A sednoid is a trans-Neptunian object with a perihelion greater than AU and a 50 semi-major axis greater than AU. 150  Only two objects are known from this population, 90377 Sedna and 2012 VP113, both of which have perihelia greater than AU, 75  but it is suspected that there are many more. These objects lie outside an apparently nearly empty gap in the Solar System starting at about 50 AU, 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, several times as far as the aphelia of the two known sednoids.
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:
- 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.
- Their orbits could have been disrupted by an as-yet-unknown planet-sized body beyond the Kuiper belt such as the hypothesized Planet Nine.
- They could have been captured from around passing stars, most likely in the Sun's birth cluster.
|Perihelion (AU)||Semimajor axis (AU)||Aphelion (AU)||Heliocentric
|Argument of perihelion (°)||Year discovered|
|90377||Sedna||995 ± 80||76.06||506||936||85.1||311.38||2003|
The two published sednoids, like all of the more extreme detached objects (objects with semi-major axes > 150 AU and perihelia > 30 AU; the orbit of Neptune), have a similar orientation (argument of perihelion) of ≈ 0° (±38°). This is not due to an 338°observational bias and is unexpected, because interaction with the giant planets should have randomized their arguments of perihelion (ω), with precession periods between 40 Myr and 650 Myr and 1.5 Gyr for Sedna. This suggests that one or more undiscovered massive perturbers may exist in the outer Solar System. A super-Earth at 250 AU would cause these objects to librate around ω = ±60° for billions of years. There are multiple possible configurations and a low-albedo super-Earth at that distance would have an 0°apparent magnitude below the current all-sky-survey detection limits. This hypothetical super-Earth has been dubbed Telisto and Planet Nine. Larger, more-distant perturbers would also be too faint to be detected.
On 10 November 2015, V774104 was announced as a third candidate sednoid, but its observation arc was only a very short 2 weeks, and thus too short to know whether its perihelion was even outside Neptune's influence.
At the Transneptunian Solar System workshop in March 2018, Scott Sheppard has announced a likely third sednoid with an observation arc of 3 years. Its perihelion is at 65 au, its current distance at 83 au and its aphelion at ~2000 au, thus it is an extreme object very suited for the analysis of a possible Planet Nine. The data is to be taken as preliminary finds only.
Sednoids might constitute a proper dynamical class, but they may have a heterogeneous origin; the spectral slopes of (474640) 2004 VN112, 2013 RF98, 2012 VP113, 2002 GB32 and 2003 HB57 are very different from that of 90377 Sedna.
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 was responsible, all such objects would share roughly the same perihelion (≈80 AU). If Sedna were 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 100–500 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 2007 OR10, 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).
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