I think your issue is that the planet's glow is actually adding to your planetary effect, which is rather easy to fix.
in the emitter color chooser, you will see three numbers, like so:
"255, 255, 255" (whatever the color is)
add a number to the front like so:
"0, 255, 255, 255"
mess around with that, I believe that adding the first number edits the alpha of your emitter's textures.
not additive shader is poorly supported in the Sins engine, and causes visible frame rate drops even on high-end machines.
hopefully that answered your question!
if the alpha values don't work for you, you can always GIMP the textures and burn them darker, then save them as a custom texture and then use those.
or you can try lowering the emit rate of the particles themselves, which in terms of additive particles will make them dimmer.