Formation of comets by radiation pressure in the outer protosun

The pressure due to the radiation from the Sun and neighboring protostars may have forced the coagulation into comets of the dust grains in the collapsing layers of the protosun at r = (1-5) x 103 AU. The grains were forced together by their self-shielding, which results in the radiation pressure due to photons coming from the direction of strong concentrations of dust being less than the pressure due to photons coming from a direction having a low concentration of dust. This causes the dust to drift toward regions of already strong dust concentration. The formation of comets under these conditions is consistent with the low rotation period of new comets and their extremely volatile chemical constituents