A sednoid is a trans-Neptunian object with a large semi-major axis and a high perihelion, similar to the orbit of the dwarf planet Sedna. The consensus among astronomers is that there are only three objects that are known from this population: Sedna,, and 541132 Leleākūhonua . All three have perihelia greater than . 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 consider the sednoids to be Inner Oort Cloud (IOC) objects, though the inner Oort cloud, or Hills cloud, was originally predicted to lie beyond 2,000 AU, beyond the aphelia of the three known sednoids.
One attempt at a precise definition of sednoids is any body with a perihelion greater than and a semi-major axis greater than .However, this definition applies to objects such as and, which have perihelia beyond 50 AU and semi-major axes over 700 AU. Despite this, these objects are thought to not belong to the sednoids, but rather to the same dynamical class as 474640 Alicanto, and .
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,, ("Buffy"), and .
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:
| data-sort-type="number" | Number | Name | data-sort-type="number" | Diameter (km) | data-sort-type="number" | Perihelion (AU) | data-sort-type="number" | Semimajor axis (AU) | data-sort-type="number" | Aphelion (AU) | data-sort-type="number" | Heliocentric distance (AU) | data-sort-type="number" | Argument of perihelion (°) | data-sort-type="number" | Year discovered (precovered) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sedna | 995 ± 80 | 76.06 | 506 | 937 | 85.1 | 311.38 | 2003 (1990) | |||||||||
| — | 300–1000 | 80.50 | 271.5 | 462 | 83.65 | 293.78 | 2012 (2011) | |||||||||
| Leleākūhonua | 220 | 65.16 | 1085 | 2126 | 77.69 | 118.17 | 2015 (none) | |||||||||
, 27 known objects have a semi-major axis greater than 150 AU, a perihelion beyond Neptune, an argument of perihelion of, and an observation arc of more than 1 year.,,,,, and are near the limit of perihelion of 50 AU, but are not considered sednoids.
On 1 October 2018, Leleākūhonua, then known as, was announced with perihelion of 65 AU and a semi-major axis of 1094 AU. With an aphelion over 2100 AU, it brings the object further out than Sedna.
In late 2015, V774104 was announced at the Division for Planetary Science conference as a further candidate sednoid, but its observation arc was too short to know whether its perihelion was even outside Neptune's influence. The talk about V774104 was probably meant to refer to Leleākūhonua even though V774104 is the internal designation for non-sednoid .
Sednoids might constitute a proper dynamical class, but they may have a heterogeneous origin; the spectral slope of is very different from that of Sedna.
Malena Rice and Gregory Laughlin applied a targeted shift-stacking search algorithm to analyze data from TESS sectors 18 and 19 looking for candidate outer Solar System objects. Their search recovered known objects like Sedna and produced 17 new outer Solar System body candidates located at geocentric distances in the range 80–200 AU, that need follow-up observations with ground-based telescope resources for confirmation. Early results from a survey with the William Herschel Telescope aimed at recovering these distant TNO candidates have failed to confirm two of them.
Each of the proposed mechanisms for Sedna's extreme orbit would leave a distinct mark on the structure and dynamics of any wider population. If a trans-Neptunian planet were responsible, all such objects would share roughly the same perihelion (≈80 AU). If Sedna had been captured from another planetary system that rotated in the same direction as the Solar System, then all of its population would have orbits on relatively low inclinations and have semi-major axes ranging from 100 to 500 AU. If it rotated in the opposite direction, then two populations would form, one with low and one with high inclinations. The perturbations from passing stars would produce a wide variety of perihelia and inclinations, each dependent on the number and angle of such encounters.
Acquiring a larger sample of such objects would therefore help in determining which scenario is most likely. "I call Sedna a fossil record of the earliest Solar System", said Brown in 2006. "Eventually, when other fossil records are found, Sedna will help tell us how the Sun formed and the number of stars that were close to the Sun when it formed." A 2007–2008 survey by Brown, Rabinowitz and Schwamb attempted to locate another member of Sedna's hypothetical population. Although the survey was sensitive to movement out to 1,000 AU and discovered the likely dwarf planet Gonggong, it detected no new sednoids. Subsequent simulations incorporating the new data suggested about 40 Sedna-sized objects probably exist in this region, with the brightest being about Eris's magnitude (−1.0).
Following the discovery of Leleākūhonua, Sheppard et al. concluded that it implies a population of about 2 million Inner Oort Cloud objects larger than 40 km, with a total mass in the range of, about the mass of Pluto and several times the mass of the asteroid belt.