Bins tell panda how to order stuff.
The background bin just sort relative it their order (that is what second param to set bin is)
opaque bin sorts every thing according to less state transformations, it start with one texture/shader then draws every thing with that texture/shader.
transparent bin sort every thing back to front. So that stuff at the back is drawn first then, stuff before it and before it. This way the transparent stuff covers what is in the back like glass. If you do front to back it will draw the glass before you first and the glass in the back will not get drawing at all because it would fail the depth test.
Now why did this sorting didnot work for you? My guess is that your planets’s bounding box extends behind object so it is drawn first. But the planet is so big it depth clips the object so the object fails the depth test and does not get drawn.
Now that unsorted bin tells every thing to draw just randomly it it turn out to be in the right order. I suggest using fixed bin with planet at bin 2 and object at bin 1 because then object will always be first.