Why Is Pluto Not a Planet? The 2006 Decision Explained

For 76 years, every schoolchild learned that the solar system had nine planets, with little Pluto at the edge. Then, in August 2006, astronomers voted, and Pluto was out. Many people were upset. But the decision was not about Pluto changing; it was about astronomers finally deciding what a planet is.

Before 2006, there was no definition

Surprisingly, until 2006 there was no official scientific definition of the word “planet”. The ancient Greek word simply meant “wanderer”: a light that moves across the sky against the fixed stars. When Clyde Tombaugh discovered Pluto in 1930, it was called the ninth planet more or less automatically.

Over time, Pluto began to look odd. It is tiny, smaller than our Moon. Its orbit is tilted and so stretched that it sometimes comes closer to the Sun than Neptune. And from 1992 onwards, astronomers found more and more icy objects beyond Neptune, in a region called the Kuiper Belt. Pluto turned out to be one of the largest members of this crowd, not a lone world.

The trigger: Eris

In 2005, a team led by Mike Brown discovered Eris, a distant icy world roughly the same size as Pluto and slightly more massive. If Pluto was a planet, Eris had to be one too, and possibly dozens more would follow. Astronomers needed a rule.

The three tests

In 2006 the International Astronomical Union (IAU) decided that a planet is a body that:

1. orbits the Sun;
2. has enough mass for its own gravity to pull it into a nearly round shape;
3. has cleared the neighbourhood around its orbit, meaning it is by far the dominant object in its zone.

Pluto passes the first two. But it fails the third: it shares its orbital zone with countless Kuiper Belt objects and its mass is tiny compared with the total of everything around it. Objects that pass the first two tests but not the third are called dwarf planets. Today the IAU recognises five: Ceres, Pluto, Eris, Haumea and Makemake.

How small is Pluto?

Pluto is about 2,377 kilometres across, smaller than our Moon and only about a fifth the width of Earth. It has five known moons; the largest, Charon, is half Pluto’s size, so large that the two orbit a point in the space between them.

A surprisingly lively world

In July 2015, NASA’s New Horizons spacecraft flew past Pluto after a nine-year journey and sent back the first close-up pictures. They showed a world far more active than expected: a huge, smooth, heart-shaped plain of nitrogen ice, mountains of water ice several kilometres high, and a thin, hazy blue atmosphere. Being a dwarf planet clearly does not mean being boring.

Still debated

Not everyone is happy with the definition. Some planetary scientists argue that any body round enough to be shaped by its own gravity should count as a planet, which would make Pluto, Eris and even some large moons planets. For now, the IAU’s 2006 definition is the official one, and Pluto remains the most famous dwarf planet.

In short

• In 2006 the IAU defined a planet for the first time.
• A planet must orbit the Sun, be round and have cleared its orbit.
• Pluto fails the third test because it shares space with the Kuiper Belt.
• It is now a dwarf planet, alongside Ceres, Eris, Haumea and Makemake.
• Pluto is about 2,377 km wide, smaller than our Moon.
• New Horizons revealed ice mountains and a heart-shaped nitrogen plain in 2015.