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Halos could be added as a 2d png texture with small shining effect which is auto-enabled if the camera is
1. In atmosphere
2. In a low temperature zone.
That's all it takes. It's one day of work.
That would be similar to implementing atmospheric rendering as changing the background colour to blue if the camera is:
1. In the atmosphere
2. On the night side of a planet.
and declare: That's all it takes. It's done in a day.
A proper implementation of halos would have to take into account the composition of the atmosphere and consider what kind of crystals could form (ice doesn't imply only water here), what kind of dust it could hold, or droplets. Plumes from cryovolcanos would require new modelling. Also, some sense of depth and density of particle clouds must be modelled and be reflected in the rendering. It could even involve original research to create those textures correctly (if implemented that way), because I don't think there has been much research done on this (hey, university students reading this - if you would like your master's thesis or whatever to offer new
research and this could fit in your field, here's an idea for you). Some limited simulations can be found here
Halos would not only offer a new dimension to the atmospheric rendering, but also bring some stunning views. Whilst pre-rendered textures with some relatively simple masking could be pragmatic way to display such halos, the graphical representation is only a small part of a proper implementation.