Pluto is not a planet but Neptune is?
Today’s IAU vote reduced the solar system’s planet count to eight. No, Pluto has not been forcibly removed from orbit for bad behavior, it has merely been demoted; Pluto has been designated a “dwarf planet” since it fails to meet the newly defined criteria for planetary status.
A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
It is the third test that Pluto fails; it has not “cleared the neighbourhood around its orbit” (whatever that means) since its orbit and Neptune’s overlap.
Fair enough. Too bad, Pluto.
But just as Pluto’s and Neptune’s respective orbits overlap, so too do Neptune’s and Pluto’s. Neptune, then, has not “cleared the neighbourhood around its orbit” either.
Perhaps I’m missing something in the highly rigorous definitions of “to clear” and “neighbourhood”, but it seems the new rules have been inconsistently applied to yield an ‘eight planet system’.
I should also point out that the new definitions do nothing to define the status of bodies orbiting other stars since all of the language used refers to “the sun” and “the solar system”. Are the IAU resolutions science, then, or just self-important verbiage?
I hope my confusion comes only from my ignorance and that there’s a good answer to all of this.