What is a Trans Neptunian Object?

A trans-Neptunian object (TNO) is a minor planet whose orbit is out beyond that of Neptune (~30AU). That is, their semimajor axes are > 30 AU.  Pluto is in fact a TNO, although it was not recognized as such when it was discovered in 1930. Not until the discovery of  (15760) 1992 QB1 did astronomers recognize this new class of solar system object. Now there are over 2700 known TNOs, which typically fall into one of two dynamical subgroups: Kuiper belt objects (KBOs) and scattered-disc objects (SDOs). The KBOs reside relatively closer in to Neptune than do the more distant SDOs (see below).

Figure above showing locations of Kuiper belt objects and Scattered disc objects, relative to major planets in the outer solar system. Several KBO-Neptune resonant relationships are also shown in the red scale.