In 2015, NASA's New Horizons spacecraft made a historic flyby of Pluto, capturing it up close for the first time. As the image data trickled back over billions of kilometres to the Earth, astronomers and the public alike fell ever deeper in love with the tiny world formerly known as ninth planet. Some sought to reignite the debate about Pluto's standing. After all, it certainly looks like a planet, and a very interesting one at that.

When this oft-posed question is turned back towards the person who asked it, the most common answer is: Pluto is too small to be a planet. While this statement does speak broadly to the reasoning behind the dwarf planet label, it is not actually correct. There is a more complex set of considerations at hand, which result in Pluto (and some other worlds) being labelled as a dwarf planet, despite the revelations of New Horizons.

In 2006 at the General Assembly of the International Astronomical Union (IAU) in Prague, astronomers from all over the world engaged in a passionate debate over a draft proposal to formally define the term ‘planet’. This might seem a rather odd thing to do, but throughout most of its long history of use, this word had no scientific definition. ‘Planet’ is derived from ‘asteres planetai’ – Greek for ‘wandering stars’. ‘Planet’ means simply ‘wanderer’ in reference to the apparent motion of the planets throughout the night sky.

In the modern world, taken strictly, this term could apply to an artificial satellite such as the International Space Station, and since the discovery of Pluto in 1930 (and particularly from the early 1990s onwards) new worlds were being found in the outer Solar System, whose size and behaviour stretched the familiar notion of a planet. They occupy a cluttered region quite distinct from the inner Solar System and Pluto, with its highly non-circular orbit, spends much of its time among them.

In 2006, the astronomers debated a definition with three requirements. Under the proposal a planet must:

Orbit the Sun
Be in hydrostatic equilibrium (a fancy way of saying close to spherical in shape)
Have cleared the neighbourhood around its orbit

In contrast, a dwarf planet is a body that satisfies the first two conditions, but fails to meet the third. Additionally, a dwarf planet cannot be a satellite of another body. Under this definition, Pluto would be labelled a dwarf planet (as would Ceres and subsequently, several other worlds.) These two proposals were both passed by votes at the 2006 IAU General Assembly. Resolution 5A cemented the definition of 'planet' with an overwhelming majority. Resolution 6A, which would ultimately determine Pluto's status as a dwarf planet, passed with a 60% majority.

Pluto does not dominate or control the material that shares its orbit, accounting for less than 10% of the total mass of its own orbital neighbourhood. Indeed, Neptune actually commands the distribution of Pluto's orbit. By comparison, the Earth is millions of times more massive than the sum total of all the debris that crosses its orbit. Pluto's diminutive size is certainly at the root of its situation - hence the label 'dwarf' - but it is a secondary effect which determines the nomenclature.

"Poor Pluto," said many at the time. The beloved ninth planet, long rumoured to have inspired the name of Walt Disney's famous canine character, was due to be downgraded. But for astronomers, a relabelling does nothing to diminish the perception of Pluto. It's not merely a fascinating world, but a system. Its large moon Charon and a scattering of smaller satellites make it truly unique. What's more, Pluto became the prototype Trans-Neptunion Object (TNO) to which other more distant neighbours are now compared. Once considered the last planet - a footnote at the end of the Solar System - Pluto is now thought of as the first discovery in a new frontier. It's a kind of ambassador from a remote, uncharted expanse far beyond the realm of the planets, where thousands of extraordinary worlds may be lurking in the darkness.

Image credits: NASA/JHUAPL/SwRI