Fifteen years ago, every child was taught that there are nine planets in the solar system: Mercury, Venus, Mars, Earth, Jupiter, Saturn, Uranus, Neptune and Pluto. The status of the planets remained largely unquestioned until 2003, when a team of astronomers led by Mike Brown discovered Eris, an object beyond Pluto that was believed to be a new planet.  In 2006, the International Astronomical Union established a formal definition of what constitutes a planet — a planet is a celestial body that orbits the sun, is round in shape and is large enough to clear the space around its orbit.  Even though Pluto orbits the sun and is round in shape, it is too small to remove all the objects from the neighborhood of its orbit, and is therefore no longer considered a planet. Celestial bodies that are planet-like but do not meet all the criteria for being classified as a planet, such as Pluto and Eris, are known as dwarf planets. 
Even though the IAU set a definition for what made a planet a planet, the standards still seemed a bit arbitrary and not very rigorous. In fact, many people aren’t ready to accept the fact that Pluto is no longer considered a planet. Recently, the Harvard-Smithsonian Center for Astrophysics brought together three experts, Owen Gingerich, Gareth Williams and Dimitar Sasselov, to debate whether Pluto should be considered a planet. Different arguments, including the historical context of defining planets, implications of adding Pluto back into the solar system, and the inability to currently define “planet”, were all discussed. At the end of the debate, the audience members voted in favor of Pluto’s return to the solar system in a landslide victory. 
At the end of the debate, the audience members voted in favor of Pluto’s return to the solar system in a landslide victory.
So is Pluto a planet or what? Should we strictly follow the IAU’s definition of a planet and continue to exclude Pluto from the solar system, or should we make an exception since society has considered Pluto a planet for so long? If we include Pluto, does that mean we have to include Eris, or other large asteroids like Ceres, as well? Or, perhaps the best question to ask, after Owen Gingerich noted that only 424 of the IAU’s 10,000 members showed up to the 2006 IAU conference, is: how much does it really matter?