The Greeks didn't actually use the word "planet" to describe unusual stars in the sky. They used the word "planetes", which means "wanderer" in Greek.

Here's why they called these celestial objects "wanderers":

* Their movement: Unlike the fixed stars that appeared to remain in the same relative positions, these objects moved across the sky in a seemingly unpredictable way. They wandered amongst the constellations, sometimes moving forward, sometimes backward, and sometimes seeming to stand still.

* Their brightness: Some of these "wanderers" were exceptionally bright, even brighter than many stars. This further set them apart from the fixed stars.

The Greeks observed five "wanderers": Mercury, Venus, Mars, Jupiter, and Saturn. They didn't know that these were planets in the modern sense of the word, but their observations and the name they gave them were essential building blocks in our understanding of the solar system.