Poor old Pluto. Demoted from planet to dwarf planet, or worse ... *panetoid*!
In 2006 the International Astronomical Union ruled that Pluto was no longer a planet, leaving just eight planets proper in our Solar System.
For the record, a planet is:
"A celestial body that (i) is in orbit around the Sun, (ii) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (iii) has cleared the neighbourhood around its orbit." (IAU Resolution B5)
Pluto satisfies (i) and (ii), but not (iii): there are other things sharing its orbit around the Sun. Bigger planets, with stronger gravitational fields, plough everything else out of the way as they go around the Sun. Pluto is just too small to make that kind of impact.
Off you go now, Pluto. It's been fun.
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When I was your age ... Pluto was a *planet*. by Kip Stewart