1857

Kirkwood Gaps

Daniel Kirkwood (1814–1895)

The discoveries of the asteroids Ceres, Pallas, Juno, and Vesta in the early nineteenth century opened up a new area of solar system study—so-called minor planet research. For more than 35 years after the discovery of Vesta in 1807, however, no new asteroids were discovered. Improvements in telescope sensitivity by the middle of the century led to a sort of mini explosion in the size of the known population, though, and by 1857 there were 50 known asteroids, all orbiting in what would be known as the main asteroid belt, between Mars and Jupiter.

This asteroid population drew the attention of the American mathematician Daniel Kirkwood. Kirkwood had gained some fame in 1846 for proposing a sort of Kepler’s law for planetary spin rates versus distance from the Sun that initially seemed to be correct. Although the idea was eventually rejected once better data became available, he continued to pursue other solar system dynamical studies. When he examined the orbital properties of the 50 or so asteroids known as of 1857 he noticed a remarkable thing: instead of a uniform or random or even bell-shaped distribution of asteroid distances from the Sun, they appeared to bunch up into clusters, with many orbiting at certain distances and none orbiting at other distances.

Kirkwood found that the places in the main belt with no asteroids are places where an object orbits a number of times that can be expressed as a simple integer relative to Jupiter’s orbit. For example, no asteroids were found near 2.25 astronomical units, where an object would orbit the Sun exactly three times for every single Jupiter orbit. He correctly proposed that the resonance between Jupiter and any objects in such places would give the objects a little extra gravitational tug from Jupiter that would cause them to eventually be nudged away from that area, or even out of the main belt entirely.

More than 150 years of data have verified the prediction, and astronomers call these “gaps” in the main belt the Kirkwood gaps, in honor of their discoverer. The gap positions are also known as mean-motion resonances. Other such gaps have been found at the 5:2, 7:3, and other mean-motion resonances, as well as in the Rings of Saturn, where they form by mean-motion resonances among embedded moons.

An artistic depiction of the Kirkwood Gaps in the main asteroid belt between Mars and Jupiter. The gaps are regions where asteroids in orbital resonances with Jupiter have been nudged out of the main belt by the giant planet’s gravity.