Sednoid

The orbits of three of the four known sednoids with Neptune's circular orbit in blue.

The apparent magnitudes of three of the known sednoids.

Discovery image of Sedna, the eponymous, first known sednoid and potential dwarf planet

A sednoid is a trans-Neptunian object with a perihelion well beyond the Kuiper cliff at 47.8 AU. Only four objects are known from this population: 90377 Sedna, 2012 VP113, 541132 Leleākūhonua (2015 TG387), and 2023 KQ14, but it is suspected that there are many more. All four have perihelia greater than 55 AU. These objects lie outside an apparently nearly empty gap in the Solar System and have no significant interaction with the planets. They are usually grouped with the detached objects. Some astronomers, such as Scott Sheppard, consider the sednoids to be inner Oort cloud objects (OCOs), though the inner Oort cloud, or Hills cloud, was originally predicted to lie beyond 2,000 AU, beyond the aphelia of the four known sednoids.

One attempt at a precise definition of sednoids is any body with a perihelion greater than 50 AU and a semi-major axis greater than 150 AU. However, this definition applies to objects such as 2013 SY99, which has a perihelion at 50.02 AU and a semi-major axis of about 700 AU but it is thought to not belong to the Sednoids, but rather to the same dynamical class as 474640 Alicanto, 2014 SR349 and 2010 GB174.

With their high eccentricities (greater than 0.8), sednoids are distinguished from the high-perihelion objects with moderate eccentricities that are in a stable resonance with Neptune, namely 2015 KQ174, 2015 FJ345, (612911) 2004 XR190 ("Buffy"), 2014 FC72 and 2014 FZ71.

Unexplained orbits

The sednoids' orbits cannot be explained by perturbations from the giant planets, nor by interaction with the galactic tides. If they formed in their current locations, their orbits must originally have been circular; otherwise accretion (the coalescence of smaller bodies into larger ones) would not have been possible because the large relative velocities between planetesimals would have been too disruptive. Their present elliptical orbits can be explained by several hypotheses:

1. These objects could have had their orbits and perihelion distances "lifted" by the passage of a nearby star when the Sun was still embedded in its birth star cluster.
2. Their orbits could have been disrupted by an as-yet-unknown planet-sized body beyond the Kuiper belt such as the hypothesized Planet Nine.
3. They could have been captured from around passing stars, most likely in the Sun's birth cluster.