What makes a planet, its orbit or its geology? The way that scientists define planets has changed over the centuries. Now, some planetary scientists say non-scientific concepts were used to exclude Pluto from the pantheon of planets, and are calling for its return.
In 2006, the International Astronomical Union (IAU) determined that Pluto was no longer a planet. But researchers who published a new study in Icarus contend that the IAU’s definition is based on astrology and other folklore rather than science.
New technology, such as the soon-to-be-launched James Webb Space Telescope, may make discoveries that challenge the current definition. “There’s an explosion in the number of exoplanets that we’ve discovered over the last 10 years, and that’s only going to increase as we put better telescopes in space,” said Philip Metzger of the University of Central Florida, lead author of the new study.
The researchers say that moons are planets too, noting that the current requirement for a planet to clear its own orbit is unnecessary, and the definition should instead focus on whether the body is now or has ever been geologically active.
Under the current definition, a planet clearing its own orbit means it is the most powerful gravitational force in its orbit and does not share or cross its orbit with other celestial bodies. Because the planet Neptune’s gravity influences Pluto, and because Pluto shares its orbit with frozen gases and objects in the Kuiper belt, the IAU determined that Pluto was not a planet.
Metzger said this change had motivated him to improve the standards of taxonomy as “we need to fix this now before we get too far in this revolution with exoplanets.”
The team of researchers reviewed four centuries’ worth of literature on planets and found that Italian astronomer Galileo Galilei’s (1564–1642) geophysical definition of a planet as a geologically active body in space had fallen out of favor over time. Galileo’s definition was based on his observations of the surface features of the moon in 1609 and was accepted well into the early 20th century.
From about 1910 to 1950, the researchers noted a decline in the number of scientific papers written on planets. “It was during that period of neglect that the transmission of… pragmatic taxonomy [handed] down from Galileo [was] interrupted.”
The team also looked at the role that almanacs played. Popular almanacs provide weather predictions based on astrology and planetary positions, relying on an orderly limited number of planets. Even as the popularity of almanacs declined, their influence continued.
“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. Around this time, astrology and its contention that moons are not planets entered into scientific literature.
“This might seem like a small change, but it undermined the central idea [of] planets that had been passed down from Galileo,” said Metzger, adding that planets were defined in astrology as simple and following idealized solar orbits. This continued into the 1960s when the space program sparked interest in planetary research.
Scientists then returned to Galileo’s geophysical definition of planets but dismissed moons and other planetary objects as being less than planets. Therefore, they continued in the belief that there was a fixed number of planets orbiting the Sun. This was the definition used by the IAU when it voted on Pluto in 2006.
Clearing the orbit as a planetary characteristic describes a planet’s current trajectory, according to Metzger, but offers no understanding of its nature. This concept had never been used before to define planets.
“It’s a current description of the status of things,” said Metzger. “But if, for instance, a star passes by and disrupts our solar system, then planets are not going [to] have their orbits cleared anymore.” The location of planets, he said, should matter less than their geophysical characteristics.
Metzger wants the IAU to retract the current definition and for scientists to revert to the geophysical definition of planets. He pointed out that Galileo was jailed for his theory that Earth is a planet. “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.”
Edited by Siân Speakman and Kristen Butler
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