Trans-Neptunian Objects isn’t a term used much any longer. “Dwarf planets” is the preferred term, if you’re being PC. (“Little planets”, if you’re being really PC.)

Anyway, these are the biggies. They’re all out beyond Neptune’s orbit. They’re all large enough to be kinda planetlike in some ways — they’re in hydrostatic equilibrium (meaning they get mostly round and stay mostly round).  They’re not themselves satellites.  But they aren’t large enough to have “cleared their neighboring region of planetesimals,” i.e., to have slurped up all other orbiting bodies unto themselves.  Which means, according to the International Astronomical Union’s Resolution 5A of 2006, they aren’t planets.

And what would Clyde Tombaugh have thought? My impression of Clyde in his later life is of a man at peace with himself.  Every picture I’ve seen of him shows him beaming, and surely he was much beloved.  Though he defended Pluto’s planetary status when it first was questioned in the 1990s, I believe he also recognized his discovery of Pluto as being something of a happy accident.  And today, I can imagine him arguing that we welcome the planetary status of such substantial bodies as Eris, Haumea, Makemake, Orcus, Quaoar, and so on.  The universe had made room for him in such an unlikely fashion, after all – I believe he’d have wanted us to return the favor.  So on Clyde’s behalf, let the IAU know we want Pluto back!

(Thanks once again to the great Laurel Kornfeld for her ongoing, informed contributions to this discussion – and for filling in some details on the IAU kerfuffle.)