Thirteen years ago, Pluto was declared not-a-planet-anymore. It was relegated to a “dwarf planet” and the

The 24th of August marks one of the more unusual anniversaries in astronomy. It’s not something that was discovered or studied, or even that something happened — it’s that something was decided. Pluto was discovered in 1930 by US astronomer Clyde Tombaugh, and the discovery quickly changed textbooks all around the world. But in 2003, textbooks were changed again, as Pluto was demoted.

The demotion didn’t come out of nowhere.

Discoveries of Kuiper Belt Objects revealed objects with masses roughly comparable to Pluto, such as Quaoar (announced in 2002), Sedna (2003) and Eris (2005). Eris, in particular, is very close in size to Pluto, and this pushed the discussion to a tipping point. Mike Brown of Caltech, who led the team that found Eris, would jokingly call himself the “man who killed Pluto”, as this discovery was the last nail in Pluto’s planethood.

Still, not everyone agrees with the demotion. NASA Administrator Jim Bridenstine has become the most recent voice calling for Pluto to become a member of the Planet Club once again.

“Just so you know, in my view, Pluto is a planet,” he said during a tour of the Aerospace Engineering Sciences Building at the University of Colorado Boulder.

“You can write that the NASA Administrator declared Pluto a planet once again. I’m sticking by that, it’s the way I learnt it, and I’m committed to it.”

Of course, this doesn’t mean anything officially — it’s a lighthearted remark. However, it keeps the conversation going, stirring spirits up about little cold Pluto, and more importantly, it showcases a problem in our classification system: we still don’t really know what can be called a planet.

Since Neptune’s gravity influences Pluto, and Pluto shares an orbit with non-planets in the Kuiper belt, the tiny world was considered a dwarf planet. However, a recent study suggests that declassifying Pluto may have been a bit hasty. Furthermore, Philip Metzger, who led the research, scoured scientific literature over the past 200 years and found only one publication that uses this same ‘clearing’ standard for classifying planets — in 1802. He also says that big moons like Titan and Europa also get called planets frequently, which suggests that the “official” International Astronomical Union (IAU) definition of planets is not reflected by modern research.

“The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be defined on the basis of a concept that nobody uses in their research,” Metzger said.

“It’s a sloppy definition,” Metzger said of the IAU’s definition twelve years ago. “We now have a list of well over 100 recent examples of planetary scientists using the word planet in a way that violates the IAU definition, but they are doing it because it’s functionally useful,” he then concluded.