Because in the heliocentric model, we claim to know the specific structure of all the forces involved (namely, gravity). In the geocentric model, there are epicycles upon epicycles, but the mathematical details of the forces that cause the planets to move in those convoluted paths are unknown. Without those details, you can't say whether you'd expect the orbits to be stable or unstable.
For that matter, why is that even a gap at all? What would cause the orbits to be unstable?
The fact that the planets interact with each other via gravity. For example, every time we swing past Jupiter on the same side of the sun, we get a little tug, and our orbit gets distorted a little bit. Those distortions pile up over time, and eventually the planets kick each other out...at least, according to the computer simulations.
Obviously, planetary orbits are stable over the long term in the real solar system. Something keeps them stable, but there is no unanimity on what that something is. The problem is enormously complicated, and the answer, subtle.
That's not true. The heliocentric model was quite well-developed before any coherent theory of gravity was put forth.
Those distortions pile up over time, and eventually the planets kick each other out...at least, according to the computer simulations.
Now that I agree is interesting. Still, however, it doesn't in any way call into question the heliocentric theory - that is, the explanation for planetary motion, particularly the epicycles, first articulated by Copernicus.
Agreed! :-)
There are still arguments about the Titius-Bode law and whether it's even a valid one. I happen to think it is. The coincidences are pretty big if not. However, the mechanisms are still not understood.