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The term fragmentation barrier refers to a particular challenge to a common assumption regarding planet formation that planets form in protoplanetary disks from the disk material. The challenge is that the growing objects (planetary embryos) would have to grow through a stage when collisions would be likely to break them apart. The regime where this appears to be a problem is somewhat less than a meter across, and the fragmentation barrier, along with the radial-drift barrier are sometimes cited as the meter size barrier.