The Pluto Problem: Is It Time to Rethink Our Definition of a Planet?

In 2006, the International Astronomical Union (IAU) voted on the definition of a planet. As is well known, Pluto no longer met the criteria and was demoted to a dwarf planet. It’s been a bit of a mess since then – so is it time to redefine the planet?

To be fair, Pluto had it coming. The word “planet“There has never been an official definition, and astronomers have always been cautious in using it. For the ancient Greeks, a planet was any “wandering star,” which included the star Sun and the moon. With the Copernican Revolution the definition changed: Earth was considered a planet in its own right, the Moon was demoted to satellite status, and the Sun was promoted.

This worked for over 200 years until William Herschel discovered it Uranus and Giuseppe Piazzi discovered Ceresthe largest object in the main asteroid belt between Mars and Jupiter. Originally both Uranus and Ceres were classified as planets. But when it turned out that other objects had similar orbits to Ceres, astronomers had to rethink their thinking – after all, planets lived alone. Herschel proposed the term “Asteroids“for the small objects between Mars and Jupiter, while Uranus remained a planet (a situation that certainly favored Herschel’s own legacy).

Even when Clyde Tombaugh discovered Pluto in 1930, astronomers were happy with these classifications. But this new planet was a challenge – it had a really crooked orbit and was much smaller than the other planets, so it didn’t take long for astronomers to consider whether they should re-examine how objects were classified.

Starting in the 1990s, astronomers began finding other objects that had similar orbits to Pluto. But the real nail in the planet’s coffin came in 2005, when astronomer Mike Brown discovered it Erisan object about the same size as Pluto orbiting beyond Neptune.

When astronomers gathered for the IAU meeting in Prague in 2006, a large group requested that this body define what a planet should be. There were two camps: the geophysicists, who argued that planets should be defined by their appearance, and the dynamists, who believed that planets should be defined by their properties.

Essentially, geophysicists argued that a planet should be anything large enough that its own gravity forces it into a near-spherical shape. The dynamists responded that a planet should be anything that can dominate its orbit and clear it mostly of debris. The first definition would allow Pluto to become planets along with Ceres and all of Pluto’s co-orbital friends. The latter definition would exclude all of these smaller entities.

In the end, the compromise encompassed both definitions, meaning the dynamists effectively won. Pluto doesn’t clean up its orbit – it just has too many neighbors – and so was, as is well known, demoted.

Related: What exactly is a planet? Astronomers want to change the definition

Pluto as seen by the New Horizons spacecraft as it flew toward the planet from a distance of 22,025 miles (35,445 kilometers) on July 14, 2015. (Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker)

But not everyone agrees with the new rules, and there are many arguments and counterarguments that can be made. For one thing, the definitions of “nearly spherical” and “largely free of orbit” are not very clear.

But proponents of the rules counter that all defined planets mercury To Neptuneare exceptionally round, while the vast majority of non-planets obviously are not. As for clearing the orbit, Mars is over 5,000 times more massive than the next largest object in its orbit. In contrast, Pluto only occupies about 7% of the total mass in its orbit. So there is a clear and huge discrepancy between these worlds.

However, the greatest confusion arises with the non-planet subtypes. When an object is large enough to be round but does not leave its orbit, like Pluto or Ceres, it is called a Dwarf planet. Astronomers often use the term “dwarf” to refer to smaller versions of the same type, such as B. Dwarf stars and dwarf galaxies. But dwarf stars are still stars, and dwarf galaxies are still galaxies; they’re just smaller. This is not the case with dwarf planets – they are emphatically not planets.

When an object is too small to be round, it is classified as a “small solar system body.” Asteroids are not officially defined as such, but astronomers generally use the word to mean small solar system bodies that live in the inner solar system. Comets are also considered small bodies of the solar system.

Then there are “minor planets,” which include the dwarf planets and all small bodies in the solar system that are not comets. And don’t forget the plutoids – dwarf planets beyond Pluto’s orbit – and the trans-Neptunian objects, which include the plutoids and other small solar system bodies orbiting beyond Neptune.

It’s a bit messy, but it’s an attempt to do justice to the complicated nature of our evolving knowledge of the solar system. We live in an active, dynamic system in which many types of objects interact and play together. We have a hard time finding clear definitions because the universe itself refuses to be neatly organized.

For now, we should probably stick with the definitions we have, even if they are imperfect. As we make new discoveries and gain new insights, we can update our definitions accordingly and hopefully streamline things a bit.

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