Composition is how you arrange things in a frame so the viewer knows where to look. Four ideas cover most of it.
Balance. Balance means the visual weight feels even, not that both sides match. A small bright shape can balance a big dark one. In Desert World the large sun sits left of center and the small blue sun sits up and to the right, so the sky feels balanced without being symmetrical.
Rule of thirds. Split the frame into a 3 by 3 grid and place the main subject where two lines cross. Off-center subjects usually feel more alive than centered ones. Centering is still fine when you want stillness or focus.
Empty space. The empty area around a subject is called negative space. It gives the subject room and makes it stand out. The dark skies in our worlds are what make single bright stars noticeable.
Direction of lines. Flat lines feel calm, vertical lines feel strong, and diagonal lines feel like movement. The lava in Volcano World runs diagonally, which is why that world feels active even when little is happening.
Try it: cover the small sun in Desert World with your thumb. The picture gets noticeably flatter.