ok, a 'few' suggestions here, first, keep in mind that there are stars EVERYWHERE in space, and maybe a little of what is messing up the transition is that you don't have any stars in FRONT of the planets too...
Second, your blending mode of disolve needs a little bit of work...(not sure actually what to suggest there, but it is a bit 'off')
also the smaller planet top left, looks a bit like it's cut-out and pasted on the image, instead of PART OF the image, you see what I am talking about?...I DO like the other two planets and nebulae and I also like your star-fields, just needs a few 'detail adjustments' is all...overall, nicely done.
You know, this work is good. The "space clouds" are cool. But I think the planets have a poor transition from "Planet" to "Space" (but I don't know what to suggest.
