Take a trip to 25143 Itokawa, a peanut-shaped asteroid that kisses the orbit of Mars, and you might be surprised by what you see: hundreds of large, rounded boulders that dot the surface of the space rock. But how did they get so round? Gravity barely exists on Itokawa, so surface collisions strong enough to shape the boulders couldn’t have taken place. Now, new research suggests another culprit: the sun. By studying the forces required to round off the sharp edges of rocks, researchers came up with a model that predicts the maximum collision speeds needed for boulder-surfacing. The relatively low speed—between 6 and 7 meters per second—suggests the process must have taken place over thousands or even hundreds of thousands of years before the asteroid was formed, when a gravitationally stable cloud of debris spun in the disk of material that would go on to build the solar system. As sunlight bounced off the orbiting boulders, photons provided a tiny push. As they radiated back outward as heat, they triggered a recoil effect that added a gentle spin. Over time, these slowly spinning boulders bumped into each other with enough force to wear their edges into smooth surfaces, write the scientists in this month’s issue of Planetary and Space Science. Because these collisions would have occurred in the early solar system, they could also have affected the process that built the planets.