It's nothing special really. The planet graphic was there originally in the rom. It was placed on the 2nd screen if you look in a level editor.
The reason it never showed up though originally is because the stage uses a special parallax scrolling effect right??
Well the NES can only scroll the current screen as it is. And it will be repeated infintely, in other words it will never draw anything to the screen above the division line between the parallax scroll, and the normal playfield. If the planet graphic was placed on the very first beginning screen, then it will appear, and stay there being scrolled very slowly, but infintely wrap from the left/right side of the screen to the opposite side.
The original game designers probably did not have it there for the final version of the game because since it's a giant graphic, it would feel really weird to keep seeing it appear on the oppositie side of the screen repeatedly as if world rotated fully every minute.
It's very close to being like parallax BG's on the SNES...except the major downside is ...because of NES limitations, you can virtually never have any level design in the "affected" area of the screen that's doing the parallax. It has to be a completely black screen, or a repeated pattern. That is why many of the NES megaman's fortress bosses like wily, use those scrolling techniques, however it can only be on pure solid black with no BG design whatsoever.
Wether some people already know this or not, i just felt like explaining for anyone that may have not actually understood or knew how it worked.