Schrödinger’s Planet

Updated at 4:15 p.m. ET on August 27, 2021.

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.

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