August 24, 2006. Today is the 13th anniversary of the decision by the International Astronomical Union (IAU) to demote Pluto to dwarf planet status. Our solar system went from having nine major planets to having eight major planets. Pluto, once considered the outermost planet, became more widely known as the largest of a number of small bodies in the outer solar system. Neptune, the eighth large planet out from our sun, is now considered outermost major planet. The IAU formulated a new definition of what it means to be a planet. The IAU’s XXVIth General Assembly formalized the decision and announced it on August 24, 2006. The public and many astronomers didn’t take it lightly, with some declaring they would still consider Pluto a planet. The word plutoed – meaning to demote or devalue something – entered the global lexicon.

Why did it happen? We still get questions about this today. Prior to 2006, astronomers hadn’t gotten around to establishing clear standards – such as a minimum size or mass, or other considerations – by which an object might be categorized as a solar system “planet” versus “dwarf planet.”

They began to see a need when many small bodies – such as Haumea and Makemake – began to be discovered in the outer solar system. Eris, also considered a dwarf planet, is even more massive than Pluto! So if Pluto is a planet, why shouldn’t Eris be granted planet status as well? That was the question the IAU asked itself, which led to its formation of a Planet Definition Committee and ultimately the 2006 decision.

The committee had a few possible roads to travel down. One would be to make the decision by size (or mass) so that Pluto would remain a planet, and therefore Eris and Ceres – the largest body in the asteroid belt in the inner solar system – would become planets, too. For a while, it looked as if that might happen with some IAU committee members favoring that decision.

Another option for the IAU might have been to define the concept of a planet without any specific logic: Earth is a planet, Pluto is a planet, but Eris is not just because.

On August 24, 2006, the IAU announced its solution to the questions raised by having so many new objects in the outer solar system. It had decided to create a scientific definition of what it means to be a planet. Unfortunately, the definition excluded Pluto from major planet status. Here’s the definition:

A planet is a celestial body that

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and

(c) has cleared the neighbourhood around its orbit.

It’s “c” that causes Pluto to fail as a planet, according to the IAU. For an object to be a major planet, according to this definition, it must be the dominant gravitational object in its orbit. It must either sling other objects away or merge with them.

Pluto is only 0.07 times the mass of the objects in its orbit. Meanwhile, Earth is 1.7 million times the mass of the objects in its orbit.

On that fateful day – August 24, 2006 – the IAU also created a new category of celestial objects for Pluto and all Pluto-like objects:

A “dwarf planet” is a celestial body that

(a) is in orbit around the sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,

(c) has not cleared the neighbourhood around its orbit, and

(d) is not a satellite.

According to the IAU, there are currently five dwarf planets: Pluto, Ceres, Eris, Haumea, and Makemake. However, there is a likely potential sixth dwarf planet candidate temporarily designated as 2007 OR10, and many other known dwarf planet candidates.

Astronomers believe there may be hundreds of undiscovered dwarf planets in the Kuiper Belt of the outer solar system. There may be up to 10,000 in the region beyond.

By the way, it’s not common knowledge that many astronomers started out quite careful about their use of the word “planet” with respect to Pluto. In 1932, for example, only two years after American astronomer Clyde Tombaugh discovered Pluto, another astronomer, Armin Otto Leuschner, wrote in a journal article:

You may observe that with extreme conservatism I am still referring to Pluto as an object rather than as a planet. There is every probability that it is a planet, as is now universally concluded, from available material … So far only an upper limit for the mass of Pluto … has been established, and such a mass is believed from gravitational considerations to be too small to affect the motions of Uranus and Neptune sufficiently … There is also a remote chance that later investigations will render its mass comparable to that of comets.

Recall why astronomers began searching for Pluto in the first place. They expected to find an object large enough to gravitationally disturb the orbit of Neptune. If Pluto had been discovered a decade or so later, when Edgeworth speculated about the existence of the Kuiper Belt, it might have never been awarded the status of planet.

As it was, like it or not, Pluto became the world’s eye-opener when it came to the classification of solar system objects.

Bottom line: August 24, 2019, is the 13th anniversary of Pluto’s demotion to dwarf planet status. According to a new definition by the International Astronomical Union, Pluto became a dwarf planet because it is has not “cleared the neighborhood around its orbit.”

Read more about Pluto here

The IAU talks about its decision to demote Pluto here