Spaceflight Insider

Ten years later, IAU Pluto vote remains controversial

Pluto and Charon: Strikingly Different Worlds. Image Credit: NASA/JHU-APL/SwRI

Pluto and Charon: this enhanced color image shows strikingly different worlds. (Click to enlarge) Image Credit: NASA / JHU-APL / SwRI

In the decade that has passed since the International Astronomical Union’s (IAU) controversial vote that demoted Pluto from planet status, people around the world have seen what was once little more than a tiny dot transformed into a complex world via NASA’s New Horizons mission.

As Principal Investigator Alan Stern told SpaceFlight Insider, the planet definition adopted by just four percent of the IAU on August 24, 2006, remains controversial and rejected by many planetary scientists ten years later.

“The controversy is about a definition that includes all planets around other stars and small planets. It is inherently biased against small planets,” Stern said.

By specifically requiring a planet to orbit the Sun instead of a star, the IAU precluded any exoplanets from being considered planets.

In 2006, the number of confirmed exoplanets was less than 300. Today, there are 3,501 and counting.

The IAU definition requires a planet to have “cleared the neighbourhood around its orbit” – which Stern and other planetary scientists see as inherently biased against both small planets and those in more distant orbits.

Pluto in true color

Four images from New Horizons’ Long Range Reconnaissance Imager (LORRI) were combined with color data from the Ralph instrument to create this global view of Pluto. (The lower right edge of Pluto in this view currently lacks high-resolution color coverage.) The images, taken when the spacecraft was 280,000 miles (450,000 kilometers) away, show features as small as 1.4 miles (2.2 kilometers), twice the resolution of the single-image view taken on July 13. Image & Caption Credit: NASA / JHU-APL / SwRI

Stern said to SpaceFlight Insider: “Put Earths in the places of all nine classical planets, the first six of nine will be able to clear their zones,” while the remaining ones will be unable to do so.

The result is that “identical objects classified differently, which is ridiculous scientifically,” Stern continued. “A star is a star no matter where it is. A cow is a cow wherever you find it. Clouds – it doesn’t matter if there are many or just one. A definition should be based on an object’s characteristics, not by what it is next to. This definition was constructed on the basis that we need to keep the number of planets small.”

That motivation is an “unscientific rationale; that we should not have too many planets because schoolchildren couldn’t memorize their names. What utter garbage.”

The IAU “created a definition that has mathematical inconsistencies. Every one of the eight planets (recognized by the IAU) [has] objects in their zones.”

New Horizons was seven months into its journey when the IAU vote took place, having been launched on January 19, 2006.

New Horizons mission to Pluto timeline graphic.

New Horizons mission to Pluto timeline graphic. (Click to enlarge) Image Credit: NASA / JHU-APL / SwRI

After the flyby in July 2015 that revealed a complex world no one had anticipated, the spacecraft is now more than three AU (astronomical unit) distant, with one AU equal to the average Earth-Sun distance – 93 million miles or 150 million kilometers. The spacecraft is now on its way to rendezvous, on January 1, 2019, with a second target – KBO 2014 MU69.

Blue skies on Pluto

Pluto’s blue sky. Image Credit: NASA / JHU-APL / SwRI

“People saw a rounded planet with mountains, glaciers, blue skies, and an atmosphere and ended up confused,” Stern told Spaceflight Insider. “Five moons, mountains, glaciers, complex geology, round and spherical – that all adds up to [a] planet.”

In addition to contradicting people’s own perceptions of Pluto, the IAU decision also sends the wrong message about the way science is done, he emphasized.

“It’s a failure on multiple levels. It did great injury to planetary science and to science in general. Science is a much grander process than voting. What the IAU did in voting made science look arbitrary.”

Stern also faults the media for its unquestioning acceptance of the IAU definition as the only “official” one.

“A few dynamicists pulled the wool over everyone’s eyes in a group of non-expert astronomers. The media is listening to the wrong experts. The science press did not look critically at the definition.”

Most of the IAU members who voted were experts in fields other than planetary science, he stated.

The question of Pluto’s planet status comes up in every talk Stern gives about Pluto and New Horizons, even if he does not raise it in his presentation, he said. In his experience, planetary scientists largely ignore the definition.

Planetary scientist Alan Stern gives the “Pluto Salute".

Planetary scientist Alan Stern gives the “Pluto Salute”. Photo Credit: Kevin Schindler

“What I see is a lot of planetary scientists naturally calling Pluto a planet.”

What planetary scientists do not have is an organization with status equivalent to the IAU. In spite of this, Stern believes the 2006 definition “will collapse under its own weight. It’s just a matter of time.”

Last year’s flyby not only revolutionized scientists’ understanding of Pluto, it also raised so many questions that some scientists want to go back, this time with an orbiter.

To promote sending an orbiter to Pluto, the scientific community will have to organize and push NASA’s Science Mission Directorate to fund a study. The issue would have to be made a priority in NASA’s next Decadal Survey, which will next be done in five or six years.

Discussing an orbiter now is premature, as 20 percent of the Pluto data still has not been returned. Once that happens, that data will be processed and analyzed via computer models that will attempt to trace the history of Pluto’s atmosphere and geology.

“We have a long list of questions that can’t be answered without an orbiter,” he acknowledged.

Meanwhile, what Pluto is and how it came to be the way it is remain unsolved mysteries.


Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

Reader Comments

There was a nefarious, dark spiritual push to reject Planet Pluto. Imagine, they only had to wait a short time before the planet itself could speak volumes, for the first time, but they preemptively devalued it in the minds of many planetary enthusiasts. Such has never before been done to a planet or the scientists who worked so hard to bring some of its mysteries to light. In fact, they couldn’t agree on the color of Pluto at the time of their vote. But now we know that it confesses blood-red regions and shows a big heart.

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