Here's why:
* Distance: Pluto is incredibly far away, averaging about 3.7 billion miles (5.9 billion kilometers) from Earth. This immense distance makes it too faint to be visible without powerful telescopes.
* Dimness: Pluto is also a relatively small and icy object, reflecting very little sunlight. This further reduces its apparent brightness.
Even with telescopes, observing Pluto can be challenging due to its faintness. It was not until 1930 that Pluto was discovered using a powerful telescope.
