Question

why is it hard to see planets in a scale model of the solar system?

Explanation:

The extreme ratio of the vast distances between planets (and between planets and the Sun) to the tiny size of the planets themselves means that when scaled down, planets become far too small to be easily seen, while the space between them remains disproportionately large, making the tiny planets hard to locate and visualize.

Answer:

It is hard to see planets in a scale model of the solar system because of the enormous difference in scale between the tiny size of planets and the immense distances between them and the Sun. When scaled down, planets become extremely small (often microscopic or nearly invisible) relative to the vast, empty scaled distances, making them difficult to spot.