Wither, I find Outline Object at 255 softness as good as, or even better than, Halo.
I'm not familiar with PS's outer glow, but If it is what I think it is, wouldn't Outline Object/Halo on a seperate layer (possibly with some curves adjustments) make an effect that is similar?
I really want to see an Inner Glow in Paint.NET though. That would make 99% of Photoshop planet tuts easier in Paint.NETtopic>
Halo: http://boltbait.googlepages.com/halo
Outline Object: viewtopic.php?f=16&t=5715