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Pluto: From Planet to Dwarf Planet

Pluto was discovered in 1930 and was counted as the ninth planet for about 76 years. In 2006 the International Astronomical Union (IAU) adopted a three-part definition of a planet. Pluto fails the third part because it has not cleared its orbital neighbourhood, so it is classed as a dwarf planet. Only the label changed. NASA’s New Horizons showed Pluto to be a complex, active world.

Discovery in 1930

Pluto was found in 1930 by Clyde Tombaugh at Lowell Observatory in Arizona. He compared photographic plates of the same patch of sky taken on different nights and spotted a faint point of light that moved against the background stars. The discovery was announced on 13 March 1930. The name was suggested by Venetia Burney, an eleven-year-old schoolgirl in Oxford, and it was accepted because Pluto is the Roman god of the underworld.

Early estimates made Pluto seem much larger than it is. As observations improved, its estimated size and mass kept shrinking. We now know its diameter is about 2,377 kilometres, smaller than that of our Moon.

What we learned afterwards

In 1978, James Christy discovered Pluto’s largest moon, Charon. It is about half the diameter of Pluto, so the two are often described as a double system. Their shared centre of mass lies outside Pluto. Four smaller moons were found later: Nix, Hydra, Kerberos and Styx.

Pluto follows an elongated, tilted orbit and takes about 248 Earth years to circle the Sun. At times it is closer to the Sun than Neptune, yet the two never collide, because Pluto completes two orbits for every three of Neptune’s (a 3:2 orbital resonance). You can read about Neptune in Neptune.

From 1992 onwards, astronomers began finding other icy bodies beyond Neptune, in a region called the Kuiper Belt. Pluto turned out not to be alone, but the brightest member of a large population of small icy worlds. In 2005 the discovery of Eris was announced, an object similar in size to Pluto and slightly more massive. The question “what is a planet?” could no longer be set aside.

The IAU decision of 2006

On 24 August 2006, at its General Assembly in Prague, the International Astronomical Union (IAU) adopted a resolution containing the first official definition of the word “planet”. Under it, Pluto is a “dwarf planet”, a category introduced by the same resolution. The IAU now recognises five: Ceres, Pluto, Haumea, Makemake and Eris.

It is worth stressing that this was a decision about classification. Pluto itself did not change; the definition astronomers use did. For more on this question see Why Pluto is not a planet and Pluto: planet or not.

The three criteria for a planet

Under the IAU definition of 2006, a celestial body is a planet when it:

1. Orbits the Sun.
2. Has enough mass to be nearly round. Its own gravity overcomes the rigidity of its material and pulls it into hydrostatic equilibrium.
3. Has cleared the neighbourhood around its orbit. It dominates its orbital zone so that no other comparable bodies share it, apart from its own moons.

Pluto meets the first two criteria but not the third. It shares its orbital region with many other Kuiper Belt objects. These criteria separate the eight planets, Mercury to Neptune, from smaller bodies. Bodies that meet the first two criteria but not the third are dwarf planets. For related small bodies see Dwarf planets and Comets.

The definition is not universally loved. Some planetary scientists prefer a classification based only on a body’s intrinsic properties, such as its shape and geological activity. The scientific debate goes on, but the IAU classification remains the official one.

What New Horizons revealed

NASA’s New Horizons spacecraft launched on 19 January 2006. On 14 July 2015 it flew past Pluto at a distance of roughly 12,500 kilometres and returned the first close-up images of the system.

The key discoveries:

• A heart-shaped region. A bright feature, named Tombaugh Regio in honour of the discoverer, has as its western lobe Sputnik Planitia, a vast plain of nitrogen ice.
• A young, active surface. Sputnik Planitia has almost no craters, which shows that its surface is geologically young and that the ice there is renewed by slow motion.
• Mountains of water ice. At Pluto’s very low temperatures, around minus 230 °C, water ice is as hard as rock and forms tall mountains.
• A thin atmosphere. It consists mainly of nitrogen and has several layers of bluish haze.
• A complex moon system. Data on Charon and the small moons showed that they too have rich histories and varied terrain.

After Pluto, New Horizons continued into the Kuiper Belt. On 1 January 2019 it flew past the object Arrokoth, one of the most distant bodies ever explored up close by a spacecraft.

Why Pluto is still fascinating

Changing the category did not reduce Pluto’s scientific value. As the best-studied body in the Kuiper Belt, it helps scientists understand how the outer parts of the Solar System formed. Astronomy is a science that revises its classifications when new data and better definitions arrive.

If you are curious how Pluto is interpreted in the cultural tradition of astrology, which is a separate subject and not part of astronomy, see Pluto in astrology. The difference between the two fields is explained in Astronomy versus astrology.

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