According to my theory of sunlight (see HERE), the fall in temperature as we move from the center to the edge of the solar system is due to the attenuation of the energy contained in spheres of energy released by the Sun’s corpuscles. The basis of this idea was that the same initial energy released by the corpuscle back on the Sun will be spread-out much, much farther by the time each released sphere of energy expands to the edge of solar system.
However, when my theory is expanded to include photon creation (see HERE), the question arises… Does the Photon retain all (or at least most) of its initial energy? If so, why is Neptune cooler than Earth?
Let us assume the harder case… that Photons retain all their energy (this is also the mainstream view). The answer here would be that the collection of photons released by the Sun are now spread out over much more area. In other words, Neptune is bathed in less energy-per-volume of space than Earth.