Doug Bonderud

Jun 13th 2020

Pluto Discovery Debacle: Is Pluto a Planet?


Pluto is making headlines again. After its much-lamented demotion to “dwarf planet” status in 2006, new research housed on Science Direct suggests that historical use of the word “planet” — not International Astronomical Union (IAU) votes — should inform the designation of astronomical objects. There’s passion and precedent on both sides of the debate, so let’s dive in to this Pluto discovery debacle: Is historical precedent superseded by the IAU’s 2006 decision? Or is Pluto a planet again?

Pluto Primer

As noted by, Pluto was discovered on Feb. 18th, 1930, by Clyde W. Tombaugh at the Lowell Observatory. Given its apparent size and distance from the sun — and the limitations of early 20th-century telescope technology — Pluto was hailed as the ninth planet. Designation aside, it’s an interesting world; according to NASA, it’s one-sixth the width of Earth, is 3.6 million miles away from the the sun and has five orbiting moons.

Changing the Game

In 2003, however, astronomers discovered an object larger than Pluto — named Eris — orbiting farther from the sun, said Cal Tech. Combined with data on other Trans-Neptunian Objects (TNO), such as Sedna, the IAU created a committee to reevaluate how we define planets.

Historically, two criteria were used: Planets must orbit around the sun and have sufficient gravity to assume a nearly-round shape. The IAU decision added a third caveat, that planets must “clear the neighborhood around their orbit,” meaning that they must be the only object of substantial size in their local region.

Pluto easily meets the first two criteria, but fails the third. The result? Pluto — along with Eris, Sedna and other large TNOs — were re-classified as “dwarf planets.”

Making the Case: Pluto Is Number Nine

Much of the drive to reclaim Pluto’s status stems from a sense of familiarity; we’ve always known Pluto as a planet, so why not reverse the decision to keep things simple?

“The Reclassification of Asteroids from Planets to Non-planets,” published on Science Direct, examines the use of the word “planet” in scientific literature published since 1802. According to researcher and lead author of the study Philip Metzger, “We now have a list of well over 100 recent examples of planetary scientists using the word planet in a way that violates the IAU definition, but they are doing it because it’s functionally useful.”

For Metzger and other Pluto-as-a-planet supporters, planetary taxonomy and geophysical differences should determine celestial body status — not IAU voting. Their verdict? Pluto was, and is, planet number nine.

Rebuttal: Not so Fast, Dwarf Planet

As noted above, the discovery of new TNOs made it clear that Pluto wasn’t the biggest, farthest object orbiting the sun. Evolving technology and detection methods revealed that Pluto is part of the Kuiper Belt, a ring of dwarf planets, ice balls and comets that number in the trillions and orbit beyond Neptune, said Popular Science. In fact, new research by Mike Brown and Konstantin Batygin, who helped discover Eris and led the down-with-Pluto movement, suggests that another, super-massive “Planet Nine” may account for odd orbits of many TNOs and be the rightful successor to Pluto’s planetary crown, according to Scientific American.

For dwarf-planet drivers, the demotion of Pluto isn’t problematic. The world remains compelling, and concern around the third criteria set out by the IAU — how we can discover if extrasolar objects have “cleared their neighborhood” — has been addressed by recent scientific work. UCLA professor Jean-Luc Margot discovered that if you know the mass of an object, its orbital distance/period and the lifetime of its planetary system, it’s possible to determine if the object meets IAU criteria with 99 percent accuracy.

Classifying the Cosmos

What’s the big deal with the planet/dwarf planet debate? It starts with the Pluto discovery: Despite more recent data, it’s hard to shake 75 years of Pluto-as-a-planet literature, research and mnemonic devices like “My Very Excellent Mother Just Served Us Nine Pizzas.”

The problem? New discoveries — such as the TRAPPIST-1 system, which boasts seven Earth-sized planets and three within the star’s “habitable zone” — demand the most accurate information possible as humankind looks to touch the stars. While Pluto and similar worlds offer massive research potential, the IAU’s decision helps narrow the definition of planet to one that more closely matches what we observe on Earth and could help inform the future of habitable planet exploration.

Is Pluto a Planet?

So what is Pluto’s status? It depends on your perspective. New IAU criteria append the “dwarf” label and remove its planetary designation; historical use suggests that geophysical features may be the ideal differentiator between terms and evolving scientific consensus will determine the best nomenclature.

Planet or not, Pluto gets plenty of attention — hopefully leading to continuing debate that helps drive public curiosity about the structure, scale and scope of our solar system.