The “sun” is the tiny yellow central square which seems to recede more than any of the “planets.” Because it is smallest—or most central? All of the “planets” either recede or press forward because of their relative sizes and colors, but not in any predictable way, subverting the theory that warm colors advance and cool colors recede, for example.
From my conversations with Robert, I would assume there was no concrete intention to paint the solar system; he is a self-described abstract painter. From the pentimenti (not very perceptible in the virtual painting shown here) quite a lot of work went into this painting. A grid was set up in part, and squares were overpainted, resized and repositioned. I would guess that the Push and Pull of Hans Hoffmann was more of an influence than Mondrian for this work, but it is so harmonious that it makes me think of Mondrian’s idea of creating universal harmony by painting (!). Looking for a nice quote online to sum up his approach I found the following (seemingly presented entirely without irony) on a website called The Art Story:
It is hard to judge if [Mondrian’s] goals of expressing universal spiritual perfection were reached, but his work did become very popular and was much used in commercial design.