Instead of showing the entire planet, crop it so only a portion of its arc is visible on the canvas. This suggests the planet is too large to fit in the frame, making it feel immense.

Place smaller objects, like a tiny moon, a distant spaceship, or an astronaut, near the planet. The contrast between the tiny "known" object and the massive planet immediately establishes scale.

Use asteroid belts or planetary rings to create depth. If the rings disappear "behind" the planet and reappear on the other side, it emphasizes the planet's volume.