Alright, Danstanta gave me an image and asked me to "make it look better". I realised I couldn't possibly fit an explanation for the process in the profile messenger, so I'm putting it here.
It started out like this:

This is a pretty nice image; it is well drawn. But I reckon Danstanta wanted the lighting effects and the glow, which is not really possible using flat colours.
Also, when looking at things from outer space, certain things are a bit blurry. So I cleaned up a couple of things in preparation:

Now, here comes the fun part. I work in many layers, adding a new one pretty much whenever I feel like it. This is because I employ different modes for different layers, mainly "addition", "multiply" and "subtract". I highly recommend that users of Photoshop and Gimp etc. try them out!
For now I will stick with adding addition layers. This is how it's done:

The next thing to do is to use different hues. Using one hue will tend to give you a flat look. Using different hues gives more shape and vibrancy as there's actual different colours for your eye to...differentiate.
So I look at a picture of Earth from far away, and I studied the shadow provided. Stands to reason there's going to be some refraction from the atmosphere, as well as some reflection from the ocean. And the ocean colour also varies! But I've kept the basic colour of the Earth the same as it was pretty.

Then I turned my attention to the stars. There are a couple of things to note:
* Starlight is made of different colours. And actually not usually yellow, but a spectrum of red and blue. Blue stars are brighter. Red stars are dimmer.
* Stars can be drawn many many different ways but drawing it as a flat dot will make the background look like a flat canvas.
So I decided to add in some light. In keeping with the piece I kept it yellow and put in a few more "addition" layers, and used a fuzzy brush.

And now for the fun part...the comet!
Again, same rule applies. Many hues, many layers. Gives a...you guessed it, layered effect!

There's like 4 or 5 layers in that. All this time think of the light, where it's going, where it came from.
And now I look at this and think is that all I should do with it? Are there any changes I should make? I have plenty of ways to make changes as I've got many layers and can easily change one bit without ruining another.

I'll leave you to figure out what I did after the last pic.
Total time: 20-30 minutes. But then again, I am using a tablet.