The Kuiper belt or Edgeworth -
Kuiper is an outer region of the Solar System that extends from the orbit of the planet
Neptune, about 30 astronomical units from the Sun, up to 50 astronomical units from
the Sun; its overall shape is somewhat reminiscent of a donut. It takes its name from the astronomers
Gerard Kuiper and Kenneth Edgeworth, who in a 1951 article hypothesized the presence of
objects beyond the orbit of Neptune, later known as trans-Neptunian objects (TNO).
Unlike the main belt between the orbits of Mars and Jupiter, essentially composed of
rocky bodies, the Kuiper belt is composed of small icy bodies (mainly water ice, ammonia
and methane), whose orbits are arranged on the plane of the ecliptic, which is the same
plane on which the orbits of the planets also lie.
The presence of Neptune has a profound effect on the structure of the Kuiper belt due to
orbital resonances: an orbital resonance occurs when two orbiting bodies have periods of revolution
such that their relationship can be expressed in fractions of small whole numbers. For example
Pluto and Neptune are in 2:3 orbital resonance: this means that every three revolutions of
Neptune around the Sun, Pluto makes two.
Over a period of time comparable to the age of the Solar System it can happen that Neptune's
gravity destabilizes the orbits of objects in orbital resonance, sending them into the
inner Solar System, or pushing them outwards, towards interstellar space. The gravitational
influence of Neptune produces pronounced gaps in the current structure of the belt, that
is areas in which there are very few or no objects: these are precisely zones of gravitational
instability produced by orbital resonances.
Over 2,000 Kuiper belt objects (or KBOs) have been discovered in the belt and it is thought
that there may be over 100,000 with a diameter greater than 100 km, and that the total mass
of all objects in the Kuiper belt could be a tenth, or even one hundredth, of Earth mass.
In total, there could be millions of celestial bodies in the Kuiper Belt. Kuiper belt objects
can be roughly divided into "classical" and "resonant".
In the first case (classical Kuiper belt) the objects can keep their orbits unchanged
over time: here we find about two thirds of the KBOs observed to date. (15760) 1992 QB1,
the first modern KBO discovered, Pluto and Charon apart, is considered the prototype
of this group and from it derives the term Cubewanos, a term that indicates the classical
KBOs and which is the phonetic translation of the English acronym " QB1-os”. According
to an IAU guideline, the name to be given to classical KBOs is that of mythological
beings associated with creation. The classical KBOs are mainly concentrated in two areas
with orbital resonance with Neptune 2:3 and 1:2, between 42 and 48 astronomical units
from the Sun, where the gravitational influence of Neptune is negligible.
The classical Kuiper belt appears to be composed of two different populations. The first, known
as the "dynamically cold" population, is composed of objects with orbits very similar to those
of the planets, ie almost circular, with an orbital eccentricity less than 0.1, and orbital
inclinations not exceeding 10 degrees. The second, the "dynamically hot" population,
has much more inclined orbits than the ecliptic, up to 30 degrees. The two populations were
named in this way not because of a difference in temperature, but because of the analogy
with the particles of a gas, which increase their relative velocity with temperature.
The hot population is believed to have formed near Jupiter and been expelled by the movements
and gravitational interactions of the gas giants. Furthermore, it is believed that the
cold population formed more or less in its present position, although it may have been
flung outwards later, during the migration of Neptune, described by the Nice model (which
owes its name to the Observatoire de la Côte d'Azur of Nice, where it was formulated):
the Nice model is a physical model that simulates the dynamic evolution of the Solar System
in the phases immediately following its formation, and according to it Neptune and Uranus were
originally much closer to the Sun than they are today. Would you have liked to see this
important moment in the evolution of the Solar System? Write it in the comments!
As for the resonant Kuiper Belt Objects, they are in orbital resonance with Neptune. Objects
in 2:3 resonance with Neptune have a semi-major axis of about 39.4 AU and about 200 of this
type are known, including Pluto and its moons, prototype of the members of this class known
as Plutinos. Plutinos have high orbital eccentricities, suggesting that they didn’t originated in
their current locations, but were hurled out to the outer Solar System by the migration
of Neptune. IAU guidelines dictate that all Plutines must, like Pluto, be named after
deities associated with the underworld.
The 1:2 orbital resonance zone, whose objects complete one orbit while Neptune makes two
and have half-axes greater than 47,7 AU, is sparsely populated. Members of this class
are known as Twotini. There are other resonances, with ratios 3:4, 3:5, 4:7 and 2:5. Neptune
also has a number of Trojan asteroids occupying its Lagrangian points L4 and L5; these are
often in 1: 1 resonance with Neptune and have generally stable orbits.
The largest body in the Kuiper belt is Pluto and the most massive is the dwarf planet Eris,
discovered in 2005, although some scientists consider Eris to be part of the diffuse disk
rather than the Kuiper belt. Since the year 2000, other objects of considerable size have
been found: 50000 Quaoar, discovered in 2002, half the size of Pluto and larger than Ceres,
the largest of the traditional asteroids. The exact classification of these objects
is unclear, because they are probably very different from the innermost asteroids. Some
satellites of the planets of the Solar System appear to come from the Kuiper belt, such
as Triton, the largest of Neptune's moons, and the Saturnian moon Phoebe. It is believed
that short-period comets, that is, those that take less than 200 years to travel their orbit
around the Sun, come from the Kuiper belt.
