Extreme trans-Neptunian object

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]

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