On Firefox (Ubuntu/Windows 7), the sun is drawn at the same size as the black circle for all planets in our solar system which are further out. This doesn't happen with Chromium (Ubuntu) or Chrome(Windows 7), but it does happen with Opera (Windows 7).
Probably some sort of rounding bug...
Changed code to draw sun so that it doesn't fill the circle (previously
it was filled with emptiness).
That comment prompted me to actually have a look at it. It seems that the sun outline was being filled in (although generally not with anything). Removing the line that fills it seems to have fixed things.
Aha, nice one!