The term “planet” is defined very strictly by the International Astronomical Union (IAU).
An astronomical body is officially a planet if it orbits the Sun, has enough mass to be spherical, and has cleared the neighborhood around its orbit, according to the definition, which was drafted, tweaked, and agreed upon in August 2006.
Only eight bodies in the Solar System can be considered planets under these rules: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
This definition effectively excluded Pluto, a move that has proven, at the very least, extremely contentious, with many scientists calling for a more inclusive redefinition based solely on the physical properties of the body in question.
That brings us to a new paper, which has strengthened those bids with an in-depth analysis of the IAU criteria. Those criteria, according to the paper, are not based on science at all, but rather on folklore and astrology.
The researchers, led by planetary scientist Phillip Metzger of the University of Central Florida, argue that the third criterion, in particular, should be rescinded and the definition of a planet simplified: that the body is, or has been, geologically active.
This would classify many Solar System bodies as planets, including Earth’s Moon and many other moons, dwarf planets, and even asteroids – an outcome that has previously been used to argue against expanding the definition.
However, the fact that these bodies are similar enough to be grouped together is a compelling reason why they should be, according to the researchers behind the new study.
“It’s like defining ‘mammals,'” Metzger says. “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.”
Over a five-year period, the team conducted a thorough review of the last 400 years of scientific literature on planets. They discovered that the definition established by Galileo in the 1630s had gradually eroded over time.
Planets, according to Galileo, are objects made of elements that change over time, just like Earth. Or, to put it another way, they are geologically active, according to the researchers. They also cite Galileo’s claim that planets reflect sunlight rather than producing their own light.
According to the researchers, this definition was in use until the twentieth century. Pluto was classified as a planet when it was discovered in 1930. But between 1910 and the 1950s, the researchers found that there was a declining interest in planetary science, at least as far as the literature goes – the number of papers published in this time dwindled.
“We’ve shown through bibliometrics that there was a period of neglect when astronomers were not paying as much attention to planets,” Metzger says. “And it was during that period of neglect that the transmission of the pragmatic taxonomy that had come down from Galileo got interrupted.”
According to the paper, folklore filled that void. Almanacs, which were annual books that made meteorological and other predictions based on the positions of a small number of planets, had become popular in the previous two centuries. In other words, astrology.
This introduced and cultivated the perception that only the largest bodies orbiting the Sun were planets. Anything else, such as moons and asteroids, were not.
And this, they suggest, crept into the scientific literature.
“This might seem like a small change, but it undermined the central idea about planets that had been passed down from Galileo,” Metzger says.
“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.”
When scientific interest in Solar System exploration was renewed in the 1960s, the geophysical definition began to rise again, causing a split in scientific thought. The IAU definition in 2006 aimed to put an end to the debate, but this clearly hasn’t happened.
One could argue that our understanding of the various types of rocks in the Solar System is far superior to that of Galileo’s day. However, the researchers argue that the criterion of “clearing the orbital neighborhood” should not be followed. Instead, this criterion was created in order to keep the number of planets small and manageable, which is bad science.
“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,” Metzger says.
“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.”
The authors, all experts in fields of space research, may have their interpretation of science history challenged by others in the research community, who are likely to have their own alternative perspectives on how voices, fashions, and beliefs from the past inform the way we now categorize nature.
But as the study authors put it, definitions matter. They shape how we observe, theorize, and think about nature on a fundamental level.
It’s a paper that will undoubtedly ruffle some feathers and keep the debate over planets going for a long time.
The paper was published in Icarus, and the supplementary data is available in full on Metzger’s website.
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