Scattered Disk Objects (SDOs)
Scattered Disk Objects (SDOs) are a specific type of Kuiper Belt object (KBO) located in a region known as the scattered disk, which lies beyond the orbit of Neptune. Unlike classical and resonance KBOs that orbit fully within the Kuiper Belt, SDOs have highly eccentric orbits that allow them to travel from 30 to over 60 astronomical units (AU) from the sun. This significant variation in distance means that SDOs spend limited time in regions where they can be easily observed, contributing to the likelihood that many remain undiscovered.
The origins of SDOs date back to the early solar system, approximately 4.6 billion years ago, and there are various theories regarding their formation—some suggest they were formed in their current locations, while others believe that Neptune's migration affected their orbits. It's estimated that the current SDO population represents only about 1 percent of the original number, with many possibly expelled from the solar system or transformed into other types of celestial bodies, such as centaurs or detached objects.
A notable SDO is Sedna, discovered in 2004, which has an exceptionally elongated orbit and may be a candidate for dwarf planet status. Overall, SDOs provide valuable insights into the early solar system and the dynamics of celestial bodies beyond Neptune.
Scattered Disk Objects (SDOs)
FIELDS OF STUDY: Astronomy; Observational Astronomy; Sub-planet Astronomy
ABSTRACT: Scattered disk objects (SDOs) are small celestial bodies of icy rock with very eccentric orbits. The points in their orbits closest to the sun, or perihelia, are in the Kuiper Belt region between Neptune and Jupiter. Scattered disk objects are important because they are thought to date from the origin of the solar system.
Origins of Scattered Disk Objects
Scattered disk objects are a type of Kuiper Belt object (KBO). KBOs are small, icy objects found in the Edgeworth-Kuiper Belt, a section of the solar system beyond Neptune’s orbit. The Edgeworth-Kuiper Belt is also commonly known as the Kuiper Belt. It is named after the men who first theorized its existence, Kenneth Edgeworth (1880–1972) and Gerard Kuiper (1905–73). KBOs are classified into three groups based on their orbits. Classical KBOs and resonance KBOs orbit fully within the belt. Scattered KBOs, or scattered disk objects (SDOs), only partly orbit within the belt. All KBOs are part of the larger group known as trans-Neptunian objects, or TNOs.
SDOs have very eccentric orbits, meaning their distance from the sun varies. They travel within the range of the other objects in the Kuiper Belt but also sometimes extend much farther. An SDO’s closest position to the sun, or perihelion, ranges from about 30 to 48 astronomical units (AU). An astronomical unit is the distance from Earth to the sun—93 million miles (150 million kilometers). However, their extremely elliptical orbits can take them beyond 60 AU from the sun at their farthest point, or aphelion. This means that SDOs spend very little time in an area where they are likely to be observed by Earth- or space-based telescopes. For this reason, it is likely that many SDOs have not yet been discovered. The distant region these objects inhabit is called the scattered disk.
Like the other objects found in the Kuiper Belt, SDOs are thought to have been formed outside Neptune’s orbit in the earliest days of the solar system, 4.6 million years ago. Scientists have different theories about where and how they were formed. Some think that they were formed in their current location. Others believe that Neptune’s migration into its current location may have moved the SDOs into the Kuiper Belt. In either case, the centers of SDOs likely contain untouched material from the origins of the solar system.
Outside Influences on Scattered Disk Objects
Scientists think that the SDOs now orbiting within the Kuiper Belt represent just a fraction—possibly as little as 1 percent—of those that were there when the solar system formed. They believe that Neptune’s gravitational force may have affected the distance between these objects and the sun. Many SDOs may have been essentially expelled from the solar system, while other SDOs may have been pulled deeper into the solar system, where they became centaurs. Centaurs are small space bodies in orbit between Neptune and Jupiter that exhibit characteristics of both comets and asteroids. Still other SDOs may have become what are known as detached objects. These are bodies that are outside of Neptune’s influence. Scientists also theorize that some periodic comets may have once been SDOs. These are comets that complete their orbits in less than two hundred years. This includes the Jupiter-family comets, with orbit periods of less than twenty years, and the Halley-type comets, with periods of twenty to two hundred years.
The Question of Sedna
In 2004, a team of researchers from the California Institute of Technology discovered a planet-like body at the far edges of the solar system. At approximately 1,700 kilometers (1,000 miles) in diameter, the object is larger than an asteroid but smaller than the dwarf planet Pluto. It is 76 AU from the sun at its perihelion and 1,000 AU from the sun at its aphelion. It is so far away that from its perspective, the sun is merely a bright star.
Scientists dubbed the planet-like body Sedna, after the Inuit goddess of the sea. Some think it may be an object from the Oort cloud, a cloud of icy objects that surrounds the solar system, while others believe that it may be a dwarf planet. In either case, the SDO has a wildly eccentric orbit. While Pluto takes about 248 years to travel around the sun, Sedna takes more than 11,000 years.
The discovery of Sedna and other icy bodies in the outer reaches of the solar system were the main catalysts for the 2006 demotion of Pluto from a planet to dwarf planet. By the 2020s, astronomers had definitively classified five dwarf planets— Ceres, Eris, Haumea, Makemake, and Pluto. Sedna and another object named Quaoar, are also possible candidates for dwarf planet status.
Principal Terms
- detached object: a celestial body in the solar system that lies beyond Neptune’s orbit and mostly outside the gravitational influence of the planets.
- eccentric orbit: an orbit that is not a perfect circle, causing the distance between the orbiting object and the body it orbits to vary.
- Edgeworth-Kuiper Belt: a ring of icy rock objects located beyond the orbit of Neptune; also known as the Kuiper Belt.
- periodic comet: a comet that takes less than two hundred years to complete an orbit; also known as a short-period comet.
Bibliography
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