Kuiper Belt
Both Arrokoth (recently visited by NASA’s New Horizons mission) and Pluto lie in the Kuiper Belt, a donut-shaped region of icy bodies beyond the orbit of Neptune. There may be millions of these icy objects, collectively called Kuiper Belt objects (KBOs) or trans-Neptunian objects (TNO), in this distant region of our solar system.
Like the asteroid belt, the Kuiper Belt is a region left over from the early history of the solar system. Like the asteroid belt, it has also been shaped by a giant planet, although it is more of a thick disk (like a donut) than a thin belt.
The Kuiper Belt should not be confused with the Oort Cloud, which is a much more distant region of icy, comet-like bodies that surround the solar system, including the Kuiper Belt. Both the Oort Cloud and the Kuiper Belt are believed to be sources of comets.
The Kuiper Belt is really a frontier in space, it is a place that we are still beginning to explore and our understanding continues to evolve.
The Kuiper Belt is a large region in the cold, outer reaches of our solar system beyond the orbit of Neptune, sometimes called the “third zone” of the solar system. Astronomers believe that there are millions of small, icy objects in this region, including hundreds of thousands more than 100 kilometers wide. Some, including Pluto, are more than 600 miles (1,000 kilometers) wide. In addition to rock ice and water, objects in the Kuiper Belt also contain a variety of other frozen compounds such as ammonia and methane.
The region is named after astronomer Gerard Kuiper, who published a scientific paper in 1951 that speculated on objects beyond Pluto. Astronomer Kenneth Edgeworth also mentioned objects beyond Pluto in articles he published in the 1940s, and is therefore sometimes referred to as the Edgeworth-Kuiper Belt. Some researchers prefer to call it the trans-Neptunian region, and refer to Kuiper belt objects (KBOs) as trans-Neptunian objects, or TNOs.
Size and distance.
The Kuiper Belt is one of the largest structures in our solar system, others are the Oort Cloud, the heliosphere and the magnetosphere of Jupiter. Its general shape is like a puffed disc, or donut. Its inner edge begins in the orbit of Neptune, about 30 AU from the Sun. (1 AU, or astronomical unit, is the distance from the Earth to the Sun.) The inner and main region of the Kuiper belt ends at about 50 UA from the Sun.
So far, more than 2,000 trans-Neptunian objects have been cataloged by observers, representing only a small fraction of the total number of objects that scientists believe are out there. In fact, astronomers estimate that there are hundreds of thousands of objects in the region that are more than 100 kilometers wide or more. However, the total mass of all material in the Kuiper Belt is estimated to be no more than 10 percent of Earth’s mass.
Formation / Origins.
Astronomers think that the icy objects in the Kuiper Belt are left over from the formation of the solar system. Similar to the relationship between the main asteroid belt and Jupiter, it is a region of objects that could have come together to form a planet if Neptune had not been there. Instead, Neptune’s gravity shook this region of space so much that the small, icy objects there weren’t able to merge into one large planet.
The amount of material in the Kuiper Belt today could be only a small fraction of what was originally there. According to a well-supported theory, the changing orbits of the four giant planets (Jupiter, Saturn, Uranus, and Neptune) could have caused most of the original material — probably 7 to 10 times the mass of Earth — to be lost.
The basic idea is that early in the history of the solar system, Uranus and Neptune were forced to orbit further from the Sun due to changes in the orbits of Jupiter and Saturn. As they veered outward, they passed through the dense disk of small icy bodies that remained after the giant planets formed.
Neptune’s orbit was the farthest, and its gravity bent the paths of countless icy bodies toward the other giants. Jupiter eventually threw most of these icy bodies, either into extremely distant orbits (to form the Oort Cloud) or out of the solar system entirely. As Neptune hurled icy objects toward the sun, this caused its own orbit to deviate even further, and its gravitational influence forced the remaining icy objects into the range of places where we find them in the Kuiper Belt.
Today the Kuiper Belt is slowly eroding. The objects that remain there occasionally collide, producing smaller objects fragmented by the collision, sometimes comets and also dust that is blown out of the solar system by the solar wind.
Structure and characteristics.
