Raise the opacity of the layer back to 100% in the Layers palette.
Click and Eye icon next to Layer 1, the earth layer to turn its visibility off for now. Use the Quick Selection tool and select the area that overlaps the earth and where we’ll need to bring in front of the earth.
Click the Eye icon next to Layer 1 again to make it visible. Go to Select>Inverse.
Now we need to mask out the area of the earth photo where the arm is. We mask out the area instead of erasing so that if we mess up, the pixels can be brought back instead of removing them. With Layer 1 selected, click the Add Layer Mask icon on the Layers palette.