I wanted to share a quick tutorial on how I generated the planets in Gimp for Ellipsoid. 

In only a few minutes you can create diverse planets like the ones shown below on gimp, and export them to your project.

After starting a new project, select a canvas size. The larger the canvas, the more detailed your planet will be. However, as the planet will be spinning, there will be a lot of frames generated which can quickly add up in your texture pages. 

All of my planets were created using either noise or fractals. To generate either, select the filters -> render option.

Then select either a noise map or a fractal that you want. Once this is generated, choose the "tile seamlessly" option. This will prevent ugly lines from the image not lining up when it is mapped to a sphere.

After that, choose the animation tab, and then the spinning globe

After selecting the number of frames you want, you will have a generated animated planet!

Attached below are some planet I created with this method!

Check out the game if you want: Ellipsoid