Shadows problem with big and small objects

Hi, I've been using Unity for severals years now, and I attempted something new : playing with planets, and the size difference between the player and the object (in VR, with the HTC Vive)

So, I used a planet model, with rings, configured a few things, set-up light. In the editor, as you can see, I have this marvelous shadow wich is exactly what I want.

In playMode, in the VR headset or on the screen, poof, no shadow.

I tried to scale up and down the planet model, same for the Vive Room, put the shadow Distance parameter to its maximum, played with all the options everywhere, made all lights static, dynamic, baked a hundred of maps, nothing. This beautiful shadow disapear the instant I enter playmode.

This is quite frustrating because I cannot identify the cause of the disapearance of this shadow I may have an idea of what's causing all this, this may be caused by the size of the objects. I duplicated y planet, made it 200x the previous size, and in the editor, the shadow on the rings doesn't show anymore.