Early Outer Solar System Planetesimals Were Mostly Heat-Formed Chondrule Rock, Study Says
New analysis of carbonaceous iron meteorites shows the first outer-Solar-System planetesimals contained just 8–17% fine, volatile-rich matrix and 83–92% chondrules—heated rock beads formed when aluminium-26 decayed, melted interiors, and drove core formation. Aerodynamic gas drag in the protoplanetary disk caused larger chondrules to drift and concentrate while fine dust stayed with the gas, effectively filtering out much ice-rich material from the earliest bodies. This early selective construction extends a pattern seen in younger meteorites and implies the outer disk’s composition was shaped from the start by particle size and gas dynamics, not by temperature alone.
