Bingowings said:
Come back Chewie's foot particles all is forgiven.
School Boy error bingo
The snow particle of which you speak dislodged itself from in between chewies toes and blew onto the lens of the camera on the miniatures set... Although it looks like a star it is in fact the very same snow particle.
Since it looks quite similar and does not stand out as being too unlike the stars in the picture ILM never bothered to remove it.