This theory, known as pebble accretion, is reshaping how scientists think about the early solar system. It also opens new research directions, such as exploring how planets form around stars other than the sun. “You can form these bodies easily and quickly,” says Michiel Lambrechts, an astronomer at Lund University in Sweden and co-author of the theory. “Pebble accretion provides a solution to so many problems.”

Chief among those problems is how planets could form before the dusty disk ran out of the raw ingredients needed to make them. Models suggest that the disk would have cleared itself out in about 1 million to 10 million years as its gas evaporated and its dust spiraled into the gravitational pull of the newborn sun. The biggest planets, such as Jupiter and Saturn, somehow packed together a core of about 10 Earth masses before the disk vanished. Making planets from planetesimals would have simply taken too long, since planetesimals typically whiz past a baby planet without being captured by its gravity.

M. Lambrechts and A. Johansen / L. Modica / Knowable

Pebbles, on the other hand, are easily captured by a protoplanet’s gravity, and their accumulation could build a planet in just a million years or so. Astronomers know that such pebbles exist, because they have seen them orbiting infant stars. Radio telescopes such as the Very Large Array near Socorro, New Mexico, have measured the size of particles in protoplanetary disks by the way the material glows in radio wavelengths. The disks often contain huge numbers of pebbles—in some cases, the equivalent of hundreds of Earth masses slowly drifting toward the star.

The pebbles form when smaller dust particles collide and coalesce. “Most of the dust in the disk becomes pebbles,” says Anders Johansen, an astronomer at Lund University and the other co-author of the theory. He calls the protoplanetary disks “pebble factories.”

Around 2010, Johansen and Lambrechts began to wonder how all these pebbles might relate to planets being born. They began a series of calculations on how pebbles might interact with other large rubbly fragments floating in the protoplanetary disk. To their surprise, Johansen and Lambrechts found that pebbles could quickly glom onto a protoplanet.

The key is friction. Imagine a stream of pebbles whizzing by a 100-kilometer-wide protoplanet. As pebbles plow through gas within the disk, friction slows them down enough to be captured by the protoplanet’s gravitational field. The pebbles begin looping around the larger rock and soon smash onto its surface. Each collision adds a tiny bit of mass—and with enough such collisions, the protoplanet grows quickly, to 1,000 kilometers across or more. “In many ways, pebble accretion is the most efficient way of adding mass to a body,” says Lambrechts.