Named for the ancient Greek god who presides over the afterlife, Pluto is now mostly known as the planet that lost its planethood 20 years ago. In August 2006, the International Astronomical Union, an organization that names and classifies celestial bodies, voted to reclassify what was once known as the ninth planet in our solar system. 

Ever since that fateful vote, Pluto has been a dwarf planet. And two decades on, it’s still a controversial decision. Earlier this year, billionaire Jared Isaacman, who now serves as NASA Administrator, voiced his support for Pluto being recognized as a planet once again. Star Trek’s William Shatner isn’t happy about it either. 

Astrophysicist David Gerdes, the dean of the College of Arts and Sciences at Case Western Reserve University, told CNET via email that our understanding of solar system objects changes as we learn more about them. “For example, large asteroids like Ceres, Pallas and Vesta that orbit between Mars and Jupiter were originally considered planets when they were discovered in the early 1800s,” Gerdes says.

“As more and more asteroids were discovered in this region, it eventually became clear that these objects were simply the biggest members of a very large population,” he added. “Today, we know of over a million asteroids — so many that we call that region the ‘asteroid belt’ — and nobody considers them planets.”

But whether Pluto is a planet — and again, right now, it’s not — comes down to how we decide to define the word “planet” in scientific terms. But the tools we use to view the cosmos, like high-powered space telescopes, are getting better all the time. Every few years, it seems like we can see farther and clearer into space. 

Using these tools to discover new celestial objects was actually what got Pluto booted from its status as a planet in the first place. Here’s what happened. 

A planet no more

In 2005, three astronomers discovered a distant, Pluto-sized celestial body and nicknamed it Xena, after the warrior princess from TV. The IAU official named it Eris, after the ancient Greek god of chaos. It’s a fitting name, as chaos ensued. 

Because Eris is roughly the same size as Pluto and shares many characteristics, Eris’s discovery set off a fierce debate around what objects in space should be considered a planet. Eventually, this led the IAU to set out conditions that a celestial body must satisfy to be classified as a planet in 2006. 

Image of the dwarf planet Eris (also known as 2003 UB313 or Xena) that was taken using the Samuel Oschin Telescope at the Palomar Observatory.Screenshot by CNET

According to the astronomers of the IAU, a celestial object needs to do three things to be considered a planet. First, it must orbit a star. So, if it’s a planet in our neck of the woods, it has to orbit the sun. 

Second, the object has to be big. If the object is big enough, it will have sufficient gravity to force it into a sphere. So, planets have to be a big ball. No flat planets allowed. 

Third, the planet also needs to be so large that it cleared anything else of similar size from its orbit. Obviously, moons and asteroids can be nearby, just not other planets. 

The IAU then set three categories for celestial bodies in our solar system: planets, dwarf planets and small solar system bodies. Dwarf planets have the same attributes as planets in this context, except they do not meet the third criterion: They have not cleared out the neighborhood around their orbits. Also, dwarf planets can’t be satellites. 

The third category, small solar system bodies, is everything else orbiting the sun that’s not a planet or a dwarf planet. Again, satellites don’t count here. 

“These definitions are somewhat arbitrary, and people still like to debate them,” Gerdes says. “But one thing is certain: There’s no way to define a ‘planet’ that includes Pluto without also adding other objects like Eris, Sedna, Makemake and Ceres to the list.”

So, unless we add more planets to the list, Pluto probably won’t be reclassified as a planet.

“Modern science provides much more knowledge than the simple fact that objects orbiting the sun appear to move with respect to the background of fixed stars,” the IAU president at the time, Ron Ekers, wrote in August 2006. 

“For example, recent new discoveries have been made of objects in the outer regions of our solar system that have sizes comparable to and larger than Pluto. These discoveries have rightfully called into question whether or not they should be considered as new ‘planets,’” he added.

Julian is a contributor and former staff writer at CNET. See full bio