Each year on August 24, the international scientific community recognizes Pluto’s historic downgrade with a holiday called Pluto Demoted Day.
In 2006, astronomers gathered in Prague to consider a very basic question: How many planets are in our solar system? Was it nine, or was it actually eight, or perhaps as many as 12? By the end of the conference, after several polite debates and “lots of heated hallway discussions,” the verdict was in. Under the new rules of planethood, the solar system had eight planets, and Pluto wasn’t one of them.
Under the new rules of planethood, the solar system had eight planets, and Pluto wasn’t one of them.
According to new rules adopted by the International Astronomical Union, a celestial body must meet the following criteria in order to qualify as a planet:
>A planet must be round.
>A planet must orbit the sun
>A planet must have “cleared the neighborhood" of its orbit. This means that as a planet travels, its gravity sweeps and clears the space around it of other objects. Some of the objects may crash into the planet, others may become moons.
Pluto follows the first two rules: It is round, and it orbits the sun. It does not, however, follow the third rule. It has not yet cleared the neighborhood of its orbit in space. Because it does not follow this rule, Pluto is no longer considered a planet.
The wider public doesn’t usually get riled up about the solar system, but this decision was quite shocking. For many Americans, the names of planets were some of the earliest scientific facts we learned, and that there were nine of them seemed like a basic truth of existence. Despite the millions of miles that separate us, the planets feel close to home, and the news that one of them had been kicked out of the group felt a bit destabilizing.
For instance, Pluto can be more than 4 billion miles away from Earth, depending on where it is in its wonky orbit, and the dwarf planet’s average temperature dips to -387 degrees Fahrenheit.