Planets and Dwarf Planets

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:

  1. The object must be in orbit around a star, while not being itself a star,
  2. The object must be massive enough for its own gravity to pull it into a nearly spherical shape, and
  3. 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.

Jeff Mangum

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