Planets do not generate their own light. Instead, they reflect the light they receive from the primary source of light in their respective solar systems, which is typically a star like our Sun. When sunlight, or starlight in the case of other star systems, reaches a planet's atmosphere, it interacts with the planet's surface, clouds, and atmospheric particles.
The reflected light is what enables us to see planets from Earth. The amount of light reflected depends on various factors such as the planet's composition, atmosphere, and surface properties. For example, a planet with a dense atmosphere or covered in reflective surfaces like ice or clouds will reflect more light, making it appear brighter.
It's important to note that planets also emit some of their own light, but this is usually in the form of thermal radiation, which is much weaker compared to the light they reflect from their star. This thermal radiation is predominantly in the infrared part of the electromagnetic spectrum and is not typically visible to the naked eye.