What is a planet? For generations, the answer was easy. A large ball of rock or gas orbiting the Sun, and there are nine in our solar system. But then astronomers began to find new objects the size of Pluto beyond Neptune. Later they found Jupiter-sized worlds circling distant stars, first dozens, then hundreds. Suddenly, the answer was not so easy. Were planets all these newly discovered things?

Pluto, the coolest dwarf planet

Pluto, the coolest dwarf planet

The International Astronomical Union (IAU), in charge of naming these newly discovered worlds, addressed the issue in a meeting in 2006, where they tried to reach a definition of a planet with which everyone could agree. They discussed, voted and chose a definition that they thought would work.

The current official definition says that a planet is a celestial body that:
– It’s in orbit around the Sun,
– It’s round or almost round
– Has “cleared the neighborhood” around its orbit.

But this definition disconcerted the public. First, it can only be applied to the planets in our Solar System. What about all the exoplanets orbiting other stars? Are they planets? And second, Pluto was kicked out of the planetary club and renamed dwarf planet. Is a dwarf planet a small planet? Not according to the IAU.

Poor Pluto...

Poor Pluto…

Eight years later, the Harvard-Smithsonian Center for Astrophysics has decided to revisit the issue with a discussion between the three “heavyweights” in planetary science, each of which presented a review. The goal: to find a definition with which everyone could be happy.

The science historian Owen Gingerich, who chaired the IAU committee definition of planets, introduced the historical point of view. Gareth Williams, associate director of the Minor Planet Center, presented the point of view of the IAU. Dimitar Sasselov, director of the Origins of Life Initiative at Harvard, presented the point of view of an expert on exoplanets.

The dance of Pluto and Charon, its satellite.

The dance of Pluto and Charon, its satellite.

Gingerich argued that “a planet is a culturally defined word that changes over time” and that Pluto is a planet. Williams defended the IAU definition, which states that Pluto is not a planet. And Sasselov defines a planet as “the smallest spherical mass of matter that is formed around stars or stellar remnants,” which means that Pluto is a planet.

Then the public voted about what is a planet or not and whether Pluto is inside or outside that definition. Following the hearing, the definition of Sasselov is the winner and therefore, yes Pluto is a planet. Surely the IAU disagree, cause “the public voted that Pluto is a planet” has the same value as “the public voted that 5 is greater than 15”.

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