Detached Object - Classification

Classification

Detached objects are one of four distinct dynamical classes of TNO; the other three classes are classical Kuiper-belt objects, resonant objects and scattered-disc objects (SDO). Detached objects generally have a perihelion distance greater than 40 AU, deterring strong interactions with Neptune, which has an approximately circular orbit ca. 30AU from the Sun. However, there are no clear boundaries between the scattered and detached regions, since both can coexist as TNOs in an intermediate region with perihelion distance between 37 and 40 AU. One such intermediate body with a well determined orbit is (120132) 2003 FY₁₂₈.

The discovery of 90377 Sedna together with a few other objects such as 2000 CR₁₀₅ and 2004 XR₁₉₀ (also known as "Buffy") has motivated discussion of a category of distant objects that may also be inner Oort cloud objects or (more likely) transitional objects between the scattered disc and the inner Oort cloud.

Although Sedna is officially considered a scattered-disc object by the MPC, its discoverer Michael E. Brown has suggested that because its perihelion distance of 76 AU is too distant to be affected by the gravitational attraction of the outer planets it should be considered an inner-Oort-cloud object rather than a member of the scattered disc. This classification of Sedna as a detached object is accepted in recent publications.

This line of thinking suggests that the lack of a significant gravitational interaction with the outer planets creates an extended–outer group starting somewhere between Sedna (perihelion 76 AU) and more conventional SDOs like 1996 TL₆₆ (perihelion 35AU), which is listed as a scattered–near object by the Deep Ecliptic Survey.

One of the problems with defining this extended category is that weak resonances may exist and would be difficult to prove due to chaotic planetary perturbations and the current lack of accuracy in the orbits of these distant objects. These objects have orbital periods of more than 300 years and most have only been observed over a short observation arc of a couple years. Due to their great distance and slow movement against background stars, it may be decades before most of these distant orbits are determined well enough to confidently confirm or rule out a resonance. Further improvement in the orbit and potential resonance of these objects will help to understand the migration of the giant planets and the formation of the Solar System. For example simulations by Emel’yanenko and Kiseleva in 2007 show that many distant objects could be in resonance with Neptune. They show a 10% likelihood that 2000 CR₁₀₅ is in a 20:1 resonance, a 38% likelihood that 2003 QK₉₁ is in a 10:3 resonance, and a 84% likelihood that (82075) 2000 YW₁₃₄ is in a 8:3 resonance. The likely dwarf planet (145480) 2005 TB₁₉₀ appears to have less than a 1% likelihood of being in a 4:1 resonance.