Shadow Became Transparent Through Background

Hi, I am new to Blender, so sorry if this is a basic question.

I am working on a space scene with cycle renderer, trying to render planet and some stars in the background.
I use the compositor to do planetary grow, flare… and the world with noise texture for the stars.
For some reason, the shadow render through as transparent and stars are showing through the planets as shown in picture below.
Is there a way to fix this?