The Harvard-Smithsonian Center for Astrophysics held a meeting last week to discuss the definition of what a planet is, and whether Pluto–which had its planet status removed in 2006 after a vote by the International Astronomical Union (IAU)–should be considered a planet. Three experts paneled the meeting, and each argued for or against Pluto as a planet. The audience then voted.

Pluto has not been considered a planet since 2006, when the IAU met for the same purpose.

In 2005, the discovery of an object, later called Eris, which was farther out than Pluto, and which was larger and more massy than Pluto, disrupted the nine-planet concept of the Solar System. Astronomers met to make a final decision on the definition of a planet at the 26th General Assembly of the IAU in the summer of 2006.

At the 2006 meeting, astronomers voted on the definition of a planet and the status of Pluto. They had three options: maintain the traditional nine-planet Solar System, add three planets of similar size to Pluto–including Eris and Ceres–or remove Pluto and adopt an eight-planet Solar System.

Controversially, they voted for an eight-planet system. Pluto and Eris became “dwarf planets.”

The IAU decided three criteria needed to be met to be considered a planet: the object must orbit the sun, it must have sufficient gravity to pull itself into spherical shape, and it must have “cleared the neighborhood” of its orbit. Pluto had not achieved the last of these criteria.

Today, many astronomers and the public are still uncertain about what exactly defines a planet, but the meeting last week reconsidered the definition, and Pluto.

Three experts presented their case, and the audience voted on the status of Pluto.

One expert, Gareth Williams, associate director at the IAU Minor Planet Center, who was opposed to making Pluto a planet, argued, “Jupiter has cleared its neighborhood. Earth has cleared its neighborhood. Ceres, which is in the main asteroid belt, hasn’t. Pluto hasn’t. In my world, Pluto is not a planet.”

However, the two other experts thought Pluto should be a planet. Historian Owen Gingerich thought that the concept of “planet” is one that is culturally defined and changes over time, and Dimitar Sasselov, director of Harvard’s planetary program, thought that a planet was the smallest spherical lump of matter formed around stars or stellar remnants, so Pluto qualified as a planet.

The audience voted, and found in favor of Pluto being counted a planet.

By Joseph Reight

The Full Debate About Planets and Pluto:

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More information: Universe Today