Figure 1 (IMAGE) Tohoku University Caption Micrometer-scale dust particles from protoplanetary disks, or sites around stars with particles and hydrogen and/or other gasses, aggregate to form planetesimals, or kilometer-scale building blocks of planets. Planetesimals, in turn, merge due to mutual gravity. Credit JAMSTEC Usage Restrictions Reporters may use freely these materials in news coverage with appropriate credit information. License Original content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.