Yesterday, August 24th, 2025, marked the 19th anniversary of Pluto’s reclassification (some prefer “demotion”) from “planet” to “dwarf planet” changing the longstanding count of planets in our Solar System from nine to eight.
I was working at the Roger B. Chaffee Planetarium at the time as the technician and producer, and the entire team had been following the discussion and proposals at that years meeting of the International Astronomical Union, the body which tries to set global standards and definitions that allow astronomers to collaborate, with rapt attention.
The discussion about Pluto had been set of by the discovery of an object then dubbed 2003 UB313 by astronomer Dr. Mike Brown at CalTech and his colleagues. This object, which was shortly dubbed “Eris,” built upon the discovery of hundreds of asteroids discovered orbiting beyond the orbit of Neptune since the 1990s, and was the first of these discovered larger than Pluto, prompting the discussion of whether Pluto really stood out from the rest of these asteroids.
Initial proposals released as the conference started envisioned a more expansive definition of planet, which would have added Eris to the list, but also restored Ceres, the largest asteroid between Mars and Jupiter, as a planet (a designation it had been stripped of when asteroids were increasingly being discovered in the mid-19th century), and treated Pluto’s largest moon, Charon, as a planet, since Charon and Pluto orbit a common center of gravity between the two bodies.
We rushed orders for slide film to be developed for our classic kodak projectors to share the proposal with our audiences.
As the week progressed though, this proposal fell out of favor with many astronomers, especially those who study the dynamics of how the solar system formed. Ultimately, in the vote that closed out the conference sessions, a three-part definition of planet was adopted.
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.
Objects that met the first two criteria, but not the third, which included Pluto, Ceres, Eris, and several other objects beyond Neptune were given a new category called dwarf planets.
In the 19 years since, Pluto has never lost the public’s love, and if anything the classification led many to personify it as a neglected underdog. This hasn’t been helped by the vague wording of cleared the neighborhood around its orbit. I’ve had the pleasure of working with many scientists who have studied Pluto and dwarf planets over my career, and I made this short explainer video to share a little more detail of what clearing the orbit really means.
In addition to working with Dr. Renu Malhotra, mentioned in the video, in the years since, I had the pleasure of working with NASA Ames Research Center and Dr. Jeffery Moore to bring high-resolution surface imagery of Pluto into planetarium software for the first time, giving a public audience the first interactive tour of the world after New Horizons flew past Pluto in 2015. I also have had the pleasure of working with the aforementioned Dr. Mike Brown, who occaionsally adds the monniker “Plutokiller” before his name to share his journey exploring the outer Solar System, and his ambitious theory that a ninth major planet may be shaping orbits in our outer solar system. I also was privileged to support New Horizons mission lead Dr. Alan Stern and Dr. David Grinspoon on their book tour for Chasing New Horizons detailing the mission to explore Pluto.
Having heard the perspectives from all sides, while I definitely will not deny that Pluto is an incredible world of fascinating landscapes, geology and chemistry… I do agree with the decision that Pluto is a different type of object than the eight major planets. Regardless of how we classify it, Pluto is a fascinating window back into the history of our Solar System, and a world with more mysteries worthy of discovery.
