A sednoid is a trans-Neptunian object with a perihelion greater than 50 AU and a semi-major axis greater than 150 AU. Only two objects are known from this population, 90377 Sedna and 2012 VP113, both of which have perihelia greater than 75 AU, 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 included 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 existence of sednoids is puzzling because their 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.
- They could have been captured from around passing stars, most likely in the Sun's birth cluster.
|Perihelion (AU)||Semimajor axis (AU)||Aphelion (AU)||Argument of perihelion (°)||Year discovered|
|90377||Sedna||995 ± 80||76.13 ± 0.01||532.3 ± 1.7||≈ 937||311.19 ± 0.02||2003|
|2012 VP113||≈ 500||80.5 ± 0.6||263 ± 7||446 ± 13||293.8 ± 2.4||2012|
The two 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° (338°±38°). This is not due to an observational bias and is unexpected, because interaction with the giant planets should have randomized their arguments of perihelion (ω). This suggests that an undiscovered massive perturber may exist in the outer Solar System. A super-Earth at 250 AU would cause these objects to librate around ω = 0°±60° for billions of years. There are multiple possible configurations and a low-albedo super-Earth at that distance would have an apparent magnitude below the current all-sky-survey detection limits. Larger, more-distant perturbers would also be too faint to be detected.
Twelve known objects have a semi-major axis > 150 AU, a perihelion beyond Neptune, and an argument of perihelion of 340°±55°.
- Trujillo, C. A.; Sheppard, S. S. (2014). "A Sedna-like body with a perihelion of 80 astronomical units". Nature 507 (7493): 471–474. doi:10.1038/nature13156. Archived from the original on 2014-12-16.
- Sheppard, Scott S.. "Known Extreme Outer Solar System Objects". Department of Terrestrial Magnetism, Carnegie Institution for Science. Retrieved 2014-04-17.
- "JPL Small-Body Database Search Engine: a > 150 (AU) and q > 50 (AU) and data-arc span > 365 (d)". JPL Solar System Dynamics. Retrieved 2014-10-15.
- Sheppard, Scott S.. "Beyond the Edge of the Solar System: The Inner Oort Cloud Population". Department of Terrestrial Magnetism, Carnegie Institution for Science. Retrieved 2014-04-17.
- Michael E. Brown, Chadwick Trujillo, David Rabinowitz (2004). "Discovery Of A Candidate Inner Oort Cloud Planetoid". Astrophysical Journal 617 (1): 645–649. arXiv:astro-ph/0404456. Bibcode:2004ApJ...617..645B. doi:10.1086/422095. Archived from the original on 2006-06-27. Retrieved 2008-04-02.
- Scott S. Sheppard; D. Jewitt (2005). "Small Bodies in the Outer Solar System". Frank N. Bash Symposium. University of Texas at Austin. Retrieved 2008-03-25.
- Alessandro Morbidelli; Harold Levison (2004). "Scenarios for the Origin of the Orbits of the Trans-Neptunian Objects 2000 CR105 and 2003 VB12 (Sedna)". Astronomical Journal 128 (5): 2564–2576. arXiv:astro-ph/0403358. Bibcode:2004AJ....128.2564M. doi:10.1086/424617.
- Rodney S. Gomes; John J. Matese; Jack J. Lissauer (2006). "A distant planetary-mass solar companion may have produced distant detached objects". Icarus 184 (2): 589–601. Bibcode:2006Icar..184..589G. doi:10.1016/j.icarus.2006.05.026.
- Lykawka, P. S. & Mukai, T. (2008). An outer planet beyond Pluto and the origin of the trans-Neptunian belt, the astronomical journal 135:1161–1200
- "JPL Small-Body Database Search Engine: a > 150 (AU) and q > 30 (AU) and data-arc span > 365 (d)". JPL Solar System Dynamics. Retrieved 2014-04-09.