Well I did not initially intend to explain orbits, but simply argue that universally expanding matter is not capable of explaining them. However according to what has previously been theorized and indeed observed regarding the formation of planetary systems, the planets are formed in the disk of dust and gas surrounding new stars. The proto planetary disk is already spinning and therefore has rotary motion. So the initial "push" is the rotation already inherent in the collapsing cloud of interstellar dust and gas that formed the star and planets at the outset. I must confess that I do not think that we have reached the final understanding of gravity and that there are inconsistencies in the current standard model.