Now, you'll notice that the edges of your picture have dashes going all the way up to the corners. Yes, unfortunately when the selected ellipse was turned into a path, it made an abrupt corner to follow the borders of the image. you'll need to select the areas where the dashes follow the border of the picture and Clear them (delete) so that you keep just the dashes you want. After this is done you should have some nice neat dashes showing the orbital paths of your planets.
Now we get to the complicated part, the planets themselves. It's best to do them all at once, so make two more layers for your map image, "Planets" and "Shadows." If you want to show moons as well, you should also make two additional layers, "Moons" and "Moon Shadows." Keeping them all on separate layers will be useful later on. Save your map (as an .xcf, remember) and close it.
Ready to go
Since there are various methods to making planets, you may prefer to use a different method. This is only a quick and easy method to make different kinds of planets which will look good when they get to their final size on the map. I'm afraid I was unable to find the original author of the tutorial I used to create these planets, so I apologise for not being able to offer credit where it is due.
Make a new image with whatever background you want at size 1000x500. Add solid noise [Filters->Renders->Clouds->Solid Noise] and give it these settings:
X Size: 6.0
Y Size: 4.0
Randomize the seed or find one you like.
If you want an oceanless planet, skip this paragraph.
Go to Brightness/Contrast and set the contrast up all the way. Adjust brightness however you see fit. The idea is that you want the image to be black blotches on a white background, or vice versa. One will be the ocean, the other the land. Give the picture an alpha channel if you need to, then select by colour whichever you want to be the ocean and Clear it. This will give you the transparency behind it. Without deselecting the ocean, create a new layer behind the current one. use the fill tool (fill whole selection) and colour it whatever you want the ocean to be, generally some form of blue. Then, invert the selection and Fill it with a lighter version of that colour. Deselect and use Gaussian Blur [filters->Blur->Gaussian Blur] at about 25 radius in both directions. This will give you a nice looking effect with lighter blue shores. Now, reselect the land on the main layer and reapply the Solid Noise with the same settings. It generally looks better if you use the same seed, but you don't really have to.
With the land still selected, apply any changes you'll want to now. You'll probably want to Colourize it, for one. Also, if you want a more dramatically rocky planet, you can turn the contrast up, and if you want a relatively smooth planet, you can lower the contrast. When you're happy with the settings, it's time to apply the Bump Map [Filters->Map-> Bump Map]. These settings are just an example:
Once you're happy with the overland map, it's time to move on. Flatten the image and go to Image->Scale Image. Resize the image to 500x500 so it looks squished in half. Then go to Map Object [Filters->Map->Map Object] and choose the sphere. Make sure you give it a transparent background, and set the light settings at "no light." Finally, set the rotational z coordinate to something like 15 degrees, and find a good y rotation. Use the preview button to play with the settings until you have something you like, and click OK.
Now you have your planet mapped to a sphere. But it still needs work, doesn't it? First you should generally Sharpen [Filters->Enhance->Sharpen] the image by 10 to 50 points. Then, create four more layers. Call them "Black," "Atmosphere," "Clouds," and "Shadow." Place Black and Atmosphere underneath the sphere and fill Black with pure black colour. This'll give you a better background to see the finished product with. Go back to the background layer and select by colour the transparency, then invert. Use a radial gradient such as the Germany Flag Smooth, centered in the middle at 250, 250 and draw down to about 410, 250. Deselect, go back to Gaussian Blur, and give it about 90 radius in both directions. This will give you a slight red blur past the sphere. Colourize the layer as you wish, generally with a light blue colour if the planet has an Earthlike atmosphere. Planets with thinner atmospheres can be given a darker colour so it fades to black quicker.
Select the planet again using the same tactic as before and this time go to the Clouds layer. Go back to solid noise [Filters->Renders->Clouds->Solid Noise] and this time set the x and y both to the same number, make sure the Turbulent box is checked, and don't worry about making it tileable. Click OK and set the Clouds layer to either Add or Screen mode in the Layers dialog box. Go back to Brightness/Contrast and set the contrast high and the brightness relatively low. You should get roughly cloudy shapes in white over your image. Play around with these settings until you're happy with the result. Leave the "Shadow" layer alone for now. This will be dealt with later. Save the planet as its own .xcf file.
Repeat for each planet in the system. Of course you might want gas giants, which can be done with a custom gradient, clever use of the iWarp filter, and a little bump-mapping for texture. You also might want planets with a very particular texture, which you can simply find, colourize, and map to a sphere. This part is where the individual creativity generally comes in, and it will take several hours to do an entire system this way, but the results are amazing. Once you have every planet and moon that will appear in the star system, move on to the next instruction.
Modra's eight orbiting bodies; Five planets, three moons. A very close hothouse planet Phoba, an overpopulated slum Hyperion, the Ghrinite homeworld Mog (now populated exclusively by the rich), a frozen rock out in space Chrio, and the artificial planet Juperos. Then the moons, a protected water reservoir moon for Hyperion called Lyons, and the two moons of Mog, Gorun and Sevique.
Back on the overall map, use your "Markers" layer to plan out the locations of each planet along the orbits. It looks best when the planets look more or less randomly staggered about the orbits, and remember that they need enough room underneath them to add the names later.
Once you know where each planet will go, open the planet pictures one at a time. Use the select ellipse tool, give it a fixed size of about 800x800, and select the part of the planet which will face the sun. Try to select a little more than half, maybe about two thirds. Now, invert the selection. Go to your shadow layer and fill the selection with pure black. Deselect and use Gaussian Blur again set to about 190 on the shadow layer. Whis should give a nice realistic-looking shadow on one side of the planet. You may need to try a few times before the effect looks right. Finally, go back to the background layer (the one which has the original sphere on it), select the transparency again, and return to the shadow layer to delete everything except the shadow on the planet itself. Now, it should have the atmosphere halo going all around it.
Now, from the layers dialog box, make the black background and the shadow invisible, right-click and select "merge visible layers" and resize the image to whatever you want it to be on the map. I keep my planets between 50x50 and 130x130 and my moons smaller than that. Remember that the image size itself includes everything around the planet itself as well, so if you want the actual body of the planet to be a certain size make sure the image size is bigger than that. Now, copy the main planet layer (which should now be the sphere, atmosphere, and clouds) and paste it onto your map, making sure the planets layer is selected first, then line it up with the correct orbit.
You can use a neat trick here. In the Paths dialog, click the left-side button next to each of the orbits to make the original path show up. Then, zoom in when you paste the planet onto the map and line up the little plus sign on the middle of your planet with the line from the path and that will line it up more or less perfectly.
Now, before deselecting anything, go back to the planet image, select the shadow layer, and paste that onto the image as well. The planet itself will automatically be pasted to the "Planets" layer. Then right-click on the "floating selection" in the layers dialog and choose "new layer." Move the layer down until it's right above the "Shadows" or "Moon shadows" layer of the map and merge down. Again, repeat for every planet and moon in the star system.
Just needs a few more touches.
And that should be it. That's the basics. You can place names underneath each planet (I use the font Zero Threes set at size 16 with Hintin and Antialiasing on) and whatever else the map needs.
Oh, and if you want to know how it's worth separating the shadow layers from the planets and moons, just click the button next to the shadow layers to make them invisible. In fact, the way I set each of my maps up, I can show or hide any combination of details, including just the moons, just the text, or everything but the star and orbits. Modra's finished .png is also included on this post, and will be added to my Emaria Star Maps thread as well.