The inverse square law is a principle in physics that describes how the intensity of a physical quantity, such as light or radiation, decreases with the square of the distance from the source. In the case of the Sun's intensity, if it followed the inverse square law, the amount of sunlight reaching distant planets like Mars, Jupiter, and Saturn would indeed decrease as the square of the distance from the Sun.
However, despite the decreasing intensity, we can still see these planets because they reflect sunlight. The planets in our solar system, including Mars, Jupiter, and Saturn, are not self-luminous like the Sun. Instead, they shine by reflecting the sunlight that falls upon them.
When sunlight reaches these planets, their surfaces scatter and reflect the light back towards our vantage point on Earth. Even though the intensity of the sunlight diminishes with distance, a significant amount of light is still reflected back to us, allowing us to observe these planets in the night sky.
It's important to note that while the intensity of sunlight decreases with distance, the planets are relatively close to Earth compared to the vast distances between stars. Therefore, they still receive sufficient sunlight to be visible to us, even when accounting for the inverse square law.