They coalesced into larger objects, forming clumps of up to a few kilometers across in a few million years, a small time with comparison to the age of the Solar System.[1] After the planetesimals grew bigger in sizes, collisions became highly destructive, making further growth more difficult. Only the biggest planetesimals survived the fragmentation process and continued to slowly grow into protoplanets by accretion of planetesimals of similar composition.[3] After the protoplanet formed, accumulation of heat from radioactive decay of short-lived elements melted the planet, allowing materials to differentiate (i.e. to separate according to their density).[3]