all objects in the game are essentially just textures wrapped around spheres, basically.
You can control the size of the sphere, its mass, and temperature with the tools in the game.
you can also make a texture, which should be 2:1 aspect ration (twice as wide as it is high), and then move png file to the media folder of the universe sandbox directory
then ingame you can select any object, with the object editing bar, type in the filename of the custom texture, and have it be applied to object. then you have created your own planet, star, comet, or any body you'd like.