* Planets don't undergo nuclear fusion: Unlike stars, planets lack the immense mass and pressure needed to initiate nuclear fusion in their cores. Nuclear fusion is the process that powers stars and produces their light and heat.
* They reflect light: Planets shine because they reflect light from their host star. We see planets because sunlight bounces off their surfaces.
* They emit some heat: Planets can emit some infrared radiation due to their internal heat, but this is usually faint and not visible to the human eye.
In summary: Planets are non-luminous because they don't generate their own light. They are visible to us because they reflect the light from their host star.
