Is Pluto a planet?
According to the IAU (International Astronomical Union) which is responsible for nomenclature, Pluto is a dwarf planet, not a planet. But the true answer is much more complex.
When Pluto was discovered in 1930 it was deemed a planet. But in the 1990’s, astronomers began to discover large bodies past Pluto, such as Eris, Sedna, Quaoar, and Makemake. Potentially hundreds of such objects could hide in the far limits of our solar system. Are these all planets? In 2006 the IAU decided that a “planet” had to meet 3 criteria:
1. It must a celestial body in orbit around the Sun
2. It must have sufficient mass to have reached “hydrostatic equilibrium” (nearly round
shape)
3. It must have “cleared the neighborhood” around its orbit
Based on Pluto not meeting criteria #3, it was demoted to “dwarf planet” status. But a strict interpretation of these criteria leads an impartial observer to conclude our solar system has just 1 planet:
Mercury fails because it does not pass criteria #2. Being made mostly of metal, it is not massive enough to have completely collapsed into a spherical shape and has not met hydrostatic equilibrium
Venus fails criteria #3 because it has at least one trojan asteroid traveling in the same orbit
Earth fails criteria #3 because it has at least two trojan asteroids traveling in the same orbit. Earth also co-orbits with over 34,000 Near Earth asteroids
Mars fails criteria #3 because it has at least 14 trojan asteroids traveling in the same orbit
Jupiter fails criteria #3 because it has at least 11,000 trojan asteroids traveling in the same orbit
Saturn meets all 3 criteria! Go Saturn!
Uranus fails criteria #3 because it has at least two trojan asteroids traveling in the same orbit
Neptune fails criteria #3 because it has at least 28 trojan asteroids traveling in the same orbit. Practically speaking, if Neptune had ‘cleared its orbit’, Pluto would not be there either, because their orbits intersect.
Pluto fails criteria #3 because of the many celestial bodies in the Kuiper belt
Supporters of the IAU definition might argue that “clearing the neighborhood” was intended to exclude trojan asteroids and other objects in gravitational resonance with the main body. But the definition is vague and does not even exclude spacecraft. Even if you allowed trojan asteroids, Mercury unquestionably fails hydrostatic equilibrium and Neptune sure didn’t clear out Pluto which sometimes orbits inside Neptune’s orbit. And what about the 34,000 Near Earth Asteroids which are not gravitational resonant with Earth?
We have discovered hundreds of “rogue planets” that travel in deep space unassociated with any one star. These are believed to be planets that were ejected from their solar systems early the system’s formation. According to the IAU these are not planets because they don’t orbit our Sun.
Arguably the people most expert on this topic are planetary astrophysicists, such as Alan Stern, who has been the principal investigator on the New Horizons and 7 NASA missions. He was quoted as saying regarding the IAU proposal: "It's an awful definition; it's sloppy science and it would never pass peer review".
Some stars have “white dwarf” companion stars, where the primary star far out-masses the companion. The white dwarf orbits the primary star. If you applied the IAU definition to such a solar system, the white dwarf is really a planet because the definition doesn’t differentiate between planetary mass and stellar mass objects.
So is Pluto a planet? Officially, the answer is no, it’s a dwarf planet. But the IAU definition is severely flawed and should be thrown out. The IAU says that Mercury, Venus, Earth, Mars, Jupiter, Uranus and Neptune are planets when each one independently fails the IAU’s own definition. And many planetary science experts like Alan Stern refuse to accept the IAU definition and maintain Pluto is still a planet.
You might think “who cares” and you’d be in good company with that opinion. But it’s a challenge for those of us involved with astronomy education. I can think of no other definition in astronomy that that so vague and inconsistently applied. I’d be open to a reclassification if a new definition was well thought-through and made sense. Until then, choose the experts you want to agree with!