If you started with the sphere rather than the background, don't be tempted to paint the background around the sphere (as shown in the top photo). Backgrounds don't do that in reality, so if you want your painting to look real, your painted background can't either.
Another thing you want to avoid is the background visibly stopping at the sphere (as on the left-hand side of the bottom sphere).
So how do you solve the problem of having painted the perfect sphere and now having to paint the background without mucking up what you're already painted? I'm afraid it comes down to brush control, and that only comes with practice.
As you develop your skills as a painter, so you'll be able to 'get' the brush to 'stop' exactly where you want it to (well, more often than not). In the meantime, if the sphere is dry, you could place your one hand over it to protect it as you paint up to it.
See Also: Background or Foreground: Which Should You Paint First?
