The scattered disc (or scattered disk) is a distant region of the Solar System The Solar System[a] consists of the Sun and those celestial objects bound to it by gravity, all of which were formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. Of the many objects that orbit the Sun, most of the mass is contained within eight relatively solitary planets[e] whose orbits are almost circular and that is sparsely populated by icy minor planets A minor planet is an astronomical object in direct orbit around the Sun that is neither a dominant planet nor a comet. The first minor planet discovered was Ceres in 1801. Since then, more than 200,000 minor planets have been discovered, most of them lying in the asteroid belt, a subset of the broader family of trans-Neptunian objects. The scattered disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia An apsis, plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system. Historically, in geocentric systems, apsides were measured from the center of the Earth greater than 30 astronomical units. These extreme orbits are believed to be the result of gravitational "scattering" by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune. While the nearest distance to the Sun approached by scattered objects is about 30–35 AU An astronomical unit is a unit of length equal to about 149,597,871 kilometres (92,955,807 miles). It is defined by the International Astronomical Union, and is defined as the mean distance between the Earth and the Sun over one Earth orbit, their orbits can extend well beyond 100 AU. This makes scattered objects "among the most distant and cold objects in the Solar System".^[1] The innermost portion of the scattered disc overlaps with a torus In geometry, a torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle. In most contexts it is assumed that the axis does not touch the circle (in this case the surface has a ring shape and is called a ring torus or simply torus if the ring shape is implicit). Other types-shaped region of orbiting objects known as the Kuiper belt The Kuiper belt , sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 55 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20–200 times as massive. Like the asteroid belt, it consists, but its outer limits reach much farther away from the Sun The Sun is the star at the center of the Solar System. It has a diameter of about 1,392,000 kilometers , about 109 times that of Earth, and its mass (about 2 × 1030 kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System. About three quarters of the Sun's mass consists of hydrogen, while the rest is and farther above and below the ecliptic The ecliptic is the apparent path that the Sun traces out in the sky during the year, appearing to move westwards on an imaginary spherical surface, the celestial sphere, relative to the fixed stars. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean than the belt proper.

Because of its unstable nature, astronomers now consider the scattered disc to be the place of origin for most periodic comets Periodic comets are comets having orbital periods of less than 200 years or which have been observed during more than a single perihelion passage (e.g. 153P/Ikeya-Zhang). (Note that "periodic comet" is also sometimes used to mean any comet with a periodic orbit, even if greater than 200 years.) observed in the Solar System, with the centaurs The centaurs are an unstable orbital class of minor planets named after the mythological race of centaurs. The name was chosen because they behave as half asteroid and half comet. Centaurs have transient orbits that cross or have crossed the orbits of one or more of the giant planets, and have dynamical lifetimes of a few million years, a population of icy bodies between Jupiter and Neptune, being the intermediate stage in an object's migration from the disc to the inner Solar System.^[2] Eventually, perturbations from the giant planets send such objects towards the Sun, transforming them into periodic comets. Many Oort cloud The Oort cloud is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The Kuiper belt and scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one objects are also believed to have originated in the scattered disc.

Discovery

During the 1980s, the introduction of the charge-coupled device A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time. Technically, CCDs are implemented as shift in telescopes A telescope is an instrument designed for the observation of remote objects by the collection of electromagnetic radiation. The first known practically functioning telescopes were invented in the Netherlands at the beginning of the 17th century. "Telescopes" can refer to a whole range of instruments operating in most regions of the in combination with higher capacity computers for image analysis allowed for more efficient deep sky surveys than was practical using photography. This led to a flood of new discoveries: between 1992 and 2006, over a thousand trans-Neptunian Objects were detected.^[3]

The first scattered disc object to be recognised as such was a (15874) 1996 TL₆₆,^[4][5] originally identified in 1996 by astronomers Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation). It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the formation and development of the universe based at Mauna Kea Mauna Kea is a dormant volcano on the island of Hawaiʻi. In Hawaiian, Mauna Kea means "white mountain," referring to its summit, which is usually snow-capped in winter. The name is derived from Mauna o Wakea, or "Mountain of (the deity) Wākea." in Hawaii. Three more were identified by the same survey in 1999: 1999 CV₁₁₈, 1999 CY₁₁₈ and 1999 CF₁₁₉.^[6] The first object presently classified as a scattered disc object to be discovered was (48639) 1995 TL₈, found in 1995 by Spacewatch Spacewatch is a project at the University of Arizona currently led by Dr. Robert S. McMillan that specializes in the study of minor planets, including various types of asteroids and comets. It was founded in 1980 by Prof. Tom Gehrels and Dr. Robert S. McMillan.^[7]

As of 2009, over 100 scattered disc objects have been identified,^[8] including 2007 UK₁₂₆ (discovered by Schwamb, Brown, and Rabinowitz),^[9] (84522) 2002 TC₃₀₂ (NEAT), Eris Eris, formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto (Brown, Trujillo, and Rabinowitz)^[10] Sedna 90377 Sedna is a trans-Neptunian object and a likely dwarf planet discovered by Michael Brown , Chad Trujillo (Gemini Observatory) and David Rabinowitz (Yale University) on November 14, 2003. It is currently 88 AU from the Sun, about three times as distant as Neptune. For most of its orbit Sedna is farther from the Sun than any other known dwarf (Brown, Trujillo, and Rabinowitz)^[11] and 2004 VN₁₁₂ (Deep Ecliptic Survey).^[12] Although the numbers of objects in the Kuiper belt and the scattered disc are hypothesized to be roughly equal, observational bias due to their greater distance means that far fewer scattered disc objects have been observed to date.^[13]

