The latest issue of POPULAR SCIENCE ( March/April 2016 ) has something about the search for a "ninth planet" or "planet X". Pluto, according to them, is not a planet. They more or less imply that it fails to meet the necessary criteria without making things too clear: "Dwarf planets are a dime a dozen. When it comes to real planets - bodies with enough mass to achieve a nearly round shape, clear nearby debris, and orbit the sun - we know of only eight in our solar system."
Pluto is more or less spherical* ( the correct term ) in shape, as pictures sent back by the New Horizons probe show.
Photomosaic of the planet Pluto as seen from different angles.
There goes that argument.
"Clear the debris" has been more precisely stated on Wikipedia as "It must have cleared the neighborhood around its orbit."
Wikipedia also states: "Alan Stern, principal investigator with NASA's New Horizons mission to Pluto, publicly derided the IAU resolution, stating that "the definition stinks, for technical reasons".[65] Stern's contention was that, by the terms of the new definition, Earth, Mars, Jupiter, and Neptune, all of which share their orbits with asteroids, would be excluded."
We don't have one rule for Pluto and another one for all the other planets.
As for the last item in the list, Pluto does orbit the sun.
Finally, "Planet X" is something that was discredited long ago when they found Pluto. Unless actual proof is found of the existence of another planet further out in the solar system, we can go on saying this one is a mistake.
*This was being denied at one time, and at one time was supposed to be one of the reasons that Pluto wasn't supposed to be a planet. Something that I guess they don't expect anyone to remember now.
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