Pluto was demoted on this date in 2006, downgraded from its status as a full-sized planet to that of a dwarf planet. Why? According to the Library of Congress, the International Astronomical Union has three criteria for a full-sized planet: It is in orbit around the Sun, its mass is sufficient to allow it to have a nearly round shape, and (this is key!) it has “cleared the neighborhood” around its orbit. In other words, the planet has become “gravitationally dominant — there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence, in its vicinity in space.” Pluto’s problem is that in all of its existence, it hasn’t cleared its neighborhood.
No performance improvement plan, just a demotion. Justified? Unjustified? Opinions on that vary, and the discussion appears to be ongoing, with NASA chief Jared Isaacman telling members of a Senate committee in April that he’s “very much in the camp of ‘make Pluto a planet again.’” Stay tuned!
