Eight days before Pluto lost its planetary status, it appeared ready to keep its place and gain three new neighbors.
According to an International Astronomical Union (IAU) draft proposal released on August 16, 2006, the Solar System would expand from 9 to 12 planets. Pluto would remain a planet. Ceres, Pluto’s largest moon Charon, and a distant body then designated 2003 UB313 would join it. Astronomers later named 2003 UB313 Eris.
The proposal also left room for more planets as astronomers evaluated other candidates. By August 24, however, IAU members had adopted a different definition of what qualifies as a planet. Rather than growing to 12 planets, the Solar System was left with eight, and Pluto was classified as a dwarf planet.
Twenty years later, researchers still disagree over whether a planet should be defined by the orbit it controls or by the physical characteristics of the world itself.
A Definition That Would Have Saved Pluto
According to the IAU, Eris appeared comparable in size to Pluto. If it counted as a planet, other distant worlds might qualify as well. If it did not, astronomers had to explain why Pluto remained on the list.
The IAU had spent nearly two years considering how to define a planet. In summer 2006, its seven-member Planet Definition Committee weighed the scientific and historical meaning of the word. Committee chair Owen Gingerich recalled that “several members admitted that they had not slept well” as the group worked toward an agreement, according to the IAU.
Explore All OptionsThe resulting draft focused largely on a body’s physical properties. It had to orbit a star without being a star itself, and it needed enough mass for its gravity to pull it into a nearly round shape. The draft also treated Charon as Pluto’s partner in a double-planet system.
Under those requirements, Pluto, Ceres, Charon, and Eris all qualified. Pluto and similar worlds in the outer Solar System would belong to a new planetary subclass called “plutons.” That version, however, did not survive the IAU gathering in Prague.
Read More: Six Giant Landslides Discovered Along Pluto's Craters Highlight the Dwarf Planet's Changing Surface
How 12 Planets Became Eight
The definition adopted on August 24 required a planet to orbit the sun, be nearly round, and have “cleared the neighbourhood around its orbit,” according to the IAU’s final resolution.
Clearing an orbit does not mean removing every nearby asteroid. Instead, a planet’s gravity must dominate its region and influence the motion of smaller bodies around it.
Pluto does not dominate its part of the Solar System. NASA places Pluto in the Kuiper Belt beyond Neptune, where it shares space with thousands of icy bodies. Pluto therefore met the first two requirements but failed the third.
The main definition passed with what the IAU called a “great majority,” although officials did not count the individual votes. A separate resolution identifying Pluto as a dwarf planet passed 237 to 157, with 17 abstentions.
Why the Pluto Debate Continues
When NASA’s New Horizons spacecraft launched toward Pluto in January 2006, Pluto was the Solar System’s ninth planet. The IAU had reclassified it by the 2015 flyby.
According to NASA, New Horizons revealed water-ice mountains, a vast nitrogen glacier, and layers of atmospheric haze. The mission also found evidence of ongoing geological activity on Pluto’s surface.
Some planetary scientists think those qualities should carry more weight than Pluto’s orbit. In a 2022 study published in Icarus, Philip Metzger and his colleagues argued for classifying planets by their internal makeup and geological behavior. Their geophysical approach would count Pluto, along with several large and complex moons, as planets.
Other researchers continue to favor orbital dominance. A 2024 study published in The Planetary Science Journal proposed measurable definitions that could also apply beyond the Solar System. That approach still separated the eight planets from Pluto and other smaller bodies.
The debate now comes down to which characteristics should determine the label: what a world is physically or how its gravity shapes the region surrounding it.
Read More: It Took Pluto Nearly 250 Years to Finally Orbit the Sun — Here's Why
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from the IAU: The IAU draft definition of "planet" and "plutons"
- This article references information from the IAU: IAU 2006 General Assembly: Result of the IAU Resolution votes
- This article references information from NASA: Pluto Facts
- This article references information from NASA: New Horizons
- This article references information from NASA: Eris
- This article references information from a study published in Icarus: Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science
- This article references information from a study published in Earth and Planetary Astrophysics: Quantitative Criteria for Defining Planets
