Extreme trans-Neptunian object
An extreme trans-Neptunian object (ETNO) is a minor planet and 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.
ETNOs can be divided into three different subgroups. The scattered ETNOs (or extreme scattered disc objects, ESDOs) have perihelia around 38–45 AU and an exceptionally high eccentricity of more than 0.85. As with the regular scattered disc objects, they were likely formed as result of "gravitational scattering" by Neptune and still interact with the giant planets. The detached ETNOs (or extreme detached disc objects, EDDOs), with perihelia approximately between 40–45 and 50–60 AU, are less affected by Neptune than the scattered ETNOS, but are still relatively close to Neptune. The sednoid or inner Oort cloud objects, with perihelia beyond 50–60 AU, are too far from Neptune to be strongly influenced by it.
Among the extreme trans-Neptunian objects are the sednoids, three objects with an outstandingly high perihelion: Sedna, 2012 VP113 ("Biden"), and 2015 TG387 ("Goblin"). Sedna and 2012 VP113 are distant detached objects with perihelia greater than 70 AU. Their high perihelia keep them at a sufficient distance to avoid significant gravitational perturbations from Neptune. Previous explanations for the high perihelion of Sedna include a close encounter with an unknown planet on a distant orbit and a distant encounter with a random star or a member of the Sun's birth cluster that passed near the Solar System.
Trujillo and Sheppard discoveries
- 2013 FT28, Longitude of perihelion aligned with Planet Nine, but well within the proposed orbit of Planet Nine, where computer modeling suggests it would be safe from gravitational kicks.
- 2014 SR349, appears to be anti-aligned with Planet Nine.
- 2014 FE72, an object with an orbit so extreme that it reaches about 3,000 AU from the Sun in a massively-elongated ellipse – at this distance its orbit is influenced by the galactic tide and other stars.
Outer Solar System Origins Survey
- 2013 SY99, which has a lower inclination than many of the objects, and which was discussed by Michele Bannister at a March 2016 lecture hosted by the SETI Institute and later at an October 2016 AAS conference.
- 2015 KG163, which has an orientation similar to 2013 FT28 but has a larger semi-major axis that may result in its orbit crossing Planet Nine's.
- 2015 RX245, which fits with the other anti-aligned objects.
- 2015 GT50, which is in neither the anti-aligned nor the aligned groups; instead, its orbit's orientation is at a right angle to that of the proposed Planet Nine. Its argument of perihelion is also outside the cluster of arguments of perihelion.
Since early 2016, ten more extreme trans-Neptunian objects have been discovered with orbits that have a perihelion greater than 30 AU and a semi-major axis greater than 250 AU bringing the total to sixteen (see table below for a complete list). Most eTNOs have perihelia significantly beyond Neptune, which orbits from the Sun. 30 AU Generally, TNOs with perihelia smaller than experience strong encounters with Neptune. 36 AU Most of the eTNOs are relatively small, but currently relatively bright because they are near their closest distance to the Sun in their elliptical orbits. These are also included in the orbital diagrams and tables below.
☊ or Ω (°)
|2013 FT28||5,050||295||43.60||546||57.0||0.86||40.2||17.3||217.8||258.0 (*)||6.7||24.4||200|
|2015 GT50||5,510||310||38.45||580||41.7||0.89||129.2||8.8||46.1||175.3 (*)||8.5||24.9||80|
|2015 KG163||17,730||680||40.51||1,320||40.8||0.95||32.0||14.0||219.1||251.1 (*)||8.1||24.3||100|
|2015 BP519 "Caju"||9,500||449||35.25||863||52.7||0.92||348.1||54.1||135.2||123.3||4.3||21.5||550|
- (*) longitude of perihelion, ϖ, outside expected range;
- are the objects included in the original study by Trujillo and Sheppard (2014).
- has been added in the 2016 study by Brown and Batygin.
- All other objects have been announced later.
The most extreme case is that of 2015 BP519, nicknamed Caju, which has both the highest inclination and the farthest nodal distance; these properties make it a probable outlier within this population.
- Given the orbital eccentricity of these objects, different epochs can generate quite different heliocentric unperturbed two-body best-fit solutions to the semi-major axis and orbital period. For objects at such high eccentricity, the Sun's barycenter is more stable than heliocentric values. Barycentric values better account for the changing position of Jupiter over Jupiter's 12 year orbit. As an example, 2007 TG422 has an epoch 2012 heliocentric period of ~13,500 years, yet an epoch 2017 heliocentric period of ~10,400 years. The barycentric solution is a much more stable ~11,300 years.
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The new evidence leaves astronomer Scott Sheppard of the Carnegie Institution for Science in Washington, D.C., "probably 90% sure there's a planet out there." But others say the clues are sparse and unconvincing. "I give it about a 1% chance of turning out to be real," says astronomer JJ Kavelaars, of the Dominion Astrophysical Observatory in Victoria, Canada.
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The statistics do sound promising, at first. The researchers say there's a 1 in 15,000 chance that the movements of these objects are coincidental and don't indicate a planetary presence at all. ... 'When we usually consider something as clinched and air tight, it usually has odds with a much lower probability of failure than what they have,' says Sara Seager, a planetary scientist at MIT. For a study to be a slam dunk, the odds of failure are usually 1 in 1,744,278. ... But researchers often publish before they get the slam-dunk odds, in order to avoid getting scooped by a competing team, Seager says. Most outside experts agree that the researchers' models are strong. And Neptune was originally detected in a similar fashion — by researching observed anomalies in the movement of Uranus. Additionally, the idea of a large planet at such a distance from the Sun isn't actually that unlikely, according to Bruce Macintosh, a planetary scientist at Stanford University.
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- Known extreme outer solar system objects, Scott Sheppard, Carnegie Science Center
- Hunt for Ninth Planet Reveals New Extremely Distant Solar System Objects, Scott Sheppard, Carnegie Science Center
- List of Known Trans-Neptunian Objects (including ESDOs and EDDOs), Robert Jonston, Johnstson's Archive