Hi Andrew,
Thanks for your answer.
I would suggest locking/clamping the value to then max then as having a slider max value does not refrain from typing in bigger values. 
Also, Yes, we can use Directionnal lights… But this will be a huge issue for us as those lights are currently used as Stars source emitters. So basically a directionnal light will do the job… On one side, but if there is another planet on the other side of the sun it will not get proper lighting as the light source would be between the two actors. (which makes it a hassle with directionnal lights)
Is there no way at all to have a point-emitted light (or even shape emitted) with very large scales such as this?