The solar system’s first asteroids were probably born big.

Rather than slowly amassing bulk over time, the original members of the asteroid belt rapidly formed into rocks hundreds of kilometers across, researchers propose. This finding, reported online August 3 in Science, may help resolve a long-standing debate over the origins of planetesimals — the giant space rocks that populated the asteroid belt and constructed the planets.

Astronomers set out to examine competing explanations for planetesimal formation. One says grains in the dusty disk surrounding the baby sun clumped together bit by bit over millions of years to form objects that ranged from meters to hundreds of kilometers across — about the size range in the asteroid belt today. The second idea argues that swarms of centimeter-sized pebbles almost instantaneously collapsed under their collective weight to create planetesimals hundreds of kilometers across. If true, then asteroids today that are mere meters or kilometers across must be fragments from asteroid collisions.

To determine which theory is correct, the researchers needed to find some of the asteroid belt’s original occupants to see how big they are. And to do that, the team first had to weed out asteroids that formed from later collisions, explains study coauthor Kevin Walsh of the Southwest Research Institute in Boulder, Colo.

Walsh and colleagues examined a region of the asteroid belt that’s between 2.1 and 2.5 times as far from the sun as Earth is. Many asteroids there had already been flagged as members of asteroid families — that is, vestiges of the same smashed asteroids. Astronomers can recognize a fleet of asteroids as belonging to a family because of their similar orbits and chemical makeup.

By combining data from several previous studies, the team identified one more asteroid family — remnants of a smashup about 4 billion years ago. The newly discovered family, along with all the previously known families, accounted for almost every asteroid in this region. But a few loner asteroids remained. These rocks were too secluded to be members of any family, which meant they couldn’t be collision fragments. Rather, they must be members of the belt’s original population, the astronomers concluded.

These outsiders are all at least 35 kilometers across. The fact that none were just a few kilometers across undercuts the any-size-goes theory of gradual planetesimal formation, Walsh and colleagues argue. After accounting for the amount of rock expected to have chipped away during collisions over the solar system’s history, the researchers estimated these asteroids were all at least 100 kilometers across when they formed.

These findings lend credence to the fast-pebble-collapse theory of planetesimal formation, says Joseph Masiero, an astronomer at NASA’s Jet Propulsion Lab in Pasadena, Calif., who was not involved in the work. “Our solar system is the closest and best example we have that shows us how planets form throughout the rest of the universe,” he adds. So these insights could also advance astronomers’ understanding of exoplanet origins.