Astrology is a Human Science and it was drafted by the Yogis of Hindus who lived in ancient Indian and in the Kumarikanda continent region.
Astrology is a knowledge base, was created for the Human and is being practiced by the Human who lives on Earth.
Since Humans live on the earth, the yogis took the Earth as an axis point to calculate and to predict Human nature, matching the Natures and their fate.
Also, Astrology studies the influence of the heavenly bodies around Earth and Humans since humans life on the Earth and not on another planet.
The Human and other living being lives on Earth are the recipients of the influence of other Solar Systems and the skies above and around.
Since Astrology studies the influence on us from the skies and solar system, Earth is not included in the list of the navagrahas. But included Rahu and Kethu which are not actually planets but demons who were given a place among the Navagrahas after consuming the Nectar. Hence the countings of Navagrahas have come to 9 instead of 8 Planets.