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Then new JWST image of Jupiter shows a small halo on the right side of the planet. What's causing this feature? I figure it could be that the planet is simply overexposed (or moving slightly during the exposure?). Or perhaps this is a phase effect, and the boundary between the light and dark limbs corresponds to the jovian terminator?

enter image description here

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    $\begingroup$ This might be totally wrong, but it’s possible that in image processing this was combined with images taken at sufficiently separated times or long enough exposures that the movement of Jupiter mattered in how everything averages out, which would be in agreement with the opposite side not having the same halo. This image shows the same effect more dramatically $\endgroup$
    – Justin T
    Commented Aug 26, 2022 at 4:20
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    $\begingroup$ That's a great hypothesis, but I'd expect such a shift to be just a translation (no atmosphere in space) to be pretty easy to correct. Of course, it could be that Jupiter's rotation has blurred out the features between exposures (amateur astronomers use tools like WinJupos to de-rotate their images) - but the atmospheric bands look exceptionally clear (except of course for the right limb). Also, if it were just a translation error, wouldn't we expect a boundary just on the 'inside' of the limb on the left side? $\endgroup$
    – user2944352
    Commented Aug 26, 2022 at 6:42
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    $\begingroup$ @user2944532 You make some excellent points, and you’re probably right. As far as the boundary on the left side, the image I linked has a bit of a boundary it seems on the left side which is what made me think of it. $\endgroup$
    – Justin T
    Commented Aug 26, 2022 at 14:59
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    $\begingroup$ My bet is on atmospheric refraction. Notice it's only visible on the dark side. The dark part is where the atmosphere is too opaque to transmit the light. Like what you see here: space.com/3747-eclipse-moon-probe-earth-blocks-sun.html $\endgroup$
    – Greg Miller
    Commented Aug 26, 2022 at 20:40
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    $\begingroup$ It covers the whole right hemisphere from north to south pole with the same thickness. That IMO rules out the translation hypothesis. And: Do we know the wavelengths shown in this picture? if it's in the far IR it could be some thermal radiation generated somewhere in the upper atmosphere. $\endgroup$
    – asdfex
    Commented Aug 27, 2022 at 18:29

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This seems to be airglow.

At least Scott Sutherland claimed this in the June 15, 2022 Weather Network article Webb bonus: Jupiter's clouds, moons, and ring shine bright in new images, and Dr James O'Donoghue gave a nice explanation on Twitter. Apparently we only see this on the right side, as it is slightly in the shadow.

click images for full size:

from https://twitter.com/physicsJ/status/1547883712569757698 from https://twitter.com/physicsJ/status/1547883712569757698

This is incredible. What we might be seeing here is the glow of Jupiter's upper atmosphere, but separated from the planet itself! What a shot, Great Red Spot, rings, moons, moon shadow and airglow like we see on Earth! Quote Tweet

And it fits the detection in UV wavelengths.

Compare this to the same phenomenon on earth and it seems plausible. Earths airglow is also detectable in the near-IR. The JWST image was taken with the NIRCam instrument 2.12 micron filter. enter image description here

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  • $\begingroup$ Great answer, thanks! If you are uncomfortable with the edit please feel free to roll back or edit further. $\endgroup$
    – uhoh
    Commented Sep 26, 2022 at 19:05
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    $\begingroup$ Thanks, edits are very much appreciated. $\endgroup$
    – SpaceCore
    Commented Sep 26, 2022 at 20:02
  • $\begingroup$ This is a great answer! Thanks for the input! $\endgroup$
    – user2944352
    Commented Oct 3, 2022 at 16:51

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