Quote Originally Posted by Firlefanz
1) How do I get animated clouds on my planets? Do I need to create a 2nd layer on my X-Object, a sphere that is a bit larger and give this layer a cloud texture? How do I make a transparent? Or is there a more simple way?
You can use the same sphere to draw the clouds. Set the depth func (D3DRS_ZFUNC) to D3DCMP_LESSEQUAL and draw the sphere again using an additive blend of a material that looks like clouds over a black background.

2) The sun only has a texture right now, how do I make a corona or another nice sun effect?
Coronas are simply a single large camera-facing quad (billboard) drawn at the sun position. Use an additive or alpha material for the corona. If you want a lens flare, then this is simply a series of billboards (usually a series of circles and hexagons) drawn along a line that passes through the screen position of the sun and the centre of the screen. The closer the sun is to the centre of the screen, the closer the billboards are spaced. The further away the sun is, the further apart the billboards are spaced along this imaginary line.

3) Is there a simple way to create some nice looking fog (Volumetric Fog?) or does anybody have other ideas for some nice effects?
Several large alpha-blended billboards in a bunch could represent a dust cloud.