On Thursday, we answer your questions!
You can use masks to create a beautiful space scene. To get started, find a picture of a starry sky to use as a background.
Now find an image of clouds and place the horizon just below the centre of the image. Go to Layer>Layer Mask>Reveal All. Use a large Soft Round brush and paint over the mask, blending the images together. Place a rocky image in the foreground and mask it to show only the rocks. Now start adding some buildings. Search for images of buildings that look futuristic and place over the canvas. Grab the Pen tool (P) and start creating a path around the images, then add a selection and apply a layer mask. Place the buildings behind the foreground image.
Now add a dome; you can use stock images or create a sphere using filters and selections. Place it over the city, now reduce the opacity setting and change the blend mode to Soft Light to create a glassy effect. Add a planet and use the adjustment layers to fine-tune the composition.