Kuiper Belt
The Kuiper belt is a ring of icy bodies beyond Neptune, from about 30 to 50 AU, that contains Pluto and is the source of short-period comets.
Definition
The Kuiper belt is a huge ring of icy objects beyond Neptune at the cold outer edge of the solar system, like the asteroid belt but much bigger and made of ice, with Pluto as its most famous member. It is a disk-shaped region from Neptune's orbit at about $30$ AU out to about $50$ AU containing the dwarf planets Pluto, Haumea, and Makemake and probably hundreds of thousands of objects larger than $100\ \text{km}$, made of frozen water, methane, and ammonia mixed with rock, and it is the source of comets with periods under $200$ years. Its objects survived from the primordial planetesimal disk and fall into distinct groups: the classical belt on low-eccentricity orbits near $42$ to $48$ AU, resonant objects such as the plutinos in Neptune's $2{:}3$ resonance near $39.4$ AU, and the scattered disk on highly eccentric orbits. This structure records Neptune's early outward migration, and the belt's present mass of only a few percent of Earth's implies most of the original material was lost.
Example
In 2015 New Horizons flew past Pluto and returned the first close-up pictures of a Kuiper belt world, with ice mountains and a heart-shaped plain. Sunlight at $40$ AU is only $1/1600$ as strong as at Earth, so surface temperatures there are around $-230^\circ\text{C}$, and objects take $250$ to $350$ years to orbit the Sun. On 1 January 2019 New Horizons passed the classical belt object Arrokoth at $43$ AU, revealing a $36\ \text{km}$ contact binary whose gently merged lobes show slow accretion in an undisturbed environment.
Key Insight
The Kuiper belt is where many comets come from; when one is nudged sunward, its ice begins to vaporize and it grows a tail. The belt was predicted before it was found, its first member after Pluto was discovered in 1992, and the flood of discoveries that followed led to Pluto's reclassification. The Kuiper belt and the far more distant Oort cloud feed different comet families: short-period comets near the ecliptic come from the belt, and long-period comets arriving from every direction come from the Oort cloud.