Using relative terms, the same stars (all of them from what I've observed on my P900) and all the planets I've observed exhibit the same fluid appearance when the focus is set as specified in an earlier post. This observation is in conflict with any statement separating this visual phenomenon being apparent in stars and not planets.
I have also observed from great distances man made light sources through a humid night sky and cannot replicate this effect. Both the man made light source and the stars use the same focus setting and when applied to one another creates a completely opposite visual feedback. When the camera puts the star in focus, the same "in focus" setting applied to the man made light source afar results in a sharp image of the light. Focal length at extreme distances are relative.
I've also dabbled in macro photography and the resolution of light or chromatic aberrations do not exhibit this fluid effect when observed. Stars and planets are the only thing that I have seen do this, except for the youtube video with LED light in water I linked earlier.
As far as I am concerned, the only "fluid" is the atmosphere. I find it hard to guess what might happen with a seriously out of focus lens.
How does that account for the fact that only the stars and planets exhibit this effect and not the moon and sun?
Because stars and planets are very dim objects in comparison.
So the luminosity of the subject is what's causing the appearance of fluid distortion?
When we're talking photography with digital cameras, auto focus, ISO settings and the settings in general needed to let in enough light, yes.
Focusing on very dim objects during night time is not easy. Even worse for auto focus algorithms.
Edit: Not the luminosity itself, but the measures taken to let in as much light as possible from distant, dim objects.
With a big enough tele lens, you won't get this with planets.