I'm just trying to understand this from a purely thermodynamical standpoint. I'm comfortable with the scenario of a blackbody cavity's interior being in thermal equilibrium with the photon gas contained, and they have identical temperature, and then the Planck distribution would apply to photon energy. However, why would such an argument extend to a radiator in space where the photon gas is obviously not in equilibrium? The photons shoot out from the surface, never crossing path with it ever again, and the photon gas' numberical density is inversely proportional to distance squared.

I'm just trying to understand this from a purely thermodynamical standpoint. I'm comfortable with the scenario of a blackbody cavity's interior being in thermal equilibrium with the photon gas contained, and they have identical temperature, and then the Planck distribution would apply to photon energy. However, why would such an argument extend to a radiator in space where the photon gas is obviously not in equilibrium? The photons shoot out from the surface, never crossing path with it ever again, and the photon gas' numerical density is inversely proportional to distance squared.