The Kuiper Belt represents a huge volume of donut-shaped space in the outer solar system. While there are many icy bodies in this region that we broadly refer to as Kuiper Belt Objects (KBOs) or Trans-Neptunian Objects (TNOs), they are quite diverse in size, shape, and color. Most importantly, they are not evenly distributed throughout space — once astronomers began discovering them in the early 1990s, one of the first surprises was that KBOs could be grouped according to shapes and sizes. of their orbits. This led scientists to understand that there are several distinct groupings, or populations, of these objects whose orbits provide clues to their history. The category an object belongs to has a lot to do with how it has interacted with Neptune’s gravity over time.
Most of the objects in the Kuiper belt are found in the main part of the belt itself or in the scattered disk:
Classic KBOs
A large fraction of KBO orbits the Sun in what is called the classical Kuiper belt. The term “classic” refers to the fact that, among KBOs, these objects have orbits more similar to the original, or classical, idea of what the Kuiper Belt was expected to look like, before astronomers began to find objects. there. (The expectation was that, if there were objects beyond Neptune, they would be in relatively circular orbits that are not tilted too much from the plane of the planets. Instead, many KBOs are found to have significantly elliptical and inclined orbits. Therefore To some extent, the KBO classification still reflects our evolving understanding of this distant region of the solar system).
There are two main groups of objects in the classic Kuiper Belt, known as “cold” and “hot”. These terms don’t refer to temperature — instead, they describe the orbits of objects, along with the amount of influence Neptune’s gravity has had on them.
All classical KBOs have a similar mean distance from the Sun between about 40 and 50 AU. Cold classic KBOs have relatively circular orbits that are not tilted far away from the plane of the planets; Hot classical KBOs have more elliptical and inclined orbits (which astronomers refer to as eccentric and inclined, respectively). This means that the cool variety spends most of its time at roughly the same distance from the Sun, while the hot ones move farther away over a larger range of distances from the Sun (meaning, in some parts of their orbits, they are closer to the Sun and sometimes they are further away).
The differences between these two types of bodies in the classic Kuiper Belt have everything to do with Neptune. The cold classic KBOs have orbits that never get very close to Neptune, and therefore remain “cool” and not disturbed by the giant planet’s gravity. Their orbits probably haven’t moved much for billions of years. In contrast, hot classical KBOs have had interactions with Neptune in the past (that is, with the giant planet’s gravity). These interactions pumped energy into their orbits, which stowed them elliptically, and tilted them slightly out of the plane of the planets.
Resonant KBOs
A significant number of KBOs are in orbits that are tightly controlled by Neptune. They orbit in resonance with the giant planet, which means that their orbits are in a stable and repeating pattern with that of Neptune. These resonant KBOs complete a specific number of orbits in the same amount of time that Neptune completes a specific number of orbits (in other words, a ratio). There are several of these groups, or resonances: 1:1 (pronounced “one to one”), 4:3, 3:2, and 2:1. For example, Pluto is in a 3:2 resonance with Neptune, which means that circles the Sun twice for every three times Neptune spins.
Scattered disk.
The scattered disk is a region that extends well beyond the main part of the Kuiper Belt, and houses objects that have been scattered by Neptune in highly elliptical orbits and very inclined to the plane of the planets. Many scattered disk objects have orbits tilted by tens of degrees. Some venture hundreds of AU from the Sun and well above the plane of the planets at the farthest point in their orbits, before falling back to the closest point near Neptune’s orbit. The orbits of many objects in the scattered disk still evolve slowly, with objects being lost over time, compared to the classic Kuiper belt, where the orbits are more stable. The scattered disk gives the classic Kuiper belt a shape of donut a much wider and thicker spread. Some astronomers speak of the two as separate regions, although their boundaries overlap and are linked in various ways. (In particular, objects from both regions are believed to have ended up there as a result of Neptune’s migration from its original orbit and closer to where it is now.)
Eris is an example of an object in the scattered disk (in fact, it is the largest known member of this population).
Additional families.
Most of the objects in the Kuiper Belt are found in the main part of the belt or in the scattered disk, but there are also a couple of additional families of objects that orbit the interior of the Sun and the exterior of the belt. These additional groups of objects probably came from the Kuiper Belt originally, but have been removed from the main regions by Neptune’s gravity or perhaps another massive planet.
Separate objects.
