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.[which?] 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.
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, and Leleākūhonua. 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.
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. Generally, TNOs with perihelia smaller than 36 AU experience strong encounters with Neptune. 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||Metastable||5,050||305||43.4||566||55.2||0.86||40.8||17.4||217.7||258.5 (*)||6.7||24.2||200|
|2013 SL102||?||5,590||326||38.1||614||39.3||0.88||265.4||6.5||94.6||0.0 (*)||7.0||23.2||140|
|2014 WB556||?||4,900||288||42.7||534||46.6||0.85||235.3||24.2||114.7||350.0 (*)||7.3||24.2||150|
|2015 GT50||Unstable||5,510||314||38.5||589||42.9||0.88||129.3||8.8||46.1||175.4 (*)||8.5||24.9||80|
|2015 KG163||Unstable||17,730||805||40.5||1,570||40.5||0.95||32.3||14.0||219.1||251.4 (*)||8.2||24.4||100|
|2018 VM35||?||4,500||252||45.0||459||54.8||0.82||302.9||8.5||192.4||135.3 (*)||7.7||25.2||140|
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.
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.
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.