YES – Pluto Is a Planet

A number of astronomers think that the IAU’s planet criteria are far too restrictive and that Pluto should regain its planet status. They point out, for example, that the IAU definition is very solar-system centric. Over 1,800 bodies equal larger (most are larger) in size to the Earth have already been discovered orbiting other stars. These exoplanets are currently excluded from official planet status by the first IAU criterion because they don’t orbit our Sun.

Critics of the IAU planet definition, however, have pointed out that it is the third criterion that is the most problematic. They state that the condition requiring a planet to have cleared its general neighborhood is not only a property of the planet candidate, but that it is very much a function of the location of the particular body in question. They cite that if the Earth, currently a planet, were to be placed in Pluto’s orbit then the Earth would lose its planetary status since it also does not have enough gravitational attraction to clear the local neighborhood. Earth would instead become a dwarf planet in this new location. Another criticism is that not all existing solar system planets have necessarily cleared their local neighborhood. Jupiter, a planet that has over 300 times the mass of the Earth, possesses two concentrations of asteroids in its orbital neighborhood that are collectively known as the Trojan asteroids. Clearly Jupiter is a planet, the biggest in our solar system, but it has definitely not “cleared the neighborhood around its orbit.”

Astronomers critical of the IAU instead suggest that a better definition is that a planet is simply a gravitationally rounded object that orbits around a star. It is not necessary for this orbiting body to have gravitationally swept out the debris in its orbit. By this definition, Pluto is a planet. Alan Stern, the principal investigator for the New Horizons mission, agrees with this simpler definition of a planet, and thinks that Pluto should regain its planet status.