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.[1][2] The orbits of ETNOs are 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.[1] 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.[3][4]
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.[1]
Among the extreme trans-Neptunian objects are the sednoids, four objects with an outstandingly high perihelion: Sedna, 2012 VP113, Leleākūhonua and 2021 RR205. 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.[5][6][7]
Most distant objects from the Sun
The chart above plots trans-Neptunian objects with a perihelion beyond Neptune (30 AU). While regular TNOs are located in the bottom left of the plot, an ETNO has a semi-major axis greater than 150–250 AU. They can be grouped by their perihelia into three distinct populations:[1]scattered ETNOs or ESDOs (38–45 AU) detached ETNOs or EDDOs (40–45 to 50–60 AU) Sednoids or inner Oort cloud objects (beyond 50–60 AU)
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.[8]
2014 SR349, appears to be anti-aligned with Planet Nine.[8]
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.[9][10][11][12]
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.[14][15]
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 TNOs have perihelia significantly beyond Neptune, which orbits 30 AU from the Sun.[16][17] Generally, TNOs with perihelia smaller than 36 AU experience strong encounters with Neptune.[18][19] 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.
TESS data search
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.[20] Their search recovered known ETNOs 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.[21][22]
List
The extreme trans-Neptunian object orbits
Close up view of 13 TNO current positions
6 original and 10 additional TNO object orbits with current positions near their perihelion in purple
Extreme trans-Neptunian objects with perihelion greater than 30 AU and a semi-major axis greater than 250 AU[23][24][25]
are the objects included in the original study by Trujillo and Sheppard (2014).[32]
has been added in the 2016 study by Brown and Batygin.[18][33][34]
All other objects have been announced later.
The most extreme case is that of 2015 BP519, nicknamed Caju, which has both the highest inclination[35] and the farthest nodal distance; these properties make it a probable outlier within this population.[2]
Notes
^The three sednoids (pink) along with the red-colored extreme trans-Neptunian object (ETNO) orbits are suspected to be aligned with the hypothetical Planet Nine while the blue-colored ETNO orbits are anti-aligned. The highly elongated orbits colored brown include centaurs and damocloids with large aphelion distances over 200 AU.
^Given the orbital eccentricity of these objects, different epochs can generate quite different heliocentric unperturbed two-bodybest-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,[26] yet an epoch 2020 heliocentric period of ~10,800 years.[27] The barycentric solution is a much more stable ~11,300 years.
^ ab"Objects beyond Neptune provide fresh evidence for Planet Nine". 2016-10-25. 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.
^Bannister, Michele T.; et al. (2016). "A new high-perihelion a ~700 AU object in the distant Solar System". American Astronomical Society, DPS Meeting #48, Id. 113.08. 48: 113.08. Bibcode:2016DPS....4811308B.
^Grush, Loren (20 January 2016). "Our solar system may have a ninth planet after all — but not all evidence is in (We still haven't seen it yet)". The Verge. Retrieved 18 July 2016. 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.
^Horizons output. "Barycentric Osculating Orbital Elements". Retrieved 4 February 2020. (Solution using the Solar System Barycenter and barycentric coordinates. (Type the target body's name, then select Ephemeris Type:Elements and Center:@0) In the second pane "PR=" can be found, which gives the orbital period in days (For Sedna as an example, the value 4.16E+06 is displayed, which is ~11400 Julian years).
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