Is there a way to have an object show up only in a reflection? I have the planet Earth set up to reflect into a habitat on the moon via a mirror set outside a window. I had to place a tiny Earth near the mirror to get this to work. From inside the habitat the faux Earth isn't visible, but I'd like to make only its reflection visible so I can take videos without the fakery showing up, and do the same thing outside other similar windows.





The ideal thing would be to be able to do this in Eevee, but Cycles would also do. Is there a way to do this?
Use the light paths node on the Earth to isolate the reflection light ray.



In this picture I have a sphere emitting light, but it is only seen on the reflection of the plane. On the right is the node set up for the Sphere. You can use that node set-up for the Earth, just replace the principled shader with your current node set up for the Earth.

Hope this helps!