What a planet is.

[oriel36]

The isolation of a planet as a term within the Ptolemaic framework was defined by the Sun moving directly through the constellations. In contrast, the planets wandered North and South of a line scribed by the Sun's annual motion and called the ecliptic.

"Moreover, we see the other five planets also retrograde at times and
stationary at either end [of the regression]. And whereas the Sun
always advances along its own direct path, they wander in various
ways, straying sometimes to the south and sometimes to the north; that
is why they are called "planets" [wanderers]. Copernicus

This is why a heliostatic system was proposed as a hypothesis insofar as the Ptolemaic framework had been used for many centuries as a predictive utility. Only today, with satellite imaging free from the daily rotational influences on observations seen from a rotating Earth, can the conclusion for direct/retrogrades be directly affirmed with the back-and-forth motions of Venus and Mercury seen from a slower-moving Earth.

Theorists, unfamiliar with astronomy and its traditions, recently attempted to define a planet in isolation.