Comets are icy bodies usually found in the outer reaches of our Solar System, though, as we recently discovered, they are also found in others. Comets become active once they get close to the Sun, its radiation heating up the nucleus, causing the frozen ice to sublimate. They can become so active that they can fall apart, so given their relatively ephemeral nature, it is surprising that so many comets exist in the Solar System with a period – the time it takes to go around the Sun – of fewer than 20 years.

These objects are known as Jupiter-family comets (JFCs). A new study published in The Astrophysical Journal suggests that beyond Jupiter there’s a region that acts as a gateway shepherding these objects into the inner Solar System.

The process has a few steps. To begin with, our comet will be just another body beyond the orbit of Neptune. Occasional close encounters with the blue giant planet can throw our comet into a closer orbit around the Sun. But not too close; it will be trapped between the orbits of Neptune and Jupiter. At this point, our comet becomes part of the centaurs family of giant comets.

In the study, simulations showed that there is a donut-shaped region that allows for the centaurs to jump in and become Jupiter-family comets. By being thrown about by the gravity of the gas giants over time they end up sneaking through this "comet gateway".

"They rattle around for a few million years, perhaps a few tens of millions of years, but none of them were there even close to the time when the Solar System formed," co-author Walter Harris at the University of Arizona said in a statement. "We know of 300 centaurs that we can see through telescopes, but that's only the tip of an iceberg of an estimated 10 million such objects."

Understanding the motion of these objects is not easy and that’s where the simulations come in. The team started with simulated trans-Neptunian objects and followed them as they moved closer and closer to the Sun.

"The chaotic nature of their orbits obscures the exact pathways these centaurs follow on their way to becoming JFCs," added Katheryn Volk, a co-author on the paper and an associate staff scientist who studies Kuiper belt objects, planetary dynamics and planets outside our Solar System, at the University of Arizona. "This makes it difficult to figure out where exactly they came from and where they might go in the future."

The most famous object in the gateway region is known as 29P/Schwassmann–Wachmann 1 or SW1 a comet 60 kilometers (40 miles) across quite the size for a comet. Based on the simulations, SW1 is destined to move forth over the next 40,000 years and the authors believe it could be a comet of incredible brilliance, more than what has been reported in recorded history.