What to do

Drag the test point from far outside the planet to its centre.
Watch the brown field arrow E grow to a maximum at the surface, then SHRINK to zero at the centre.
Predict first: is the field strongest at the surface, or deep inside?

What to observe

Outside the planet the field falls as E = GM/r² — the inverse-square law.
Inside a uniform sphere only the mass closer to the centre than you pulls (shell theorem), so E = GMr/R³ grows linearly from 0 at the centre to g at the surface.
The field is largest exactly at the surface, r = R.

What it means

The shell theorem is why g rises linearly with depth toward the centre and falls as 1/r² with altitude — one idea explains both variations of g.

Why this matters

The shell theorem is what lets you treat a whole planet as a point: outside, only the total mass matters, which is why g = GM/R² works at all and why orbits do not care about a planet's internal structure.
Inside, the mass further out than you pulls in every direction and cancels exactly, so only the sphere below your feet acts. That is the reason g falls to zero at the centre rather than diverging, and it is why a tunnel through the Earth gives simple harmonic motion.
So the field peaks at the surface — because that is the one place where all the mass is still pulling and you are as close to it as you can get.

🌐 Gravitational Field — E = GM/r², and the shell theorem