Reclassifying Pluto

Young Astronomers Blog, Volume 29, Number 18.

Remember Pluto? Yes, Pluto. It was a planet for a long time. Then it wasn’t a planet (and many people were not happy).

Pluto was discovered in 1930 and for many years was thought to be the only object in the distant reaches of the Solar System. That is until QB₁ was found in 1992. This object, now named Albion, was followed by the discovery of other objects including Quaoar, Haumea, Makemake, and eventually Eris. Suddenly the outer part of the solar system beyond the orbit of Neptune was getting very crowded. People started asking questions. How many of these new objects were planets? Was Pluto still a planet?

Central to this discussion was the International Astronomical Union (IAU). The IAU is the group of Ph.D. level astronomers with the responsibility for keeping track of and naming objects in the Solar System. If you discover something new, the first thing you do is contact the IAU’s Minor Planet Center (MPC). They also have very specific guidelines for naming objects. Once you decide on a name, you submit it to the IAU MPC for approval.

One of the first to argue for the “reclassification” of Pluto was Brian Marsden, the longtime director of the Minor Planet Center, who offered to give Pluto a dual status of planet and minor planet (number 10000) on February 2, 1999. The idea was immediately rejected by the IAU with a press release assuring everyone that there were no plans to change the status of Pluto.

Next came the Hayden Planetarium in New York, which opened a new exhibit and relegated Pluto to just one of several Kuiper Belt objects. This led to a 2001 headline in the New York Times announcing: “Pluto’s not a planet? Only in New York.”

As the IAU was approaching their 2006 General Assembly in Prague, they decided something must be done and they would officially address the definition of a planet.

The process wasn’t as smooth as they would have liked. Much of the problem stemmed from a split between two opposing viewpoints. (see Gingerich,  “Perplexed in Paris” and Boyle “The Fight for the Ninth Planet”).

• There were those who wanted to include considerations of how an object’s dynamics affected other objects around it.
• Others wanted to focus only on the object’s structure and the size required to reach hydrostatic equilibrium (i.e., to become spherical).

In March 2004, The Working Group on the Definition of a Planet was formed. This was a committee of 19 that met only by e-mail for 18 months. They were unable to develop a final definition, however, in the end, they offered three possibilities (see Williams and Burnell,  “What it takes to make a planet.”).

1. A planet is anything with a radius greater than 1,000 km.
2. Trans-Neptunian objects with unusual orbits would fall into a new category, distinct from the existing planets.
3. A planet is an object that dominates its locality.

In April 2006, The Planet Classification Committee was formed. It included only eight members (Andre Brahic, Iwan Williams, Junichi Watanabe, Richard Binzel, Owen Gingerich, Dava Sobel, and Catherine Cesarsky). The committee was asked to develop a definition of a planet in time for the IAU August meeting in Prague. The committee met in June 2006 and after a fruitless first day, they came to a consensus on the second day. However, the IAU decided to keep the proposed resolution secret until the general meeting in August.

At the beginning of the Prague meeting, the IAU announced the committee’s recommendation (see iau0601 press release). It was momentous! It would change the number of planets forever. They determined that if an object was round and orbited the Sun it would be considered a planet. Not only Pluto, but Ceres (the largest asteroid), Eris, and even Pluto’s moon Charon were going to be planets. And the Solar System would have twelve planets in all!

But alas, after ten days of intense argument and debate the final decision was in. It was a red-letter day for Pluto. The debate was heated. Astronomer Owen Gingerich described the entire process in an article, “Losing it in Prague.” In the end, Pluto, Eris, and Ceres didn’t make the cut. They would forever be known as Dwarf Planets. Charon didn’t even make that cut; Charon was back to being just a moon.

Under the IAU definitions, both a planet and dwarf planet had to be spherical, and both had to directly orbit the Sun (sorry no moons). The distinction between the two is that a planet clears its orbit of other objects, while a dwarf planet does not. This, in a sense, resulted in the classification of larger spherical objects as planets and smaller ones as dwarf planets. See Resolution B5 for the official definition and IAU press release iau0603 for the final vote and additional information.

My way of thinking about the new definitions is that planets live by themselves in their own house, while dwarf planets live in apartments with others around.

At this time, Pluto was also classified as a minor planet and official received minor planet number 134340. Maybe number 10000 would have been okay after all?

As you may know, the results of the IAU meeting were not met with overwhelming support from the general public. People all over the country were up in arms! How could this happen to poor old Pluto.

The reaction was loud and immediate.

• Articles appeared in newspapers across the country.
• There were public outcries and protests.
• Some lawmakers passed resolutions in favor of Pluto’s planetary status.
• Three hundred astronomers signed a petition, organized by Mark Sykes and others, protesting the IAU resolution.

One of the criticisms of these new definitions is that only around 400 astronomers, out of 2,700 who attended the meeting and the 10,000 members of the IAU, were around to vote on the final resolutions. Another is that these are astronomers, not planetary geologists, who determined the definition of a planet.

Moreover, the definition of a dwarf planet is not quite consistent with other definitions. A dwarf planet is not a planet, although a dwarf star is a star, and a dwarf tree is a tree. There was a proposal to define the existing planets (except for Pluto) as “Classical Planets” to distinguish them from “Dwarf Planets,” and allow both to be “planets.” However, this resolution was defeated.

Mark Alford in “The Dog Star, uh Planet, uh Whatever…” reported on the reaction of Pluto’s demotion from some friends of Pluto.

Despite the multitude of objections, many astronomers supported the new definitions. Some have a concern that leaving things as they were, would result in a 100 or more planets down the road as more small spherical objects were discovered. And, in general, the new definitions have stayed with us.

In June 2008, the IAU threw Pluto a bone by declaring that dwarf planets outside the orbit of Neptune would be called Plutoids. Maybe the third time is a charm. They initially considered the term “Plutons” for these objects, then “Plutonian objects”, and now we officially have “Plutoids”. Later in the same year, the IAU added two more Kuiper Belt objects to the official list of Dwarf Planets; Makemake in July and Haumea in September. See IAU press releases iau0804, iau0806, and iau0807.

In 2008, following the legacy of the Great Debate of 1920, Mark Sykes and Neil deGrasse Tyson squared off for The Great Planet Debate, this one about the definition of a planet and the status of Pluto. I don’t think a clear winner emerged from the debate.

In December 2021, several astronomers, including Mark Sykes and Alan Stern, published a paper purposing that all geologically active solar system objects should be considered planets. This would add Pluto and many moons to a much-expanded list of planets. 

So far, as of mid-2023, none of the other trans-Neptunian or Kuiper Belt objects have been officially classified as dwarf planets. The IAU did adopt a rule that if an object has an absolute magnitude of 1 or brighter, then they will go about the formal process to classify it as a dwarf planet (see IAU press release iau0804). No other object has yet to meet this criterion. 

Astronomer Mike Brown does maintain a list of potential dwarf planets and, as of February 2021, there are six others that he considers “near certain” to be dwarf planets (Gonggong, Quaoar, Sedna, Orcus, 2002 MS₄, and Salacia).

Appendix (Absolute Magnitude)
The absolute magnitude of a solar system object is defined as the apparent magnitude of the object if positioned one astronomical unit from the Sun and viewed at a distance of one astronomical unit. The absolute magnitude of an interstellar object is the apparent magnitude of the object from a distance of 10 parsecs (32.6 light years).