Kirkwood

This is our asteroid belt. The horizontal axis is the orbital period, labelled with semi-major axis (loosely, average distance from the sun) and with period expressed as a simple fraction of Jupiter's period. The vertical axis is the eccentricity of the orbit. An eccentricity of zero is a circle, higher numbers are more oval. The color represents the inclination of the orbit, or how it is tilted out of the plane of the solar system.

One of the most striking features of this plot is the gaps, called Kirkwood gaps after Daniel Kirkwood (1814-1895). The scale of the plot highlights the fact that simple fractions of Jupiter's period (with the notable exceptions of 2/3 and 2/7) tend not to be the orbital periods of asteroids. Kirkwood hypothesized that resonances, with the asteroid repeatedly being in the same place, both in its orbit and relative to Jupiter, would destabilize the orbit.

This is Kirkwood's asteroid belt. These are the asteroids he knew about when he announced his discovery in 1866, the others have been discovered since his time. The patterns are there, some of the gaps can be discerned, but would you spot them if you didn't already know where to look? I suspect I wouldn't. Kirkwood did.

The 2/3 resonance orbit was much further away than any known asteroids at the time. The asteroid Thule was the first to be discovered that far away, in 1888.

The 2/3 resonance is now known to be stable, and several Kuiper Belt objects (including Pluto) bear its opposite, a 3/2 relationship to the planet Neptune.