The Earth-centered system, also known as the geocentric model, doesn't actually define a specific number of planets. It's a model of the universe where the Earth is considered the center, and everything else revolves around it.

Here's why it's tricky to count planets in this system:

* Historical context: Historically, the geocentric model included the Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn as planets. These were the celestial bodies that were visible to the naked eye and appeared to move against the background of stars.

* No defined boundary: The geocentric model was later disproven by the heliocentric model, which places the Sun at the center of the solar system. There's no strict definition of what constitutes a "planet" within the geocentric framework.

In short, while the geocentric model historically considered seven celestial bodies as planets, the modern understanding of planetary systems renders the concept of a fixed number of planets within that model obsolete.