I have two layes - one of the sun and stars and the other of a planet. I have rendered them separately from a 3D program and now I want to bring them back together. What I am trying for is the halo or fringing of the sun around the edges of the planet, but I am unsure of how to do this. I tried the Add, Sub, Mult, burn etc but nothing seems to give me that look. All of the planet material is greenscreen with the sun and stars underneath it.
Is this something I can accomplish in Vegas or I have to try AE?

Would something like 'glow' work in your 3D app? Which app do you use?

What I am trying for is the halo or fringing of the sun around the edges of the planet, but I am unsure of how to do this.

Use Sony Light Rays FX on the planet layer. You may have to add the Sony Cookie Cutter before it to just affect the circle of the planet. You can also add a Generate Media Solid Color on the track below the planet, then Cookie Cutter the generated media to match the planet and add Light Rays to that. This way you can control the color of the rays with the color of the Generated Media.

Alternately you could have just dropped a light behind the planet in the 3D app that you used.

~jr