By Emily Wearing
Introduction
The Edgeworth-Kuiper belt, otherwise known as the Kuiper belt, is a flat ring consisting of many small celestial bodies and is one of the largest structures in our solar system. Whether life as we know it can be supported in this far-off, frigid zone remains unknown. The belt extends from Neptune’s orbit at 30 astronomical units, to roughly 50 astronomical units (AU*) from the sun. It is home to up to millions of icy bodies called Kuiper belt objects or ‘KBOs,’ as well as Pluto, numerous other dwarf planets and comets. This area is sometimes referred to as the ‘third zone’ of our solar system.
The belt has some similarities with the Milky Way’s main asteroid belt between Mars and Jupiter - since both of these consist of bodies and remnants from the formation of the solar system and both are relatively disc-shaped. However, the Kuiper belt is much greater in size: approximately 20 times wider.
*1 AU= distance from Earth to Sun
History and Formation
The region is named after an astronomer called Gerard Kuiper, who published a paper in 1951 hypothesising that there were objects beyond Pluto. This suggestion was also mentioned in papers published in the 1940s by astronomer Kenneth Edgeworth, hence the fact the belt is sometimes referred to as the Edgeworth-Kuiper belt. However, Julio Angel Fernandez was the first researcher to expand Kuiper’s theory of objects beyond Pluto into the existence of a comet belt in 1980, making him the first to predict the presence of the Kuiper Belt.
Despite this, Pluto was the first object found in the Kuiper Belt in 1930 by Clyde Tombaugh, before astronomers had any cause to believe there was a population of cold bodies beyond Neptune. The dwarf planet is referred to as “King of the Kuiper Belt” and it is the largest body in the region. Eris, another dwarf planet of comparable size, is slightly greater in mass. Astronomers questioned whether Pluto should actually be regarded as a planet or as one of the largest bodies in the Kuiper belt. As evidence grew that Pluto was a KBO that was coincidentally discovered before the belt, the International Astronomical Union decided to designate both Pluto and Eris as dwarf planets in 2006.
12 years after Fernandez’s hypothesis, astronomer David Jewitt and student Jane Luu discovered 15760 Albion from the Mauna Kea observatory in Hawaii, which was shortly dubbed by the media as the tenth planet of our solar system. Despite this, it was soon categorised as the first KBO. Furthermore, it was the first trans-neptunian object (TNO) to be found post Pluto and Charon. From its brightness, the diameter of the body is estimated to be around 200-250 km. This discovery alerted astronomers of the possibility of more KBOs, therefore as a result over 1500 more have been detected within the subsequent 20 years, although there are an estimated trillion or more comets left to discover within the belt. ‘New Horizons’ is the first mission to investigate the Kuiper Belt. In 2015, it passed Pluto while travelling to investigate another planet in the Kuiper Belt.
Kuiper Belt’s Formation
The frozen objects are believed by astronomers to be leftovers from the solar system’s origin. It is an area of objects that, in the absence of Neptune, might have formed a planet, similar to Jupiter and the main asteroid belt’s relation. Instead, the icy, small particles in this region were agitated by Neptune’s gravity to such an extent that they were unable to form a single, massive planet. There is the possibility that the current amount of material in the Kuiper belt is simply a tiny fraction of what once was present, seeing that it is currently gradually disappearing. The remnants occasionally collide, creating dust that is blasted out of the solar system by solar wind, as well as smaller objects broken apart by the impact. Occasionally, comets can also be produced as a result of the collision.
Structure & Characteristics
This belt is one of the largest structures in our solar system, and its overall shape is reminiscent of a doughnut, or a puffed-up disk. A second region known as the ‘Scattered Disk’ overlaps the outermost part of the main Kuiper Belt, extending outwards to almost 1000 AU (1000 times the distance between the Earth and Sun) with some orbital bodies that extend greatly beyond. Furthermore, astronomers anticipate that hundreds of thousands of objects greater than 100 km in diameter are situated in the region, with bodies like Pluto exceeding 1000 km. Nonetheless, it is estimated that the mass of all material in the belt as a whole makes up no more than 10% of Earth’s mass. Alongside rock and water ice, numerous other frozen compounds are contained within this belt, for instance methane and ammonia.
KBOs in this region are located in either the belt’s central region, or in the scattered disk. These are rather varied in terms of size, form and colour, and, significantly, are not equally dispersed over space. In the early 1990s, one surprise to astronomers was that these could be grouped in relation to orbital shape and size.
Classical KBOs
KBOs whose orbits most closely resemble the original, or classical, theory of the Kuiper Belt's composition before astronomers began actually discovering objects within it. The expectation of astronomers was that the orbits would be circular, however many have been found to have noticeably elliptical orbits.
The classical Belt is divided into two primary object groups, "cold" and "hot." However, these categories don't relate to temperature, but instead to how the orbits and Neptune's gravity has affected them. The “cold” KBOs are "cool" and unaffected by Neptune's gravity, since their orbits have never passed extremely close to the planet. For billions of years, their orbits most likely haven't shifted all that much. On the other hand, the “hot” KBOs have already interacted with Neptune's gravity. The orbits were pumped with energy from these interactions, which caused them to elongate and tilt slightly away from the planet plane.
Resonant KBOs
A large fraction of KBOs are in orbit under strict Neptune control. They are in a steady, repetitive pattern in accordance with Neptune, indicating they are in resonance with the planet. There are several different resonances, for instance Pluto and Neptune are part of a 3:2 resonance, meaning that Pluto orbits the Sun twice for every three times Neptune does. There are many different ratios for different KBOs, but Astronomers have classified the plutinos (3:2) as a distinct group, since there are so many objects on orbits with this resonance.
Detached Objects
The orbits of detached Kuiper Belt objects never go closer to the Sun than roughly 40 astronomical units, distinguishing them from the majority of other KBOs, which spend most of their orbits 40-50 AU from the Sun. Some experts believe that an undiscovered massive planet orbiting a far-off star could be the cause for these detached bodies. One illustration of these is the dwarf planet Sedna, which travels up to 1200 AU to reach its farthest point from the Sun, with 76 AU being its closest approach.
Final note
There is still much to learn about the Kuiper belt, with researchers continuing to unveil new information regarding the region and whether or not some bodies could hold the capability of supporting human life.
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