How to animate the opacity of a shadow in Photoshop

1.) Make sure you click the shadow layer before even starting a timeline 

2.) Click Create Timeline then look for shadow on the left side of the window 

3.) Click the arrow on shadow then click Opacity after click the little diamond shape object that comes up and congratulations you've made one key frame

4.) Next thing is to add more key frames along the timeline and for each one change the Opacity located at the right of the screen to whatever you feel like looks good on the animation 

For example for my planet my first keyframe was 0% then after that I did 60% then 0% so I just went back and forth and in my opinion it turned out pretty nice looking