An extreme outer solar system object or, an extreme trans-Neptunian object (ETNO), is
any minor planet in the Solar system with a perihelion (closest approach to the Sun) larger
than 30 astronomical units (AU) and the semi-major axis larger than 150 AU. There are
currently about 19 known ETNOs, 8 of them have a perihelion larger than 40 AU, which
makes them only weakly aﬀected by Neptune.
There are several irregularities related to the orbits of ETNOs, in particular, large eccen-
tricities and inclinations to the ecliptic (T. Mukai et al, R. Gomez et al), and the fact that
arguments of the perihelion are clustered in the range of 280 to 40 degrees.
S. Sheppard and C. Trujillo, and later R. and C. de la Fuente Marcos suggested that
the irregularities are caused by a large Neptune size body which turns around the Sun at
a distance 200 to 1500 AU via the so called Lidov-Kozai mechanism.
mechanism is a perturbation of an orbit of a minor body by large distant body that causes
oscillations about a constant value of the orbit’s argument of pericenter.
The Lidov-Kozai scenario has been modeled by K. Batygin and M. E. Brown for six ETNOs:
their model explained most of the irregularities as the eﬀect of large planet whose orbit has
a semi-major axis between 200 and 2000 AU.
Currently, S. Sheppard, C. Trujillo, K. Batygin and M. E. Brown are using the Subaru
Telescope of the National Astronomical Observatory of Japan to search for the proposed
planet. Their research has been widely publicized in media as a pre-discovery of Planet Nine.
Very few alternative mechanisms (which do exist) have been studied. One possibility, is a
massive, yet undiscovered, belt of minor planets (A-M. Madigan, M. McCourt).
The project would involve a critical mathematical study of all mechanisms which could be
responsible for the observed orbits of the ETNOs. Hopefully, as in the nearest future, more
ETNOs will be discovered to make any model statistically more meaningful (currently, the
ETNOs are discovered at a rate of about one per two years).
red and green. The arguments of their perhilion are clearely clustered. (taken