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The orbits of the three known sednoids with Neptune's 30 AU circular orbit in blue. Sednoid orbits.png
The orbits of the three known sednoids with Neptune's 30 AU circular orbit in blue.
The apparent magnitudes of the three known sednoids. Sednoid apparent magnitudes.png
The apparent magnitudes of the three known sednoids.
Discovery image of Sedna, the eponymous and first known sednoid Sedna-NASA.JPG
Discovery image of Sedna, the eponymous and first known sednoid

A sednoid is a trans-Neptunian object with a perihelion well beyond the Kuiper cliff at 47.8  AU . Only three objects are known from this population: 90377 Sedna, 2012 VP113 , and 541132 Leleākūhonua (2015 TG387), but it is suspected that there are many more. All three have perihelia greater than 64 AU. [1] 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, [2] 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 three 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. [3] [4] 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 2004 VN112 , 2014 SR349 and 2010 GB174 . [5]

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 , 2004 XR190 , 2014 FC72 and 2014 FZ71 . [6]

Unexplained orbits

The sednoids' orbits cannot be explained by perturbations from the giant planets, [7] nor by interaction with the galactic tides. [3] 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. [8] 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. [9] [10]
  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. [11] [12]
  3. They could have been captured from around passing stars, most likely in the Sun's birth cluster. [7] [13]

Known members

Sednoids and Sednoid candidates [1] [14]
Perihelion (AU) Semimajor axis (AU) Aphelion (AU)Heliocentric
distance (AU)
Argument of perihelion (°)Year discovered (precovered)
90377 Sedna 995 ± 8076.0650693685.1311.382003 (1990)
2012 VP113 300–1000 [15] 80.5026144183.65293.782012 (2011)
541132 Leleākūhonua 220 [16] 64.941094212377.69118.172015 (none)
Orbits and positions of the three sednoids (labeled in pink) and various other extreme trans-Neptunian objects as of 2021 Distant object orbits and positions closeup.png
Orbits and positions of the three sednoids (labeled in pink) and various other extreme trans-Neptunian objects as of 2021

The three 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° (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), [3] with precession periods between 40 Myr and 650 Myr and 1.5 Gyr for Sedna. [13] This suggests that one [3] or more [17] 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 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. [3]

As of 2016, 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. [18] 2013 SY99 , 2014 ST373 , 2015 FJ345 , 2004 XR190 , 2014 FC72 , and 2014 FZ71 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 semimajor 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. [19] 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 .

Sednoids might constitute a proper dynamical class, but they may have a heterogeneous origin; the spectral slope of 2012 VP113 is very different from that of 90377 Sedna. [20]

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. [21] 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 WHT aimed at recovering these distant TNO candidates have failed to confirm two of them. [22] [23]

Theoretical population

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 100–500 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. [24]

Acquiring a larger sample of such objects would therefore help in determining which scenario is most likely. [25] "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." [26] 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. [25] 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). [25]

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 1×1022 kg, about the mass of Pluto and several times the mass of the asteroid belt. [27]

See also

Related Research Articles

Planets beyond Neptune Hypothetical planets that orbit beyond Neptune

Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century and continued at the start of the 20th with Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the giant planets, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.

Trans-Neptunian object Solar system objects beyond Neptune

A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (AU).

90377 Sedna Large minor planet in the outer reaches of the Solar System

Sedna (minor-planet designation 90377 Sedna) is a dwarf planet in the outer reaches of the Solar System that is currently in the innermost part of its orbit; as of 2021 it is 84 astronomical units (1.26×1010 km; 0.00041 pc) from the Sun, almost three times farther than Neptune. Spectroscopy has revealed that Sedna's surface composition is similar to those of some other trans-Neptunian objects, being largely a mixture of water, methane, and nitrogen ices with tholins. Its surface is one of the reddest among Solar System objects. To within estimated uncertainties, Sedna is tied with Ceres as the largest planetoid not known to have a moon.

(148209) 2000 CR105 is a trans-Neptunian object and the tenth-most-distant known object in the Solar System as of 2015. Considered a detached object, it orbits the Sun in a highly eccentric orbit every 3305 years at an average distance of 222 astronomical units (AU).

Scattered disc Collection of bodies in the extreme Solar System

The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc objects (SDOs) have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 astronomical units (4.5×109 km; 2.8×109 mi). These extreme orbits are thought to be the result of gravitational "scattering" by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune.

Detached object Dynamical class of minor planets

Detached objects are a dynamical class of minor planets in the outer reaches of the Solar System and belong to the broader family of trans-Neptunian objects (TNOs). These objects have orbits whose points of closest approach to the Sun (perihelion) are sufficiently distant from the gravitational influence of Neptune that they are only moderately affected by Neptune and the other known planets: This makes them appear to be "detached" from the rest of the Solar System, except for their attraction to the Sun.

Hills cloud Vast theoretical circumstellar disc

In astronomy, the Hills cloud is a vast theoretical circumstellar disc, interior to the Oort cloud, whose outer border would be located at around 20,000 to 30,000 astronomical units (AU) from the Sun, and whose inner border, less well defined, is hypothetically located at 250–1500 AU, well beyond planetary and Kuiper Belt object orbits—but distances might be much greater. If it exists, the Hills cloud contains roughly 5 times as many comets as the Oort cloud.

(445473) 2010 VZ98, provisional designation 2010 VZ98, is a trans-Neptunian object of the scattered disc, orbiting the Sun in the outermost region of the Solar System. It has a diameter of approximately 400 kilometers.

