Comets are considered to be the most ancient and least changed objects in the Solar System. The question of the formation of these bodies may shed light on the process of evolution of the protoplanetary cloud. In this paper, we substantiate the assumption that a clearly defined layered (onion-like) structure of comets has closed gas-impermeable shells between the layers. A model is proposed that explains the possibility of the formation of space objects equipped with these heat-resistant shells. The main process of formation of space objects in this model is the accumulation of matter as a result of the adhesion of granules. This process begins at the periphery of the protoplanetary disk as temperatures drop. Volatile organic matter on the ice surface of the granules is modified by ultraviolet light, forming “glue” allowing the granules to unite. The object gains mass and, under the influence of gravity, repeatedly crosses the heated zone of the central plane, moving up and down across the entire thickness of the protoplanetary disk. In the cold zone, the substance accumulates on the object. And in the heated zone, at a distance of 3.5 to 5.0 AU from the axis of rotation (where the temperature can reach ~1000 K), closed shells are formed, separating the layers. An approximate estimate of the thickness of the layers in the cometary body has been made. It was found that at the initial stage of comet formation the thickness of the layers reached 1 - 2 m.
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