Subdivisions of trans-Neptunian space

Known trans-Neptunian objects are often divided into two subpopulations: the Kuiper belt and the scattered disc.^[14] A third reservoir of trans-Neptunian objects, the Oort cloud The Oort cloud is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The Kuiper belt and scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one, is believed to exist, although no confirmed direct observations of the Oort cloud have been made.^[15] Some researchers further suggest a transitional space between the scattered disc and the inner Oort cloud, populated with "detached objects".^[16]

Scattered disc versus Kuiper belt

The Kuiper belt is a relatively thick torus In geometry, a torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle. In most contexts it is assumed that the axis does not touch the circle (in this case the surface has a ring shape and is called a ring torus or simply torus if the ring shape is implicit). Other types (or "doughnut") of space, extending from about 30 to 50 AU^[17] comprising two main populations: the classical Kuiper belt objects (or "cubewanos"), which lie in orbits untouched by Neptune, and the resonant Kuiper belt objects; those which Neptune has locked into a precise orbital ratio such as 3:2 (the KBO goes around twice for every three Neptune orbits) and 2:1 (the object goes around once for every two Neptune orbits). These ratios, called orbital resonances, allow KBOs to persist in regions which Neptune's gravitational influence would otherwise have cleared out over the age of the Solar System, since the objects are never close enough to Neptune to be scattered by its gravity. Those in 3:2 resonances are known as "plutinos In astronomy, a plutino is a trans-Neptunian object in 2:3 mean motion resonance with Neptune. For every 2 orbits that a plutino makes, Neptune orbits 3 times. Plutinos are named after Pluto, which follows an orbit trapped in the same resonance, with the Italian diminutive suffix -ino. The name refers only to the orbital resonance and does not", because Pluto Pluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun. Originally classified as a planet, Pluto is now considered the largest member of a distinct population known as the Kuiper belt.[note 9] is the largest member of their group, whereas those in 2:1 resonances are known as "twotinos".

In contrast to the Kuiper belt, the scattered disc population can be disturbed by Neptune.^[18] Scattered disc objects come within gravitational range of Neptune at their closest approaches (~30 AU) but their farthest distances reach many times that.^[16] Ongoing research^[19] suggests that the centaurs The centaurs are an unstable orbital class of minor planets named after the mythological race of centaurs. The name was chosen because they behave as half asteroid and half comet. Centaurs have transient orbits that cross or have crossed the orbits of one or more of the giant planets, and have dynamical lifetimes of a few million years, a class of icy planetoids A minor planet is an astronomical object in direct orbit around the Sun that is neither a dominant planet nor a comet. The first minor planet discovered was Ceres in 1801. Since then, more than 200,000 minor planets have been discovered, most of them lying in the asteroid belt that orbit between Jupiter and Neptune, may simply be SDOs thrown into the inner reaches of the Solar System by Neptune, making them "cis-Neptunian" rather than trans-Neptunian scattered objects.^[20] Some objects, like (29981) 1999 TD₁₀, blur the distinction^[21] and the Minor Planet Center The Minor Planet Center operates at the Smithsonian Astrophysical Observatory , which is part of the Center for Astrophysics (CfA) along with the Harvard College Observatory (HCO) (MPC), which officially catalogues all trans-Neptunian objects A trans-Neptunian object is any object in the Solar System that orbits the Sun at a greater distance on average than Neptune. The Kuiper belt, scattered disk, and Oort cloud are three divisions of this volume of space, now lists centaurs and SDOs together.^[8]

The MPC also makes a clear distinction between the Kuiper belt and the scattered disc; separating those objects in stable orbits (the Kuiper belt) from those in scattered orbits (the scattered disc and the centaurs).^[8] However, the difference between the Kuiper belt and the scattered disc is not clearcut, and many astronomers Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation). It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the formation and development of the universe see the scattered disc not as a separate population but as an outward region of the Kuiper belt. Another term used is "scattered Kuiper belt object" (or SKBO) for bodies of the scattered disc.^[22]

Morbidelli and Brown propose that the difference between objects in the Kuiper belt and scattered objects is that the latter bodies "are transported in semi-major axis by close and distant encounters with Neptune",^[14] but the former experienced no such close encounters. This delineation is inadequate (as they note) over the age of the Solar System, since bodies "trapped in resonances" could "pass from a scattering phase to a non-scattering phase (and vice versa) numerous times".^[14] That is, trans-Neptunian objects could travel back and forth between the Kuiper belt and the scattered disc over time. Therefore they chose instead to define the regions, rather than the objects, defining the scattered disc as "the region of orbital space that can be visited by bodies that have encountered Neptune" within the radius of a Hill sphere An astronomical body's Hill sphere is the region in which it dominates the attraction of satellites. For a planet to retain a moon, the moon must have an orbit that lies within the Hill sphere of the planet. That moon would, in turn, have a Hill sphere of its own. Any object within that distance would tend to become a satellite of the moon, rather, and the Kuiper belt as its "complement ... in the a > 30 AU region"; the region of the Solar System in which an object's farthest distance from the Sun is greater than 30 AU.^[14]

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