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Sednoid
A sednoid is a trans-Neptunian object with a large semi-major axis, a distant perihelion and a highly eccentric orbit, similar to that of the dwarf planet Sedna. The consensus among astronomers is that there are only four objects that are known from this population: Sedna, 2012 VP113, 541132 Leleākūhonua, and 2023 KQ14. All four have perihelia greater than 60 AU. The sednoids are also classified as detached objects, since their perihelion distances are large enough that Neptune's gravity does not strongly influence their orbits. Some astronomers consider the sednoids to be Inner Oort Cloud (IOC) objects. The inner Oort cloud, or Hills cloud, lies at 1,000–10,000 AU from the Sun.
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 the objects 2013 SY99, 2020 MQ53, and 2021 RR205 which have perihelia beyond 50 AU and semi-major axes over 700 AU. Despite this, astronomers do not classify these objects as sednoids because their orbits still experience gradual orbital migration as a result of perturbations by galactic tides and Neptune's weak gravitational influence.
With their high eccentricities (greater than 0.8), sednoids are distinguished from the high-perihelion objects with moderate eccentricities that are not affected by perturbations from Neptune, namely 2015 KQ174, 2015 FJ345, (612911) 2004 XR190 ("Buffy"), (690420) 2014 FC72 and 2014 FZ71.
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:
The first three known 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), 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 = 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. This hypothetical super-Earth has been dubbed Planet Nine. Larger, more-distant perturbers would also be too faint to be detected.
As of 2016[update],[needs update] 27 known objects have a semi-major axis greater than 150 AU, a perihelion beyond Neptune, an argument of perihelion of 340°±55°, and an observation arc of more than 1 year. 2013 SY99, 2014 ST373, 2015 FJ345, 2021 RW209, (612911) 2004 XR190, (690420) 2014 FC72, 2014 US277, 2014 FZ71, and 2021 RR205 are near the limit of perihelion of 50 AU, but are not considered sednoids.
On 1 October 2018, Leleākūhonua, then known as 2015 TG387, 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 (2015 TG387) even though V774104 is the internal designation for non-sednoid 2015 TH367.
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Sednoid
A sednoid is a trans-Neptunian object with a large semi-major axis, a distant perihelion and a highly eccentric orbit, similar to that of the dwarf planet Sedna. The consensus among astronomers is that there are only four objects that are known from this population: Sedna, 2012 VP113, 541132 Leleākūhonua, and 2023 KQ14. All four have perihelia greater than 60 AU. The sednoids are also classified as detached objects, since their perihelion distances are large enough that Neptune's gravity does not strongly influence their orbits. Some astronomers consider the sednoids to be Inner Oort Cloud (IOC) objects. The inner Oort cloud, or Hills cloud, lies at 1,000–10,000 AU from the Sun.
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 the objects 2013 SY99, 2020 MQ53, and 2021 RR205 which have perihelia beyond 50 AU and semi-major axes over 700 AU. Despite this, astronomers do not classify these objects as sednoids because their orbits still experience gradual orbital migration as a result of perturbations by galactic tides and Neptune's weak gravitational influence.
With their high eccentricities (greater than 0.8), sednoids are distinguished from the high-perihelion objects with moderate eccentricities that are not affected by perturbations from Neptune, namely 2015 KQ174, 2015 FJ345, (612911) 2004 XR190 ("Buffy"), (690420) 2014 FC72 and 2014 FZ71.
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:
The first three known 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), 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 = 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. This hypothetical super-Earth has been dubbed Planet Nine. Larger, more-distant perturbers would also be too faint to be detected.
As of 2016[update],[needs update] 27 known objects have a semi-major axis greater than 150 AU, a perihelion beyond Neptune, an argument of perihelion of 340°±55°, and an observation arc of more than 1 year. 2013 SY99, 2014 ST373, 2015 FJ345, 2021 RW209, (612911) 2004 XR190, (690420) 2014 FC72, 2014 US277, 2014 FZ71, and 2021 RR205 are near the limit of perihelion of 50 AU, but are not considered sednoids.
On 1 October 2018, Leleākūhonua, then known as 2015 TG387, 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 (2015 TG387) even though V774104 is the internal designation for non-sednoid 2015 TH367.
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