Separate objects in the Kuiper Belt have orbits that never get closer to the Sun than around 40 AU. This sets them apart from most other KBOs, which spend at least part of their orbits in the region between 40 and 50 AU from the Sun. Because their orbits do not approach Neptune’s distance from the Sun (30 AU), it seems unlikely that the separated objects have been removed from the Kuiper Belt by interactions with the giant planet. Scientists believe that some other force is likely responsible, such as an undiscovered giant planet (in a very distant orbit), the gravity of passing stars, or gravitational disturbances such as the Kuiper Belt was forming long ago.
Sedna is an example of a separate KBO. The closest it gets to the Sun is 76 AU, while at its furthest point it travels at 1200 AU.
Centaurs.
Centaurs are objects with orbits that travel through space between the orbits of Jupiter and Neptune. In these orbits, they interact strongly with the gravity of the giant planets. Due to these powerful gravitational encounters, most are destined to be ejected from the solar system or pushed into the inner solar system where they become comets or crash into the Sun and planets.
This process the removal of the Centaurs is ongoing, taking tens of millions of years for the typical Centaur object. Therefore, the fact that there are centaurs around today is evidence that they are being actively supplied from elsewhere. Astronomers think the most likely explanation is that they are relatively recent fugitives from the Kuiper Belt. In fact, it is understood that the centaurs are scattered objects, like those of the scattered disk, the difference is that the Centaurs have been scattered closer to the Sun by Neptune, rather than further.
Pluto’s place in the Kuiper belt.
Pluto was the first Kuiper Belt object to be discovered, in 1930, at a time before astronomers had reason to expect a large population of icy worlds beyond Neptune. Today it is known as the “King of the Kuiper Belt” it’s the largest object in the region, even though another object similar in size, called Eris, has a slightly higher mass. Pluto’s orbit is said to be in resonance with Neptune’s orbit, which means that Pluto’s orbit is in a stable, repeating pattern with Neptune’s. For every three orbits completed by Neptune, Pluto makes two orbits. In this situation, Pluto never gets close enough to Neptune to be greatly affected by its gravity. In fact, even though its orbit crosses the orbit of Neptune, Pluto is physically closer to Uranus than to Neptune.
Kuiper belt moons and binaries.
A fairly large number of KBOs have moons that is, bodies significantly smaller than they orbit — or are binary objects. Binaries are pairs of objects that are relatively similar in size or mass that orbit around a point (a shared center of mass) that lies between them. Some binaries actually touch, creating a kind of peanut shape, creating what is known as a contact binary.
Pluto, Eris, Haumea, and Quaoar are all Kuiper Belt objects that have moons. Observations from the telescope suggest that the target for NASA’s New Horizons spacecraft 2019 flyby, known as 2014 MU69, may be a contact binary.
One thing that makes binary KBOs particularly interesting is that most of them can be extremely old, or primordial, objects that have been little altered since their formation. The various ideas about how these pairs are formed require many more objects than the current Kuiper Belt seems to contain. One main idea is that binaries can result from low-speed collisions between KBOs, allowing them to survive the impact and stay together due to their mutual gravity. These collisions were probably much more common billions of years ago, when most KBOs were in similar orbits that were more circular and close to the plane of the planets (called the ecliptic). Today such collisions are much rarer. They also tend to be destructive, as many KBOs are now in orbits that are inclined or elliptical, which means they collide with each other more strongly and break apart.
Relationship with comets.
The Kuiper Belt is a source of comets, but not the only source. Today the Kuiper Belt is believed to be very slowly eroding. Objects there occasionally collide, with the collision fragments producing smaller KBOs (some of which can become comets), as well as dust being blown out of the solar system by the solar wind. Pieces produced by colliding KBOs can be pulled by Neptune’s gravity into orbits that send them toward the sun, where Jupiter corners them in short loops that last 20 years or less.
These are called Jupiter-family short period comets. Given their frequent trips to the inner solar system, most tend to deplete their volatile ice fairly quickly and eventually go dormant, or dead, comets with little or no detectable activity. Researchers have discovered that some near-Earth asteroids are burned-out comets, and most of them would have started in the Kuiper Belt. Many comets collide with the Sun or planets. Those who have close encounters with Jupiter tend to get ripped apart or pulled out of the solar system entirely.
