jupiter, an orbiting mass of tens of thousands of rocky fragments that have orbited the Sun for billions of years. Within the belt there are some larger objects—such as Ceres, 580 miles in diameter—but also plenty of much, much smaller ones, fragmented over many millennia by collisions. When an asteroid breaks up this way, its fragments tend to hang around together as a family of smaller asteroids that orbits together. As time goes by, like members of some human families, these asteroids tend to drift apart. The oldest asteroid families, then, are the ones that have drifted the farthest apart. Astronomers have understood this for awhile, but it is a difficult phenomenon to study.

This week, an international team of astronomers reported that they have identified a very early, primordial asteroid family by correlating the size of its members and how far apart they are. That family is nearly as old as the solar system itself—over four billion years. The scientists now plan to use the same approach on other parts of the asteroid belt to dig even deeper into the early stages of our solar system.

“By identifying all the families in the main belt, we can figure out which asteroids have been formed by collisions and which might be some of the original members of the asteroid belt,” said Kevin Walsh, an astronomer at the Southwest Research Institute and a coauthor of the paper, published in Science. “We identified all known families and their members and discovered a gigantic void in the main belt, populated by only a handful of asteroids. These relics must be part of the original asteroid belt. That is the real prize, to know what the main belt looked like just after it formed.”

Reaching that far back will take just a little more time.