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Hi Frank
In terrestrial cameras, auto exposure is (generally) not suited to astronomical scenes. Auto exposure algorithms attempt to integrate the tones in a scene and average them out to a mid-grey. On my Canon 40D this works very well on most day time scenes where there is a nice spread of tones from blacks through greys through whites.
With Jupiter, all you have is a black sky and a white blob for the planet with few (none?) in-between tones so this fools the exposure meter which then over exposes when it “sees” all that black and attempts to let more light in, to lighten the blacks to make them approach a tonal value of mid-grey.
Cheers
Dennis
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