You can change planets surface by putting textures into the media folder and typing in the name in game.
"In the appearance tab, the first control is the color. There is a colored and rounded box to the right of this. Clicking on the box will open up a color palette. After selecting a color from either the presets or the gradient chart, you can click Ok and the window will close. Selecting a color does 3 things. It changes the color of the trail, the color of the highlight mark, and most importantly, it can change the color of the planet.
To the far-right of the color box selector is a check-box for tint. Tint is the option that controls whether or not the planet will turn your color. For example, if you selected blue as a color, checking tint would turn your planet blue. Unchecking tint would return the object to its previous appearance which is determined by its texture.
In the middle of color and tint is an even smaller check-box labeled auto. Checking this will let the computer automatically calculate a balance between the color you selected and the color the object is.
There is a text-box labeled texture below. Here is where you can see which texture a model is using, or even input your own. To use your own texture, you must put it in the folder. The file should be at Documents\Universe Sandbox\Media
Next, type in the name of the texture exactly the same way you name it. The object should now me that texture. If the object is a star, the texture may appear lighter and will seem to glow. If your object has turned into something that looks like the Moon, you've done something wrong.
To the right of the text-box is a button labeled random. This button will spit out another texture each time you press it. Although the textures are in black and white, the color and tint settings above can quickly turn a bleak world into a tropical paradise. If you do not have a lot of memory on your computer, it is a good idea to clear out these textures once in a while. They do not delete themselves and instead of cycling textures, a new one is created every time you press the button. The textures from this should be located at Documents\Universe Sandbox\Media"
You can't spawn comets, you can make two objects rub together creating dust and simulating a comet, kinda.
When two objects collide, they're produce dust by default, switch the collision mode to bounce and make dust. It's extremely glitchy though; the comet often randomly explodes