<span class="nowrap">2012 VP<sub>113</sub></span> Trans-Neptunian object

2012 VP113, also known by its nickname "Biden", is a trans-Neptunian object of the sednoid population, located in the outermost reaches of the Solar System. It was first observed on 5 November 2012 by American astronomers Scott Sheppard and Chad Trujillo at the Cerro Tololo Inter-American Observatory in Chile. The discovery was announced on 26 March 2014. The object probably measures somewhere between 300 and 1000 km in diameter, possibly large enough to be a dwarf planet.

Extreme trans-Neptunian object Solar system objects beyond the known trans-Neptunian objects

An extreme trans-Neptunian object (ETNO) is a trans-Neptunian object orbiting the Sun well beyond Neptune (30 AU) in the outermost region of the Solar System. An ETNO has a large semi-major axis of at least 150–250 AU. Its orbit is much less affected by the known giant planets than all other known trans-Neptunian objects. They may, however, be influenced by gravitational interactions with a hypothetical Planet Nine, shepherding these objects into similar types of orbits. The known ETNOs exhibit a highly statistically significant asymmetry between the distributions of object pairs with small ascending and descending nodal distances that might be indicative of a response to external perturbations.

Planet Nine Hypothetical Solar System planet

Planet Nine is a hypothetical planet in the outer region of the Solar System. Its gravitational effects could explain the peculiar clustering of orbits for a group of extreme trans-Neptunian objects (ETNOs), bodies beyond Neptune that orbit the Sun at distances averaging more than 250 times that of the Earth. These ETNOs tend to make their closest approaches to the Sun in one sector, and their orbits are similarly tilted. These alignments suggest that an undiscovered planet may be shepherding the orbits of the most distant known Solar System objects. Nonetheless, some astronomers question the idea that the hypothetical planet exists and instead assert that the clustering of the ETNOs orbits is due to observing biases, resulting from the difficulty of discovering and tracking these objects during much of the year.

2013 GP136 is a trans-Neptunian object from the scattered disc in the outermost reaches of the Solar System, approximately 212 kilometers in diameter. It was discovered on 8 February 2013, by the Outer Solar System Origins Survey at the Mauna Kea Observatories on the island of Hawaii, United States.

<span class="nowrap">2013 SY<sub>99</sub></span> Trans-Neptunian object

2013 SY99, also known by its OSSOS survey designation uo3L91, is a trans-Neptunian object discovered on September 29, 2013 by the Outer Solar System Origins Survey using the Canada–France–Hawaii Telescope at Mauna Kea Observatory. This object orbits the Sun between 50 and 1,300 AU (7.5 and 190 billion km), and has a barycentric orbital period of nearly 20,000 years. It has the second largest semi-major axis yet detected for an orbit with a perihelion beyond the zone of strong influence of Neptune (q > 38), second only to 541132 Leleākūhonua, but exceeding the semi-major axes of Sedna, 2012 VP113 and 2010 GB174. 2013 SY99 has one of highest perihelia of any known extreme trans-Neptunian object, behind sednoids including Sedna (76 AU), 2012 VP113 (80 AU), and Leleākūhonua (65 AU).

<span class="nowrap">2013 FT<sub>28</sub></span> Trans-Neptunian object

2013 FT28 is a trans-Neptunian object. The existence of the TNO was discovered on 16 March 2013 at Cerro Tololo Observatory, La Serena and revealed on 30 August 2016.

(506479) 2003 HB57, is an extreme trans-Neptunian object of the extended scattered disc in the outermost region of the Solar System, approximately 180 kilometers in diameter. It was discovered by astronomers at the Mauna Kea Observatory on 26 April 2003.

541132 Leleākūhonua Sednoid in the outermost part of the solar system

541132 Leleākūhonua, provisionally designated 2015 TG387, is an extreme trans-Neptunian object and sednoid in the outermost part of the Solar System. It was first observed on 13 October 2015, by astronomers at the Mauna Kea Observatories, Hawaii. Based on its discovery date near Halloween and the letters in its provisional designation 2015 TG387, the object was informally nicknamed "The Goblin" by its discoverers and later named Leleākūhonua, comparing its orbit to the flight of the Pacific golden plover. It was the third sednoid discovered, after Sedna and 2012 VP113, and measures around 220 kilometers (140 miles) in diameter.

The hypothetical Planet Nine would modify the orbits of extreme trans-Neptunian objects via a combination of effects. On very long timescales exchanges of angular momentum with Planet Nine cause the perihelia of anti-aligned objects to rise until their precession reverses direction, maintaining their anti-alignment, and later fall, returning them to their original orbits. On shorter timescales mean-motion resonances with Planet Nine provides phase protection, which stabilizes their orbits by slightly altering the objects' semi-major axes, keeping their orbits synchronized with Planet Nine's and preventing close approaches. The inclination of Planet Nine's orbit weakens this protection, resulting in a chaotic variation of semi-major axes as objects hop between resonances. The orbital poles of the objects circle that of the Solar System's Laplace plane, which at large semi-major axes is warped toward the plane of Planet Nine's orbit, causing their poles to be clustered toward one side.

2014 ST373 (prov. designation:2014 ST373) is a trans-Neptunian object and a detached object from the outermost region of the Solar System. With a perihelion of 50.2 AU, it belongs to the top 10 minor planets with the highest known perihelia of our Solar System. and is neither a scattered disc nor an extreme trans-Neptunian object. It measures approximately 370 kilometers (230 miles) in diameter and was first observed on 25 September 2014, by astronomers using the Dark Energy Camera (DECam) at Cerro Tololo Inter-American Observatory in Chile.


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