What is a Planet and Why Does it Matter?

New Research Paper Says it is Time to Return to Original Definition made by Galileo 

A group of researchers are pushing the International Astronomical Union (IAU) to change its current definition of a planet from one based on its orbital mechanics to one focused on its inherent composition. This change, they note, is long overdue, and would remove the legacy of folklore and astrology that they say created the current definition while also giving exoplanet hunters a clearer definition to frame their work.


In 2006, the International Astronomical Union redefined what it means to be a planet. This decision not only removed Pluto from the planetary ranks, dropping the solar system’s count from nine to eight, but broke the hearts of many schoolchildren who found the playfully named astronomical body to be their favorite.

Now, a group of researchers from the University of Central Florida’s Florida Space Institute have written a paper challenging the IAU to dispense with the current definition, one they say is rooted in folklore and astrology, and return to the original definition laid out by Galileo over 400 years earlier.


Published in the journal Icarus, the UCF team notes that Galileo’s original definition of a planet was simple. It is an astronomical body that is geologically active. However, they say, the IAU’s 2006 definition added a new criteria that these bodies must clear their own orbits or they are not planets.

“This might seem like a small change, but it undermined the central idea about planets that had been passed down from Galileo,” said the study’s lead author and UCF planetary scientist Phillip Metzger in a press release announcing the paper’s publication. “Planets were no longer defined by virtue of being complex, with active geology and the potential for life and civilization. Instead, they were defined by virtue of being simple, following certain idealized paths around the Sun.”

In fact, the team says, the new definition was added to satisfy the desire by the IAU to streamline the solar system down to eight planets, and that this idea is rooted in astrology and folklore, not science. Specifically, the idea that moons and other smaller bodies were not planets came from astrologers, who require a small set of planets for their models to work, and that this idea grew and flourished during the first half of the 20th century when almanacs designed to aid farmers reinforced this belief.

“This was a key period in history, when the public accepted that the Earth orbits the Sun instead of the other way around, and they combined this great scientific insight with a definition of planets that came from astrology,” said Metzger.

Instead, the UCF team says, planets should be defined by their inherent characteristics, just like is done in the rest of mainstream science.

“It’s like defining ‘mammals,’” said Metzger. “They are mammals whether they live on the land or in the sea. It’s not about their location. It’s about the intrinsic characteristics that make them what they are.”

“For the term planet, myself and most planetary scientists consider round icy moons to be planets,” added study co-author Charlene E. Detelich, a geologist with the Johns Hopkins University Applied Physics Laboratory. “They all have active geologic processes that are driven by a variety of internal processes, as does any world with enough mass to reach hydrostatic equilibrium. As a geologist, it is immensely more useful to divide planets by their intrinsic characteristics than by their orbital dynamics.”


400 hundred years have passed since Galileo was jailed for saying that the Earth orbited the Sun and not the other way around. This injustice, the team notes, still exists in the legacy of the 2006 IAU decision. And, they add, it is one that should be corrected before our rapidly increasing ability to study exoplanets in more detail makes the issue of what is a planet a more serious consideration for scientists.

“When Galileo proposed that planets revolve around the Sun, and reconceptualized Earth as a planet, it got him jailed under house arrest for the rest of his life,” said Metzger. “When scientists adopted his position, he was vindicated, in a sense, let out of jail. But then around the early 1900s, we put him back in jail again when we went with this folk concept of an orderly number of planets. So, in a sense, we rejailed Galileo. So, what we’re trying to do, in a sense, is get Galileo out of jail again, so that his deep insight will be crystal clear.”

Follow and connect with author Christopher Plain on Twitter: @plain_fiction