Pluto was reclassified from the ninth planet to a “dwarf planet” on August 24, 2006, when the International Astronomical Union (IAU) voted to create a formal definition of the word “planet” for the first time. Pluto met two of the three new criteria but failed the third, and it has remained a dwarf planet ever since. The story of how that happened, and what we’ve learned about Pluto in the years after, is more interesting than most people realize.
Why Pluto Lost Its Planet Status
For decades, astronomers had no official definition of “planet.” The word was used loosely, and Pluto held its spot as the ninth planet mostly out of tradition. That changed when the IAU established three requirements a body must meet to qualify:
- It orbits the Sun.
- It has enough mass for gravity to pull it into a roughly round shape.
- It has “cleared the neighborhood” around its orbit.
Pluto passes the first two tests easily. It orbits the Sun and it’s spherical. But it fails the third. “Clearing the neighborhood” means a body has become the gravitationally dominant object in its orbital zone, either by absorbing nearby objects, flinging them away, or capturing them as moons. Earth, Jupiter, and the other planets have done this thoroughly. Pluto has not. It shares its region of space, the Kuiper Belt, with thousands of other icy bodies of similar size and composition. It’s one object among many rather than the ruler of its orbit.
The Discovery That Forced the Decision
The real catalyst was Eris, a distant icy world discovered in 2005. Eris is 1.27 times the mass of Pluto and was initially measured as slightly physically larger (though later observations put their diameters very close together). If Pluto was a planet, Eris would have to be the tenth planet. And astronomers knew more objects like it were likely waiting to be found in the outer solar system. The IAU faced a choice: keep adding planets every time a new Kuiper Belt object turned up, or draw a clear line. They drew the line, and both Pluto and Eris fell on the other side of it.
The Dwarf Planet Category
Pluto wasn’t simply demoted to nothing. The IAU created a new classification, “dwarf planet,” for round bodies orbiting the Sun that haven’t cleared their neighborhoods. Our solar system currently has five recognized dwarf planets. In order of distance from the Sun, they are Ceres (in the asteroid belt), Pluto, Haumea, Makemake, and Eris. Many astronomers suspect dozens more objects in the outer solar system could eventually qualify.
Not Everyone Agrees
The 2006 vote was contentious, and the debate hasn’t fully settled. A group of planetary scientists, including Alan Stern, the lead researcher behind NASA’s New Horizons mission to Pluto, has argued for a “geophysical planet definition.” Under this framework, any body massive enough to be round counts as a planet, regardless of what else shares its orbit. Stern and his colleagues point out that the “clearing the neighborhood” criterion is about where an object is, not what it is, and that a body’s geology and complexity should matter more than its orbital real estate.
The New Horizons flyby in 2015 gave this argument significant ammunition. The spacecraft revealed a world far more complex than anyone expected: ice mountains, floating glaciers, windswept dunes, evidence of cryovolcanism (volcanoes that erupt ice instead of lava), varied terrain types, a layered atmosphere, and signs of a subsurface ocean. As supporters of the geophysical definition put it: it looks like a planet. The IAU has not revisited its definition.
What New Horizons Revealed
Before 2015, the best images of Pluto were blurry smudges from the Hubble Space Telescope. New Horizons changed everything. The most iconic feature it found is a vast heart-shaped region on Pluto’s surface. The left lobe of that heart, informally called Sputnik Planitia, is a basin roughly 620 miles (1,000 kilometers) wide filled with a deep layer of solid nitrogen ice. This nitrogen ice is actively churning in slow convection cells, meaning Pluto’s surface is geologically alive despite surface temperatures around negative 230 degrees Celsius.
The mission also returned detailed data on Pluto’s atmosphere, a thin envelope of nitrogen, carbon, and traces of other gases that rises hundreds of miles above the surface. Haze layers in the atmosphere scatter blue light, giving Pluto a faint blue sky when viewed from certain angles. Pluto’s outflowing atmosphere interacts with the solar wind, exchanging electrons with charged particles of oxygen, carbon, and nitrogen streaming from the Sun.
A World of Rock and Ice
Pluto is far denser than you might expect for something so far from the Sun. Its interior is roughly 68 to 80 percent rock by mass (relative to its combined rock and water-ice content), making it much more rock-rich than other large icy moons like Jupiter’s Ganymede or Saturn’s Titan. Scientists believe Pluto is a differentiated body, meaning its rock and ice have separated into layers over time, with a rocky core surrounded by an icy mantle. Its closest structural cousin in the solar system may be Europa, Jupiter’s ocean moon.
Evidence from New Horizons strongly suggests a liquid water ocean exists beneath Pluto’s icy shell. The formation of the Sputnik Planitia basin and the way Pluto’s surface has deformed over time are difficult to explain without a subsurface layer of liquid. A reservoir of methane near the surface also points to internal activity that has been ongoing for billions of years.
Pluto’s Unusual Relationship With Charon
Pluto has five known moons, but Charon is by far the largest. It’s so big relative to Pluto (about half Pluto’s diameter) that the two bodies orbit a shared center of gravity, called a barycenter, that sits in the empty space between them rather than inside Pluto. This is unlike any planet-moon pair in the solar system, where the barycenter always falls within the larger body. Some astronomers have suggested calling Pluto and Charon a “double dwarf planet” system. The four smaller moons, Nix, Hydra, Kerberos, and Styx, orbit around this same barycenter. Charon itself likely formed from a giant impact, similar to how Earth’s Moon is thought to have formed.
Pluto’s Atmosphere Is Fading
Pluto’s atmosphere isn’t permanent. It depends on sunlight warming surface ices enough to sublimate into gas. Pluto reached its closest approach to the Sun in 1989, and since then it has been slowly moving farther away on its 248-year orbit. Observations from 1988 through 2016 showed atmospheric pressure steadily increasing, likely due to thermal lag (the surface takes decades to respond to changing solar input, much like how the hottest day of summer comes well after the longest day).
That trend now appears to be reversing. Recent stellar occultation measurements indicate Pluto’s atmospheric pressure has begun decreasing. Models disagree on what comes next. Some predict a gradual contraction over the coming decades, while others suggest a more dramatic collapse, where the atmosphere freezes out onto the surface entirely. A few models argue the atmosphere could persist, in a thinner form, throughout Pluto’s entire orbit. Resolving this question will require continued observations from Earth-based telescopes, since no new spacecraft missions to Pluto are currently in development.