Short answers, pulled from the story.
Detached objects are a dynamical class of minor planets in the outer Solar System whose closest approach to the Sun is too distant for Neptune or any other known planet to significantly influence their orbits. They belong to the broader family of trans-Neptunian objects but have perihelia greater than 40 astronomical units, setting them apart from classical Kuiper belt objects, resonant objects, and scattered-disc objects.
Sedna, officially designated 90377 Sedna, is the largest, most distant, and best-known detached object. Discovered in 2003 by Michael E. Brown, Chad Trujillo, and David Rabinowitz, it has a perihelion of 76 astronomical units and an estimated diameter of about 995 kilometres.
Sednoids are detached objects with large semi-major axes and high perihelion orbits similar to that of Sedna. As of 2025, four sednoids are confirmed, including Leleakuhonua, which was discovered in 2015 and has a perihelion of 65 AU and a semi-major axis of about 1,042 AU.
Detached objects have perihelia much larger than Neptune's aphelion and travel on highly elliptical orbits that cannot have been created by gravitational scattering from the giant planets. Proposed explanations include a close encounter with a passing star, the gravitational influence of an undiscovered distant planet, Neptune having once had a more eccentric orbit, or early rogue planets that were later ejected from the Solar System.
The Planet Nine hypothesis proposes that the orbits of several detached objects can be explained by the gravitational pull of a large, unobserved planet located somewhere between 200 and 1,200 AU from the Sun. Mike Brown noted that all known distant detached objects with semi-major axes greater than 100 AU and perihelia greater than 42 AU appear to cluster together, consistent with the influence of such a body.
The Deep Ecliptic Survey team formally distinguishes scattered-near objects, which Neptune can still gravitationally reach, from scattered-extended objects, which it cannot, using a Tisserand parameter value of 3 as the boundary. Detached objects fall into the scattered-extended category and have also been called extended scattered disc objects, distant detached objects, or simply scattered-extended objects in the scientific literature.