Currently, the discussion about Pluto shows the problems in defining the concept of “planet”. The International Astronomical Union defines a planet as a celestial body that orbits the Sun with a nearly spherical appearance and, in most cases, clears the vicinity of its orbit of debris from other bodies. However, this set of criteria has not been universally agreed upon.

Earth and even Jupiter, despite their large size, have not cleared many asteroids from their orbital regions. In addition, there are small worlds such as Ceres that are spherical and revolve around the Sun, and are not considered planets.

Pluto’s demotion raises larger issues about how to define everybody in the solar system, or even space more generally. This incident shows that science sometimes cannot divide objects into easy categories; Because if the definition of planet is expanded again, it is not clear how we should evaluate the numerous non-spherical bodies that orbit the Sun. Decisions about this may even call into question the asteroid belt (the huge band of small objects between Mars and Jupiter). Or what happens if a planet somehow breaks into pieces?

The word planet in English (Planet) goes back to ancient times and is derived from the Greek word Planetes meaning “wandering star”. The five classical planets—Mercury, Venus, Mars, Jupiter, and Saturn—are visible to the naked eye and move in strange paths across the sky compared to the much more distant background stars.

After the advent of telescopes, astronomers discovered two new planets, Uranus and Neptune. These two distant worlds are very dim and cannot be seen with the naked eye. It should be kept in mind that the discussed definition of a planet follows the Greek-Roman tradition and the definitions of the International Astronomical Union are based on it. In ancient times, the planets were observed with the naked eye all over the world and had different names in each culture.

When astronomers discovered Ceres in the asteroid belt in 1801, it was classified as a “planet” by the scientific community at the time. But the situation began to change; Because further measurements showed that Ceres is smaller than any other planet seen so far. This mass then entered a group of rocky bodies called “asteroids”, of which we now know hundreds of thousands of examples in the asteroid belt alone. Today, Ceres is known as a dwarf planet.

Pluto was discovered and classified as a planet in 1930 (11 years after the founding of the International Astronomical Union). At the time, Clyde Tamba of the Lowell Observatory in Arizona compared photographic plates of the sky on separate nights and noticed a small dot moving back and forth across the starscape. However, the latest candidate for the ninth planet of the solar system was immediately considered a strange object. Pluto’s orbit is so elliptical, or eccentric, that it brings the object closer to the Sun than Neptune in 20 years of its 248-year journey. Pluto’s orbit is also tilted relative to the ecliptic, or the plane on which the other planets in the solar system rotate.

After the discovery of Eris, researchers had to ask themselves these questions: If Pluto is a planet, then is Eris also considered a planet? What about all those other icy bodies in the Kuiper Belt or smaller bodies in the Asteroid Belt? Where exactly is the dividing line for classifying an object as a planet? A word that once seemed straightforward and simple suddenly became strangely complicated.

Then intense debates ensued and new proposals were made to define the planet. Brian Marsden, a member of the IAU executive committee responsible for finding a new meaning for the planet, told Space.com in 2005: “Every time we think some of us are reaching a consensus, then someone says something and shows that it’s clear.” It’s not like that.”

A year later, astronomers were still nowhere near a solution, and the dilemma hung over the IAU General Assembly in Prague in 2006 like a dark cloud. At this conference, the researchers had eight days of intensive discussion and presented four different proposals. A controversial proposal would have brought the total number of planets in the solar system to 12 by adding Ceres, the largest asteroid, and Pluto’s moon, Charon.

Michael Brown, an astronomer at Caltech University and discoverer of Eris, called the proposal “complete confusion.”

Near the end of the conference, the remaining 424 astronomers voted to create three new classifications for objects in the solar system. From then on, only Mercury and Neptune and the large worlds in between were considered planets. Then Pluto and its counterparts (round bodies that shared their orbits with other bodies) were called dwarf planets. All other objects that orbit the Sun are known as minor solar system bodies.

A group of experts did not take the decision of their colleagues seriously. Alan Stern, the senior researcher of NASA’s New Horizons spacecraft, which passed by Pluto in 2015, regretted the demotion of the former ninth planet of the solar system and said that less than five percent of the world’s 10,000 astronomers participated in the International Astronomical Union vote.

New Horizons was considered an important turning point in the planetary debate. The spacecraft’s quick flyby of Pluto revealed a world far more dynamic than anyone imagined. Large mountains, impact craters, and signs of liquid nitrogen flowing on the surface all suggest a world that has undergone significant geological changes since its formation. People like Stern have argued that Pluto should be considered a planet on that basis alone.

Many people share views with Stern and other like-minded experts. In 2014, shortly before New Horizons flew past Pluto, experts at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, debated different definitions of the planet. Owen Gingerich, a science historian who chairs the IAU’s Planet Definition Committee, stated that “planet is a culturally defined term that changes over time.” But most of the audience watching the CfA debate chose a different definition that would have put Pluto back among the planets.

Alternative classification schemes continue to be proposed. A 2017 proposal defined a planet as “a spherical body in space that is smaller than a star.” This definition makes Pluto a planet again; But it does the same with Earth’s moon, as well as many other moons in the solar system, bringing the total number of officially recognized planets to 110. A year later, Stern wrote an op-ed in The Washington Post with David Greenspoon, senior scientist at the Planetary Science Association, arguing that the International Astronomical Union’s definition was hastily adopted and problematic and that astronomers should rethink their ideas.

Numerous requests from experts have so far been ignored, and the International Astronomical Union is unlikely to address the dispute anytime soon. “The simple fact is that Pluto was misclassified at the time of discovery,” wrote American astrophysicist Ethan Siegel in response to Stern and Greenspoon. “This crime has never been in the same position as the other eight worlds.”

Michael Brown also says: “As a result, Pluto is still not a planet, and in fact it never was.” We just got it wrong for 50 years and now we know better. Missing Pluto is not really a very good argument. “The reality is something else and we have to deal with it.”

These days, children who weren’t even born when Pluto was a planet, ask what the definition of a planet even matters. Why do we have to discuss whether Pluto is a planet or not? Astronomers say there’s no simple answer, and we may have to look beyond our own solar system to understand what makes an object a planet or not.

More than five thousand exoplanets or worlds beyond the solar system have been discovered so far. This vast collection ranges from Earth-sized “super-Earths” to Uranus and “hot Jupiters” orbiting their star closely, to a range of worlds of other sizes. The types of planetary environments that must be considered are changing rapidly.

Even if planets are lucky enough to grow large, how they interact with other planets early in their formation is poorly understood. The worlds interact with each other, and the mutual gravitational effect between them causes the planets to move away from their parent star, close to it, or in some cases, fall out of the system altogether.

What all these explanations suggest is that our definition of a planet should probably be more contextual to account for the number of possible scenarios for the formation of worlds. Perhaps the planets depend on a specific formation condition or specific regions. All we seem to know for sure is that as more and more data is collected, the planet definition and the debate that Pluto has sparked will continue for some time to come.