Yeah fair enough - both things are subject to atmospheric distortion changing the way they look. I am thinking in the context of ufo-ology though - planets are large enough that distortions in the atmosphere between the viewer and the planet can make the planet look like it's doing stuff - different shadows and warbles on the disc of the planet can look like shit changing form. a star will shift with atmospheric distortion but because it's only a single point of light instead of a disc, it'll appear to move a little in position but there isn't the same intra-object change.