10 Interesting Facts about the Kuiper Belt

10 Interesting Facts about the Kuiper Belt
Image Credit: NASA

Also known as the Edgeworth–Kuiper belt, this structure extends from just beyond the orbit of the planet Neptune to a distance of roughly 50-55 astronomical units from the Sun. Composed primarily of small icy bodies that include the minor planets Pluto, Haumea and Makemake, much about the origin of the Kuiper belt remains unknown. What is known, however, is that Triton and Phoebe, satellites of the planets Neptune and Saturn respectively, are likely Kuiper belt objects that were captured by their host planets. Below are 10 more interesting facts about the Kuiper belt you may not have known.

Gerhard Kuiper did not discover the Kuiper belt

Although the Kuiper belt is named after the Dutch astronomer Gerhard Kuiper, he did not discover it, nor was he the first to hypothesize or predict its existence. While the historical record is somewhat unclear on who first suggested or predicted the existence of the structure, it is generally accepted that its existence was proposed as long ago as the 1930’s, soon after Pluto was discovered by Clyde Tombaugh. However, the astronomers David Jewitt and Jane Luu are credited with the official discovery of the Kuiper belt in 1992.

Short period comets do not originate in the Kuiper belt

While it was long thought that the Kuiper belt was the source of all, or most short-period comets, studies have shown that this is not the case. Improved methods have led to the discovery that short-period comets mostly originate in the scattered disc, a population of icy bodies that is linked to the Kuiper belt. This population of bodies was created by the planet Neptune as it moved through the present-day Kuiper belt as migrated away from the Sun, scattering a large number of objects that are still subject to gravitational perturbations by Neptune.

The Kuiper is torus-shaped

Excluding the scattered disc portion of the structure, but including its outlying regions, the Kuiper belt spans an area between 30 AU and 55 AU from the Sun. However, its densest part stretches from the 2:3 mean-motion resonance* to the 1:2 resonance* at about 48 astronomical units from the Sun. Overall, the Kuiper belt is very thick; the main concentration of objects extend to about ten degrees on either side of the ecliptic plane, with a more diffuse population extending several tens of degrees further. In general, the Kuiper belt is inclined with respect to the ecliptic by 1.86 degrees, and looks more like a torus (doughnut) than a flat belt or ring.

* In this context, “resonance” refers to an area or distance from the Sun at which objects in the Kuiper belt are locked into an orbital resonance with the planet Neptune.

The classical Kuiper belt has two distinctly different populations

The area between 42 and 45 astronomical units from the Sun is referred to as the “classical Kuiper belt, and this area is inhabited by two distinctly different object populations. The first is known as the “cold” population, and these objects have nearly circular orbits that are generally inclined with respect to the ecliptic by less than 10 degrees. The second population is known as the “hot” population, and consists of objects that have orbits that can be inclined by as much as 30 degrees or sometimes more. Note that the words “cold” and “hot” do not refer to the relative temperature of the two populations, but to the differences in their orbital velocities, after an analogy with how gas particles behave when they are heated up.

More than 99% of the Kuiper belt’s mass is missing

Although standard models of how the solar system was formed predict that the Kuiper belt should have about 30 Earth-masses, the structure is observed to weigh only about 1/25 to 1/10 as much as the Earth. This discrepancy is important, since the missing mass is required for objects that are bigger than 100 km (62 miles) to exist at all. Put simply, if the Kuiper belt had always had its observed low mass, the large bodies in the structure could not have formed. Studies have shown the Neptune’s’ current influence could not have expelled so much mass from the structure, although it may have expelled significant amounts of mass in the distant past as it migrated outward from the Sun. Although the issue of the missing mass remains unresolved, current conjectures range from passing stars that disrupted the structure of the Kuiper belt, to violent collisions that reduced large bodies to dust that was subsequently blown out of the solar system by the Sun’s solar wind.

Triton is a captured Kuiper belt object

Since Neptune’s moon Triton is only 14% bigger than Pluto (a confirmed Kuiper belt object), and appears to also have a composition similar to Pluto, most investigators agree that Triton is also a Kuiper belt object that was captured fully formed as Neptune migrated outward from the Sun. However, since capturing such a large object is not easy, there is still some debate about the mechanisms that allowed Neptune to do this. Current speculations include the possibility that Triton was one component of a binary object, and that one object was expelled from the system when Neptune captured both.

Neptune largely controls the structure of the Kuiper belt

Neptune’s gravitational field has a pronounced effect on the structure of the Kuiper belt. In fact, this effect is so powerful that at distances between 40 and 42 astronomical units from the Sun, no object can maintain a stable orbit due to the influence of Neptune’s gravity, and over timescales that compare to the age of the solar system, Neptune has either expelled objects in this region outward from the Kuiper belt into the scattered disc region, or have caused them to plunge into the inner solar system. This has resulted in pronounced gaps in the Kuiper belt that can be roughly compared to the Kirkwood-gaps in the asteroid belt that exists between Mars and Jupiter.

The Kuiper belt ends in a “cliff”

While the 1:2 resonance area in the Kuiper belt corresponds to a distance of about 50 astronomical units from the Sun, and theory predicted that the number of objects bigger than about 100km in diameter beyond this region should increase two-fold, no such increase has been observed. In fact, this area resembles a “cliff”, beyond which very few objects of any appreciable size exists, and has since its discovery become known as the “Kuiper Cliff”. Moreover, Bernstein, Trilling, et al. have found compelling evidence that the drop off in the number of large objects is not due to observational bias, but to date, nobody has yet proposed a credible mechanism that can explain the Kuiper Cliff.

The exact origins of the Kuiper belt remain unknown

While much is known about the Kuiper belt, even more remains unknown. For instance, while it is known that the structure is composed of millions of icy planetesimals, nobody has yet worked out how these bodies came to be where they are now. Although a large-scale investigation by the Pan-STARRS system was completed in 2014, investigators are still sifting through the data in the hope of decoding the exact origin of the Kuiper belt.

Kuiper belt-like structures are not unique to the solar system

Structures like the Kuiper belt appear to occur frequently around other stars. In fact, as of 2006, nine other stars have been observed to be surrounded by similar features that fall into two distinct classifications. One group have radii of over 50 astronomical units, while another group have radii of around 20-30 astronomical units, which is about the width of our solar system. Moreover, observations have shown that 15-20% of observed Sun-like stars are surrounded by areas of excess infrared radiation, which is indicative of hot discs of dust that surround these stars.

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