Hello here's my test scene who tries to make a circle to glow like a sun.

Please don't pay attention to the last 2D node. It does nothing for the moment.
I plan to make glow this object only that's why I gave him a Viewport node (but a simpler solution is welcome).
And here's the Viewport properties.

The script draws a white circle.
func _draw():
draw_circle(Vector2(0, 0), diameter, Color(1.0, 1.0, 1.0))
But the circle has no glow. What am I doing wrong ?