You can make them in Maya. It's covered in one of the projects in Learning Maya [X] - Foundations.

Basically what you wanna do is model and texture a few planets and render them on a black background. Then import the rendered image into Maya's PaintFX and use it as a canvas, and then paint some stars and stuff using the PFX brushes in Maya... It's a pretty simple way to do it, and it also looks good