Scattered disc
The scattered disc is a distant circumstellar disc in the Solar System that is sparsely populated by icy small Solar System bodies. These objects have orbital eccentricities ranging as high as 0.8 and inclinations as high as 40 degrees. Their perihelia remain greater than 30 astronomical units from the Sun. Although the closest scattered-disc objects approach the Sun at about 30 to 35 AU, their orbits can extend well beyond 100 AU. This makes scattered disc objects among the coldest and most distant known objects in the Solar System. The innermost portion of the scattered disc overlaps with a torus-shaped region of orbiting objects traditionally called the Kuiper belt. Its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the Kuiper belt proper.
Traditionally, devices like a blink comparator were used in astronomy to detect objects in the Solar System. These tools required time-consuming steps like exposing and developing photographic plates or films. People then used a blink comparator to manually detect prospective objects. During the 1980s, the use of CCD-based cameras in telescopes made it possible to directly produce electronic images. Because the CCD captured more light than film, surveys allowed for higher throughput. A flood of new discoveries was the result: over a thousand trans-Neptunian objects were detected between 1992 and 2006. The first scattered-disc object to be recognised as such was discovered in 1996 by astronomers based at Mauna Kea in Hawaii. Three more were identified by the same survey in 1999. As of 2011, over 200 SDOs have been identified.
The Kuiper belt is a relatively thick torus of space extending from about 30 to 50 AU. It comprises two main populations of Kuiper belt objects known as classical Kuiper-belt objects and resonant Kuiper-belt objects. Classical objects lie in orbits untouched by Neptune. Resonant objects are locked into precise orbital ratios such as 2:3 or 1:2. These ratios allow KBOs to persist in regions which Neptune's gravitational influence would otherwise have cleared out. In contrast to the Kuiper belt, the scattered-disc population can be disturbed by Neptune. Scattered-disc objects come within gravitational range of Neptune at their closest approaches but their farthest distances reach many times that. Some objects blur the distinction and the Minor Planet Center now lists centaurs and SDOs together.
The Minor Planet Center classifies the trans-Neptunian object 90377 Sedna as a scattered-disc object. Its discoverer Michael E. Brown has suggested instead that it should be considered an inner Oort-cloud object rather than a member of the scattered disc. With a perihelion distance of 76 AU, it is too remote to be affected by the gravitational attraction of the outer planets. Under this definition, an object with a perihelion greater than 40 AU could be classified as outside the scattered disc. Sedna is not the only such object. Another object discovered before Sedna and 474640 Alicanto have a perihelion too far away from Neptune to be influenced by it. This led to a discussion among astronomers about a new minor planet set called the extended scattered disc. More recently, these objects have been referred to as detached or distant detached objects.
The scattered disc is a very dynamic environment where orbits are always in danger of disruption. Either being sent outward to the Oort cloud or inward into the centaur population and ultimately the Jupiter family of comets. SDOs are typically characterized by orbits with medium and high eccentricities with a semi-major axis greater than 50 AU. Their perihelia bring them within influence of Neptune. Having a perihelion of roughly 30 AU is one of the defining characteristics of scattered objects. The classical objects are very different from the scattered objects. More than 30% of all cubewanos are on low-inclination near-circular orbits whose eccentricities peak at 0.25. Although motions in the scattered disc are random, they do tend to follow similar directions which means that SDOs can become trapped in temporary resonances with Neptune.
The scattered disc formed when Kuiper belt objects were scattered into eccentric and inclined orbits by gravitational interaction with Neptune and other outer planets. One hypothesis estimates a period equal to the entire age of the Solar System while a second posits that the scattering took place relatively quickly during Neptune's early migration epoch. Computer simulations have also suggested a more rapid and earlier formation for the scattered disc. Modern theories indicate that neither Uranus nor Neptune could have formed in situ beyond Saturn as too little primordial matter existed at that range. Instead these planets may have formed closer to Jupiter but were flung outwards during the early evolution of the Solar System. Once the orbits of Jupiter and Saturn shifted to a 2:1 resonance their combined gravitational pull disrupted the orbits of Uranus and Neptune sending Neptune into the temporary chaos of the proto-Kuiper belt.
Scattered objects like other trans-Neptunian objects have low densities and are composed largely of frozen volatiles such as water and methane. Spectral analysis has revealed signatures of similar compounds. Both Pluto and Eris show signatures for methane. Astronomers originally supposed that the entire trans-Neptunian population would show a similar red surface colour. Specifically SDOs were expected to have large amounts of surface methane chemically altered into tholins by sunlight from the Sun. This would absorb blue light creating a reddish hue. Most classical objects display this colour but scattered objects do not instead they present a white or greyish appearance. One explanation is the exposure of whiter subsurface layers by impacts while another suggests greater distance creates a composition gradient.
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Common questions
What is the scattered disc and how does it differ from the Kuiper belt?
The scattered disc is a distant circumstellar disc in the Solar System that is sparsely populated by icy small Solar System bodies. Its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the Kuiper belt proper.
When was the first scattered-disc object discovered and who found it?
Astronomers based at Mauna Kea in Hawaii discovered the first scattered-disc object to be recognised as such in 1996. Three more were identified by the same survey in 1999, and over 200 SDOs have been identified as of 2011.
How did technology change the discovery of trans-Neptunian objects between 1992 and 2006?
CCD-based cameras made it possible to directly produce electronic images during the 1980s, allowing surveys to capture more light than film. This resulted in over a thousand trans-Neptunian objects being detected between 1992 and 2006.
Why do astronomers debate whether Sedna belongs to the scattered disc or the inner Oort cloud?
The Minor Planet Center classifies the trans-Neptunian object 90377 Sedna as a scattered-disc object, but its discoverer Michael E. Brown suggests it should be considered an inner Oort-cloud object. With a perihelion distance of 76 AU, it is too remote to be affected by the gravitational attraction of the outer planets.
What causes the formation of the scattered disc according to modern theories?
The scattered disc formed when Kuiper belt objects were scattered into eccentric and inclined orbits by gravitational interaction with Neptune and other outer planets. Computer simulations suggest this scattering took place relatively quickly during Neptune's early migration epoch.