This isn't an exact answer to your question, but another way to accomplish this effect is to use a texture without alpha information. Instead use black (color 0,0,0) for the transparent part around the outside of the planet image. Then create your planet Object3D without alpha, and setTransparency to some high number (say 100). This will make all the black areas completely transparent, and all the non-black areas completely opaque. I've tested this method in GL software and hardware rendering modes.
Example applet (
source code)
I was too lazy to draw a planet for the above example applet - I just threw together a tree model, sky sphere, and star tile background that I had sitting around from other projects, but the transparent areas around the tree's leaves are the same thing that you are trying to do for the transparent areas around your planet.
Note, if there are other opaque areas of your texture that are currently 0,0,0, you'd need to change them to some other black (say 5,5,5).