Open the Background image and delete the mountains. Create a layer mask and use the Brush Tool and begin to delete this area as shown below.
Place the "Cosmos" stock image below Background layer and rasterize holding (Shift+ALT) and press enter.
Open the "Space Hole" and place it on the cosmos (planet) like in the picture. Press CTRL+T to transform it and right click and choose flip horizontal>. Now create a layer mask and use a brush tool to delete unnecessary part (shown in red). Blend mode is set to Linear Dodge.
Linear Dodge.