Detached objects are a dynamical class of bodies in the outer Solar System beyond the orbit of Neptune. These objects have orbits whose points of closest approach to the Sun (perihelion) are sufficiently distant from the gravitational influence of Neptune that they are essentially unaffected by Neptune and the other planets: this makes them appear to be "detached" from the Solar System.
In this way, they differ substantially from the majority of the known trans-Neptunian objects (TNOs), which form a loosely defined set of populations that have been perturbed to varying degrees onto their current orbit by gravitational encounters with the gas giants, predominantly Neptune. Detached objects have larger perihelia than those other TNO populations.
At least nine such bodies have been securely identified, of which the largest, most distant, and best known is Sedna.
Orbits
Detached objects have perihelia much larger than Neptune's aphelion. They often have highly elliptical, very large orbits with semi-major axes of up to a few hundred astronomical units (AU).
Such orbits cannot have been created by gravitational scattering by the gas giants (in particular, Neptune). Instead, a number of explanations have been put forward, including an encounter with a passing star or a distant planet-sized object.
Classification
Detached objects generally have a perihelion distance greater than 40 AU, deterring strong interactions with Neptune, which has an approximately circular orbit 30 AU 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.
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.
As they are so far away, one of the problems with Detached Object classification is that weak resonances with Neptune 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.
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