Question: Although Pluto has already been classified as a dwarf planet, I do know that there are still some people out there who consider Pluto to be a planet. What is your take on it? Do you think Pluto should still be considered a planet? Why? — Ansley
Answer: In 2006 the International Astronomical Union decided that celestial objects must meet the following conditions in order to be classified as planets:
- The object must be in orbit around a star, while not being itself a star,
- The object must be massive enough for its own gravity to pull it into a nearly spherical shape, and
- The object must have cleared the neighborhood around its orbit.
The last criterion is the one that Pluto fails to meet, mainly because Pluto has a moon, Charon, that is about half its size. In the end, this is all just a refinement of the term “planet” which allows for the proper characterization of objects which are not really asteroids, but also not quite planets. There are currently 5 IAU-recognized dwarf planets: Pluto, Ceres, Eris, Haumea, and Makemake.