In addition to the classic and resonant Kuiper Belt objects, the diffuse disk should also
be mentioned, a sparsely populated region whose innermost part overlaps the Kuiper Belt
and extends up to 100 AU from the Sun and beyond. Unlike what happens for the objects
of the innermost Kuiper belt that we have spoken about now, the objects of the diffuse
disk travel in very elliptical orbits and often very inclined with respect to the ecliptic.
Most models of the solar system's formation show that both KBOs and disk objects were
part of a belt of primordial comets and that subsequent gravitational interactions, particularly
with Neptune, sent these objects outward, some in stable orbits (KBOs) and others in
unstable orbits, forming the diffuse disk.
Among the objects of the Kuiper belt, even if present mainly in its outermost part, there
are also the "detached objects", whose orbit is almost entirely included between 40 and
50 astronomical units from the Sun. Given their gigantic distance from Neptune, it is
unlikely that these objects will suffer the gravitational perturbations. An example of
a detached Kuiper belt object is Sedna, whose orbit has perihelion at 76 astronomical units
from the Sun, and aphelion at about 1200 astronomical units from our star.
The Centauri asteroids deserve a separate mention: their orbits extend between Jupiter
and Neptune, from which they suffer the intense gravitational attraction. Many of them, due
to these gravitational perturbations, end up being ejected from the Solar System or
pushed towards the innermost areas of the Solar System where they become comets or crash
into the Sun and planets.
The first probe that explored a celestial body in the Kuiper belt was the New Horizons
probe: launched on January 19th, 2006 from the Cape Canaveral base, on July 14th, 2015
it flew by Pluto. Would you like to see this dwarf planet up closely? Write it in the comments!
The primary objective was to study the geology and morphology of the dwarf planet Pluto and
its satellite Charon, create a map of the surface of the two celestial bodies and analyze
their atmosphere. Other objectives were the study of the atmosphere of the two celestial
bodies over time, the high resolution analysis of some areas of Pluto and Charon, the analysis
of the ionosphere and charged particles, the search for the atmosphere around Charon, the
study of the four minor satellites Styx, Nix, Kerberos and Hydra, the search for any unknown
satellites or rings. Given the good condition of the probe after the primary mission, NASA
technicians decided to make it continue the journey in the Kuiper belt to send data on
the belt to Earth. On January 1st, 2019, the spacecraft crossed the orbit of asteroid 486958
Arrokoth (also known as Ultima Thule) in the Kuiper belt, flying over it at a minimum distance
of about 3,500 km. Discovered on June 26th, 2014 by the Hubble Space Telescope, Arrokoth
is a primordial body of the Solar System 36 kilometers long and composed of two planetesimals
of 21 and 15 kilometers of diameter, called "Ultima" and "Thule" - respectively the lower
and upper lobe, united along their main axes at the level of the "neck", hence the classification
of binary asteroid in contact. Precisely this bilobed shape, in the days following the overflight
of New Horizon, triggered the toto-nicknames all over the world: some called it a snowman,
some peanut, etc. But apart from the shape, what is most surprising about Arrokoth is
the silhouette of the two lobes, surprisingly flat: there is no other known celestial body
that has a similar shape.
According to a study published on Nature Astronomy, it is likely that Arrokoth initially had an
ordinary "chubby" shape, formed 4.6 billion years ago by the condensation of particles
from the same primordial nebula that formed the other celestial bodies of the Solar System.
The particles of this cloud would subsequently agglomerate to form increasingly larger bodies,
two of which, one spherical and one oblate - that is, pot-bellied at the equator - colliding
would have produced the asteroid. The flattened shape would eventually be acquired over millions
of years by sublimation of the condensed grains, a process that began when the nebula in that
region of space thinned out, allowing the Sun's rays to increase the cold temperatures
present and to to sublimate Arrokoth.
Its strange shape would therefore be the natural result of a process of mass loss due to degassing
of the volatile compounds that constituted it, to which however a favorable combination
of intrinsic characteristics of the object such as the particular axial inclination and
the small orbital eccentricity of the celestial body should be added, resulting in an almost
symmetrical erosion between the northern and southern hemispheres. These characteristics
explain why the celestial body has flattened and not shortened or shrunk.
As Arrokoth's axis of rotation is almost parallel to the orbital plane, during its 298 years
of orbit around the Sun, one polar region of Arrokoth faces our star for almost half
the time, while the other faces towards the opposite side. The regions at the equator
and in the lower latitudes are dominated by diurnal variations throughout the year.
With a speed of 58536 kilometers per hour (16260 m / s), reached when the third stage
was turned off, New Horizons is the man-made object that has reached the fastest speed
in leaving the Earth.
The probe contains part of the ashes of Clyde Tombaugh, the astronomer who discovered Pluto
in 1930, a CD-ROM with the names of 434,000 people who signed up for the project, two
coins, two US flags and a 1991 postage stamp which reads: «Pluto: not yet explored».
“If you haven’t already did it, “Drop a line below and let us know what you think
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