The Oort Cloud: The Edge of Our Solar System
TL;DR Summary
Trillions of icy comets orbit the Sun in a massive, spherical shell located halfway to the nearest star.
Where does the solar system end?
Many people think it ends at Pluto, or the Kuiper Belt. In reality, the Sun's gravitational dominance extends vastly further, encompassing a massive, theoretical sphere of icy bodies known as the Oort Cloud.
A Sphere of Ice
Unlike the planets, which all orbit the Sun in a relatively flat disk (the ecliptic), the Oort Cloud is believed to be a giant, hollow, spherical shell surrounding the entire solar system.
It is incredibly distant. The inner edge of the Oort Cloud begins roughly 2,000 to 5,000 Astronomical Units (AU) from the Sun (one AU is the distance from the Earth to the Sun). The outer edge extends all the way out to 100,000 AU. That is over a light-year away—more than halfway to Proxima Centauri, the nearest star.
The Home of Long-Period Comets
The Oort Cloud is composed of trillions of icy bodies—chunks of water, ammonia, and methane ice. These are the pristine leftovers from the formation of the solar system 4.6 billion years ago.
Occasionally, a passing star, a giant molecular cloud, or the tidal forces of the Milky Way galaxy itself will gravitationally disturb the Oort Cloud. This perturbation knocks an icy body out of its orbit and sends it plummeting toward the inner solar system.
When it approaches the heat of the Sun, the ice begins to vaporize, forming a glowing coma and a spectacular tail. This is the origin of "long-period comets," such as Comet Hale-Bopp, which can take thousands or even millions of years to complete a single orbit around the Sun.
Have We Reached It?
Because the Oort Cloud is so dark, distant, and sparse, no telescope has ever directly observed an object within it. Its existence is entirely inferred from the trajectories of incoming long-period comets.
NASA's Voyager 1 spacecraft is currently the fastest human-made object fleeing the solar system. Even traveling at over 38,000 mph, it will take Voyager 1 about 300 years just to reach the inner edge of the Oort Cloud, and nearly 30,000 years to cross it and finally leave the solar system entirely.