In 1866 the American astronomer Daniel Kirkwood was studying all the known asteroids, which at the time amounted to several hundred. If you just look at a snapshot of the solar system, there’s nothing that particularly stands out about the asteroids. They appear to have all sorts of random positions and random orbits within the main asteroid belt, which sits halfway between Mars and Jupiter.
But when Kirkwood mapped out all their orbits, he noticed something odd: gaps. Orbits that by all rights should have had asteroids, but simply didn’t. These gaps happened at very precise distances from the sun, for example at 2.065 AU, 2.502 AU, and 3.279 AU.
Helpfully for future astronomers, Kirkwood was able to provide an explanation for these gaps, and his explanation stands even today.
The problem? The biggest bully in the solar system: Jupiter.
Jupiter’s gravity is so massive that it affects pretty much everything in the system, including the Sun, causing it to wobble as the giant planet orbits around it.
imagine you’re an asteroid, minding your own business in the asteroid belt. You have your own orbit with your own orbital speed and your own orbital period. Everything’s great. But if you’re at a precise orbital distance from the Sun, then you and Jupiter can end up at the same side of the solar system every few orbits. These are called resonances. For example, if you have a 2:1 resonance with Jupiter, then every second orbit you will find yourself close to Jupiter. If you have a 5:1 resonance, then every 5th orbit you’ll align.
If you’re not in one of these special orbits, then your close encounters with Jupiter will happen randomly, and so you’ve got nothing to worry about.
But if you do happen to be in resonance, then over the course of millions of years the gentle gravitational tug of Jupiter can destabilize your orbit, sending you flying into the Sun or getting kicked out of the solar system altogether.
The result: gaps, where asteroids tried to make a home for themselves, but Jupiter didn’t let them.
