What changed in 2006
For decades after its discovery in 1930 by Clyde Tombaugh, Pluto was counted as the ninth planet. In August 2006, the International Astronomical Union (IAU), the main body that names and classifies astronomical objects, voted on the first formal definition of “planet” and placed Pluto in a new category: dwarf planet. Pluto did not change; the way astronomers classify objects did.
The reason was that, from the 1990s on, astronomers had found many icy objects orbiting beyond Neptune, in a region called the Kuiper Belt. In 2005, one called Eris was announced that appeared about as large as Pluto, and a little more massive. If Pluto was a planet, then Eris and possibly dozens of other objects might need to be too. Astronomers needed a clear rule.
The three criteria
Under the IAU definition, a planet in our solar system must meet three conditions.
- It orbits the Sun. It is not a moon of another body.
- It has enough mass for its own gravity to pull it into a roughly round shape. Small lumpy objects such as many asteroids fail this test.
- It has cleared the neighborhood around its orbit. This means it is by far the dominant body in its orbital zone, having swept up or flung away other material of comparable size.
A dwarf planet meets the first two conditions but not the third, and it is not a moon. Pluto orbits the Sun and is round, but it shares its region with many other icy bodies in the Kuiper Belt, so it has not cleared its orbit. By contrast, Earth and Jupiter dominate their orbits, which is what the third criterion expresses.
Where Pluto fits
Pluto is small. Its diameter is roughly 2,400 kilometers, which is smaller than Earth’s Moon (about 3,500 kilometers). Its orbit takes around 248 years, and it is tilted and oval-shaped, bringing it closer to the Sun than Neptune for some of that time. The two never come near each other, because they are locked into a repeating orbital pattern in which Pluto goes around twice for every three Neptune orbits.
Pluto’s largest moon, Charon, is big in proportion, around half Pluto’s diameter, so the two are sometimes described as a double system. Pluto has four smaller moons as well.
Other dwarf planets
Pluto is not alone in its category. The IAU recognizes a handful of dwarf planets, and others are suspected.
- Ceres lies in the asteroid belt between Mars and Jupiter and is the smallest, about 950 kilometers wide.
- Eris is in the far outer solar system and is of similar size to Pluto.
- Haumea and Makemake are in the Kuiper Belt region.
The exact count depends on how many distant objects are measured accurately enough to confirm they are round, so scientists speak of several recognized and many candidate dwarf planets. It is a good example of how classification rules follow the discoveries. Some of the most common solar system myths concern this very subject, such as the idea that Pluto was “demoted” because of its size alone.
What New Horizons found
In July 2015, NASA’s New Horizons spacecraft flew past Pluto after a journey of more than nine years. Images showed a world far more varied than expected, with mountains of water ice, wide plains of frozen nitrogen, a heart-shaped bright region now informally called Tombaugh Regio, hazy layers in a thin atmosphere and signs of ongoing geological activity. This shows that a dwarf planet is not a lesser or boring object. Pluto’s ices, such as nitrogen and methane, change between solid and gas depending on the temperature, which links to the states of matter. The flyby is also one of the milestones in the history of space exploration.
Is the debate settled?
For the IAU, yes, but not everyone is satisfied. Some planetary scientists argue that the third criterion is vague. For example, how completely must an object clear its zone? Under a strict reading, even Earth shares its orbit with thousands of near-Earth asteroids, and Jupiter shares its orbit with the Trojan asteroids, though both dominate by a huge margin. Critics have also pointed out that the final vote in 2006 was taken by a relatively small number of the astronomers attending the meeting, a fraction of the profession worldwide.
Some researchers prefer a definition based on intrinsic properties, such as being large enough to be round, which would make Pluto and many moons count as planets. Others defend the IAU approach as useful, since it separates the handful of dominant planets from a large number of smaller bodies. Both sides generally agree that the real facts about Pluto are the same regardless of the label. It is a distinct and interesting world, whatever name is attached.
Why labels matter in science
Naming categories helps scientists organize and compare objects. A planet in this system is one that formed and grew in a way that dominated its orbital zone, which is a clue to its history. A dwarf planet like Pluto is a leftover body from the outer regions where such growth stalled. The label therefore carries scientific meaning about origin, not a judgment of importance. As new objects are found and measured, definitions may continue to be discussed and refined.