Why did astronomers decided to group the plates inner and outer ?

Question

Why did astronomers decided to group the plates inner and outer ?

Answer 1

Astronomers divide the planets into terrestrial, which are small and rocky like the Earth, and gas giant, which are big and made of gas, like Jupiter.

As it happens, the four inner ones are terrestrial and the four outer ones are gas giants, so it is easier just to talk about them as "inner" and "outer".

Answer 2

Excuse me. This question needs to be rephrased. It make no sense as it is written

Answer 3

There is an asteroid belt between Mars and Jupiter. The planets on the side of the asteroid belt that is closer to the sun are the inner planets and the planets on the side of the asteroid belt that is further away from the sun are the outer planets.

Also astronomers must have put the closest 4 planets to the sun (Mercury, Venus, Earth & Mars) into one category because they are similar in size and are much smaller than the outer planets and they are really close together.