Last week at the Harvard-Smithsonian Center for Astrophysics, three experts - two for Pluto being a planet and one against - debated the issue in front of an audience of scientists, teachers and the public.
The debate recalls a decision made in 2006 by the International Astronomical Union (IAU), who felt it necessary to redefine what constitutes a 'planet'. The highest profile casualty ended up being Pluto, demoted because it was just too small to compete with its eight larger and more impressive peers in our Solar System. The redefinition was based on the fact that many worlds similar to Pluto had been discovered floating on the fringe of their respective solar systems - some even bigger than Pluto - and they didn’t enjoy the status of 'planet', so why should Pluto?
The IAU decided to classify Pluto and other small, fringe-dwelling bodies as 'dwarf planets', and the new category was soon filled by the likes of Ceres, located in the asteroid belt in between the orbits of Mars and Jupiter; and Eris, Haumea, and Makemake, all located in the Kuiper Belt, just past Neptune’s orbit.
Which sounds fine, because Pluto was still technically a planet, just a little one, right? Wrong. And that’s where the controversy surrounding the IAU decision comes from, as Nadia Drake explains at National Geographic:
"In the same confusing way that king cobras are not actually cobras, dwarf planets are not, in fact, planets. They meet two of the three IAU criteria for a planet: They're round, and they orbit the sun. But unlike every world from Mercury through Neptune, the dwarfs haven't grown massive enough to dominate their orbits and clear those paths of other solar system debris, by either knocking it away or reeling it in with their gravity.
"Jupiter has cleared its neighborhood. Earth has cleared its neighborhood. Ceres, which is in the main asteroid belt, hasn't. Pluto hasn't," said Gareth Williams, associate director of the IAU's Minor Planet Center, who presented the IAU definition at the Harvard debate. "In my world, Pluto is not a planet.””
Also adding to the highly complex nature of the debate is the fact that the definition of a planet has been through several changes over the millennia that humans have been aware of the existence of other planets in our Solar System. "'Planet' is a culturally defined word that has changed its meaning over and over again," one of the debaters, Historian Owen Gingerich from the Harvard-Smithsonian Center for Astrophysics, argued. "My feeling is that in retrospect, the IAU should not have attempted to define the word 'planet.'"
With the complicated history of the debate in mind, three experts put forward their opinions, and according to The Irish Examiner, they went as follows:
1. Historian Owen Gingerich argued: A planet is a culturally-defined word that changes over time. Pluto IS a planet.
2. Gareth Williams, from the IAU's Minor Planets Center argued: A planet is a spherical body that orbits the Sun and has cleared its path. Pluto is NOT a planet.
3. Dimitar Sasselov, the director of Harvard’s planetary program, the Origins Of Life Initiative, argued: A planet is the smallest spherical lump of matter that formed around stars or stellar remnants. Pluto IS a planet.
Argument number 3 was voted by the audience as most convincing, but with the understanding that things change in science where more information is discovered, and when it comes to, you know, THE UNIVERSE, there is still much to be learned. So Pluto should be considered a planet right now, the audience decided, but that could change in the future as our knowledge about planets and other cosmic bodies grows.
You can watch the debate in full here: