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The term radial-drift barrier (or drift barrier) refers to one challenge to theories of planet formation, that small objects forming within a protoplanetary disk material tend to drift toward the star, the drift sufficiently rapid that such an object nears the star and is vaporized, effectively becoming disk gas, to be accreted by the star or pushed back out into the disk. The regime of object-sizes that presents this challenge is the range of a millimeter to a meter.
The gas and dust of the disk are not moving as fast as a Keplerian orbit since an outward force of gas pressure contributes to the equilibrium. The gas pressure affects gas and dust more than more massive objects, and if something grows larger and more massive, the pressure is insufficient to hold it that distance from the star. Equivalently, the Earth's atmosphere holds up the higher portion of the atmosphere as well as very small dust grains, but a rock falls, and when water droplets reach a certain size, rain results.