We had to rewrite textbooks. The public was outraged. As a result of the International Astronomical Union's (IAU's) vote on Pluto reclassifying it as a dwarf planet on Aug. 24, 2006, our understanding of the solar system was forever changed. This relegation, which was widely considered a demotion and that continues to have repercussions today, has still been felt today. 

Today, the debate about Pluto exposes difficulties in the definition of "planet." The IAU defines a planet as a celestial body orbiting the sun, with a nearly spherical appearance, and that has (for the most part) cleared waste from its orbital neighbourhood. But even this set of metrics is not universally agreed upon. 

In spite of their large sizes, Earth and even Jupiter have not cleared many asteroids from their orbital regions. Ceres, for instance, is a circular world that orbits the sun but is not considered a planet.

The History of Pluto

Pluto's demotion from planet status raises broader questions about what constitutes a solar system object or any object  in space. In some cases, science cannot categorize objects easily. If the definition of a planet is once again widened, it becomes unclear how to assess the many non-circular objects that circle our sun. This may even raise questions about the asteroid belt, which consists of a massive band of small objects between Mars and Jupiter. What would happen to a planet if it were broken up into pieces?

Despite this, many don't quite understand why Pluto was knocked from its planetary position nearly two decades ago. However, the solar system's transformation from nine planets to eight (at least by the standard IAU definition) was a long time in the making and helps encapsulate one of the greatest strengths of science — the ability to alter seemingly steadfast definitions in light of new evidence.

The word planet (in English) stretches back to antiquity, deriving from the Greek word "planetes," which means "wandering star." There are five classic planets visible to the naked eye, Mercury, Venus, Mars, Jupiter and Saturn, which move across the sky in strange pathways as compared to distant background stars in the distant background.  

In 1930, when Clyde Tombaugh of the Lowell Observatory in Arizona compared photographs of the sky on separate nights and noticed a tiny dot drifting back and forth against the backdrop of stars, Pluto was discovered and classified as a planet (the International Astronomical Union was founded in 1919). It remained an oddball for some time, however. During its 248-year journey, it actually gets closer to the sun than Neptune for 20 years because its orbit is so eccentric. As well as tilting toward the ecliptic, the plane on which other planets orbit in the solar system, it also revolves relative to it.

The first Kuiper Belt object was discovered in 1992, 1992 QB1, a tiny body orbiting beyond Neptune's orbit. In the next few months, many more smaller, frozen worlds were discovered, similar to the asteroid belt between Mars and Jupiter. Astronomers discovered the distant body Eris in July 2005, which was initially thought to be even larger than Pluto, in this region.

Redrawing the lines

When Pluto was classified as a planet, did that mean Eris was as well? How about the other icy objects in the Kuiper belt or the smaller ones in the asteroid belt? Where exactly was the cut-off line for classifying a body as a planet? A word that had seemed straightforward and simple was suddenly shown to be oddly slippery.

An intense debate followed, with many new proposals for the definition of a planet being offered.

During the Prague conference, the 424 astronomers who remained voted on three new categories for objects in the solar system. From that point forward, only Mercury through Neptune would be considered planets. Pluto and its neighbours - round objects with orbits that share space with other entities - were renamed dwarf planets. Objects orbiting the sun are known as small solar system bodies. 

Pluto is a Planet.

An embarrassing less than 5 percent of the world's 10,000 astronomers participated in the vote.

Despite Pluto's classification as a dwarf planet, it is a world that is far more dynamic than anyone imagined. Its large mountains, battered craters, and signs of liquid flowing upon its surface indicate a world that has undergone an extensive geological change. In these terms alone, people like Stern have argued that Pluto should be considered a planet because its surface is not static and is not disturbed by micrometeorites alone.

Pluto's moon Charon also appears to be a very dynamic place, including a red cap on its pole that changes appearance with the slow seasonal change so far out in space. Pluto possesses several moons, whereas Mercury and Venus lack moons. Asteroids and dwarf planets often have moons, which complicates the definition of a planet further.

Answers that raise more Questions

There are a growing number of alternative classification schemes. According to a proposal published in 2017, planets are "round objects in space smaller than stars." This would reclassify Pluto as a planet, but would also reclassify Earth's moon, as well as many other moons in our solar system, and increase the number of planets recognized. In the Washington Post, Alan Stern, a leader of NASA's New Horizons mission which flew past Pluto in 2015, and planetary scientist David Grinspoon argued that the IAU's definition was "hurriedly drawn" and "flawed" and astronomers should reconsider it.

QUESTION: Do you think we’ll see Pluto reinstated?

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