The meaning of “planet” has changed greatly over the centuries as our understanding improved. It originally meant “wanderer” and referred to the five naked-eye planets known since antiquity plus the sun and moon, but NOT the Earth.

As astronomers and astrophysicists learned more about them, it became clear that classifying all stellar objects as the same category was unworkable. Earth was re-classified as a planet, the moon as a moon, and the sun as a star.

Likewise, Pluto was reclassified because the old classification made little sense.