It was not an error. The notion that science should be decided by decree of some "authority" makes absolutely no sense.

That's just one view, not any sort of gospel truth. According to advocates of the geophysical planet definition, including the majority of the New Horizons team, Pluto is a planet.

There is no "official" position. There are two competing, equally legitimate positions: the dynamical one, in which an object must clear its orbit to be considered a planet, the position of the IAU; and the geophysical one, in which any non-self-luminous spheroidal body orbiting a star or free-floating in space is a…

No, it isn't incorrect. It's one view in an ongoing debate, a matter of interpretation.

Advocates of the geophysical planet definition, which include many planetary scientists, consider all spherical Kuiper Belt Objects to be planets.

Pluto is not a planetoid, and neither is any object large enough to be in hydrostatic equilibrium. The geophysical planet definition includes all objects large enough and massive enough to be rounded by their own gravity as planets. Planetoid is a synonym for asteroid, meaning that word refers to tiny objects shaped…

The small chunks of rock in the Kuiper Belt are not planets because the overwhelming majority of them are not large enough to be rounded by their own gravity. The few that are rounded by their gravity are planets too.

That is true. Walt Disney named the dog the same year that Pluto was discovered, 1930, and chose the name to capitalize on the new planet's popularity.

Mike Brown's book is not a great book. It is mostly about himself rather than about astronomy. He does not even study Pluto, yet he has used this ridiculous notion that he "killed" Pluto as a means of gaining money and fame. He did not "kill" Pluto because Pluto as a planet is not dead. There are many inaccurate…

That does not make Pluto a comet. Pluto is much larger than the largest comet, and it never comes into the inner solar system the way comets do. It also does not lose mass at each perihelion the way comets do. Comets are largely "dirty snowballs" while Pluto is estimated to be 70 percent rock. We have already…

There are also two moons bigger than Mercury. That does not make Mercury—or Pluto—not a planet. An object should be classified by its own intrinsic properties, not by the properties of another object.

That was the intention of Dr. Alan Stern when he coined the term "dwarf planet"—that dwarf planets are a subclass of planets the same way dwarf stars are a subclass of stars, and dwarf galaxies are a subclass of galaxies. Also, Eris is not bigger than Pluto. It was initially thought to be so, but in November 2010,…

How long it lasts is really a matter of how much public consensus there is in favor of it. If enough astronomers and members of the public ignore the IAU definition, as many already are doing, it will gradually become irrelevant.

World Book should know better than to report something as fact just because four percent of a self-appointed body say so. The more appropriate response would have been to state that the definition of planet is a matter of ongoing debate and briefly discuss the positions of both sides.

And the corollary question is, who should make such a decision?

Are these professional astronomers planetary scientists or other types of astronomers?

As of now, that really depends on the individual teachers.

Who said having hundreds of planets in our solar system doesn't make sense to non-specialists? It makes perfect sense. The solar system has whatever number of planets it has, and if that is hundreds, then so be it. We can distinguish the different types of planets by using subcategories. One idea is to create a…

The issue isn't being exactly spherical; it's a matter of the object being shaped by its gravity rather than only by chemical bonds. This happens only when an object reaches a certain threshold for size and mass. As far as spherical moons, some scientists have proposed calling them "satellite planets." Many of these…

Pluto has five moons, one of which (Charon) is half its size, also spherical, and can be considered part of a double planet system with Pluto.