Sednoid

• 1. Defining The Sednoid Class

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  Astronomers classify a specific group of distant solar system bodies as sednoids based on strict orbital criteria. These objects possess large semi-major axes and perihelia exceeding 50 AU. Their highly eccentric orbits resemble that of the dwarf planet Sedna, which serves as the eponymous first known member. All four confirmed sednoids maintain perihelion distances far beyond Neptune's gravitational influence. This distance classifies them as detached objects within the trans-Neptunian population. Some experts further categorize these bodies as Inner Oort Cloud objects located between 1,000 and 10,000 AU from the Sun. A precise definition requires any body to have a perihelion greater than 50 AU and a semi-major axis over 700 AU. This definition applies to objects like 2013 RF98 and 2014 FE72 but excludes others with similar parameters. Astronomers do not label those excluded objects as sednoids because their orbits still experience gradual migration. Galactic tides and Neptune's weak gravity continue to perturb these other bodies. High eccentricities above 0.8 distinguish true sednoids from high-perihelion objects with moderate eccentricities. Objects such as 2013 FT28 and 2014 SR349 fall into this latter category.

• 2. The Four Known Members

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  Sedna remains the largest and most famous of the four known sednoids discovered so far. It was identified in 2003 with an orbital period spanning approximately 11,400 years. The second member, Leleākūhonua, appeared in 2015 with a semi-major axis reaching 1,090 AU. Its aphelion extends beyond 2,100 AU, placing it further out than Sedna itself. A third object, 2013 RF98, features a perihelion of 65 AU and an inclination of 11.7 degrees. The fourth confirmed member, 2014 FE72, has a diameter ranging between 220 and 380 kilometers. These four bodies share similar orientations in their arguments of perihelion near 0 degrees. This alignment is unexpected because interactions with giant planets should have randomized these angles over time. Precession periods for Sedna alone span from 40 million to 650 million years. V774104 was announced as a candidate in late 2015 but lacked sufficient observation data. Its internal designation differs from its final classification status. Malena Rice and Gregory Laughlin used TESS sector data to find new candidates at distances of 80 to 200 AU. Their search recovered known objects like Sedna but failed to confirm two specific targets. Early results from the William Herschel Telescope survey also proved inconclusive.

• 3. Unexplained Orbital Dynamics

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  The current elliptical orbits of sednoids cannot be explained by standard gravitational models involving giant planets. Galactic tides fail to account for the observed orbital configurations either. If these objects formed in their present locations, their original orbits must have been circular. Accretion requires low relative velocities between planetesimals to allow coalescence into larger bodies. Large relative velocities would prove too disruptive for accretion processes to succeed. Several hypotheses attempt to explain how these orbits became so elliptical. One theory suggests nearby stellar passages lifted perihelion distances when the Sun resided in its birth cluster. Another proposes capture from around passing stars within that same early solar environment. A third hypothesis points to disruption by an unknown planet-sized body beyond the Kuiper belt. This hypothetical entity has been dubbed Planet Nine. A fourth idea involves a temporarily-present rogue planet lifting perihelion distances during the early solar system. The alignment of arguments of perihelion near 0 degrees suggests one or more undiscovered massive perturbers exist. A super-Earth at 250 AU could cause these objects to librate around specific angles for billions of years. Such a planet would remain undetected due to its low albedo and faint apparent magnitude.

• 4. Hypotheses For Origin

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  Each proposed mechanism for Sedna's extreme orbit leaves distinct marks on wider population structure. If a trans-Neptunian planet caused these orbits, all such objects would share roughly the same perihelion near 80 AU. Capture from another planetary system rotating with the Solar System implies low inclinations and semi-major axes between 100 and 500 AU. Opposite rotation directions would create two populations with differing inclination levels. Perturbations from passing stars produce wide varieties of perihelia and inclinations depending on encounter numbers and angles. Acquiring larger samples helps determine which scenario best explains current observations. Brown called Sedna a fossil record of the earliest Solar System in 2006. He stated that future discoveries would reveal how the Sun formed and the number of nearby stars during formation. A 2007, 2008 survey by Brown, Rabinowitz, and Schwamb searched for additional members but found none. The survey detected Gonggong but failed to locate new sednoids despite sensitivity out to 1,000 AU. Subsequent simulations incorporating this data suggest about 40 Sedna-sized objects likely exist in this region. The brightest among them would reach Eris magnitude at negative 1.0.

• 5. Statistical Population Estimates

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  Sheppard and colleagues concluded after Leleākūhonua's discovery that millions of Inner Oort Cloud objects probably exist. Their analysis suggests a population of approximately 2 million objects larger than 40 kilometers. Total mass estimates fall within ranges comparable to Pluto itself. This total exceeds several times the combined mass of the asteroid belt. Simulations indicate the brightest objects would have magnitudes near negative 1.0. These figures help constrain theories regarding solar system formation and early stellar encounters. The existence of such a massive hidden population supports hypotheses involving planetary perturbations or stellar interactions. Future surveys may confirm these statistical predictions through direct detection. Current technology limits observation capabilities for faint, distant bodies beyond 1,000 AU. New instruments promise improved sensitivity for detecting low-albedo super-Earths or rogue planets. Malena Rice and Gregory Laughlin continue analyzing TESS sector data for additional candidates. Their work focuses on geocentric distances between 80 and 200 AU requiring ground-based follow-up observations. Early results from William Herschel Telescope surveys failed to confirm two specific targets. Further research remains essential to validate theoretical models against empirical evidence.