People still get upset about Pluto. There are T-shirts. There are mugs that say “Never Forget”, with a little sad planet on them. I’d like to argue that what happened to Pluto in 2006 is one of the best things science has ever done in public, and that it should make us happier, not sadder.
First, a confession on behalf of astronomers: for most of history, nobody had a definition of “planet”. The word comes from the Greek for “wanderer”, and it meant the bright points that moved against the stars. When telescopes showed more of them, they were added to the list, and nobody needed to write down a rule, because there were so few that you could recognise one when you saw it.
Then, in 1801, an astronomer in Sicily found a new world between Mars and Jupiter, called Ceres. It was called a planet. Within a few years, three more had been found in the same region, and they were called planets too. By the 1850s there were dozens, all small, all in the same belt, and astronomers quietly realised they were looking at a new kind of object. They called them asteroids. Ceres lost its title, nobody made a mug, and science moved on. Keep that in mind, because history was about to repeat itself.
Pluto was found in 1930 by a young man named Clyde Tombaugh, comparing photographs of the same patch of sky taken nights apart and looking for a dot that moved. It was so far away and so faint that, for years, nobody knew how big it was. Early guesses made it as big as Earth. As measurements improved, it kept shrinking. Today we know it is smaller than our own Moon.
And from the 1990s, astronomers began finding objects like Pluto out beyond Neptune. Hundreds of them, then thousands: a whole belt of icy bodies, now called the Kuiper belt, with Pluto as one of its largest members. In 2005 they found Eris, which is slightly smaller than Pluto but more massive. Now there was a real choice. Either Eris was the tenth planet, and probably dozens more would follow, or astronomers needed a rule.
The rule they agreed in 2006 has three parts. A planet must orbit the Sun. It must be massive enough for its own gravity to have pulled it into a round shape. And it must have cleared its neighbourhood, which means it must be by far the dominant thing in its orbit. That third test is not vague. You can measure it. Earth is about a million and a half times more massive than all the other bits and pieces that share its orbit. Even Mars, the weakest of the eight, outweighs everything near it thousands of times over. Pluto, by the same measure, is outweighed by the icy bodies around it. It isn’t the ruler of its neighbourhood. It’s one of the larger residents.
Pluto didn’t change in 2006. It didn’t shrink or move or lose a moon. Our understanding changed, because we had looked harder and found what was around it. That is exactly what is supposed to happen. If new evidence never changed our categories, it would mean we weren’t learning anything. The Pluto decision was the same move that made Ceres an asteroid, a scientific community saying, in public, “we know more than we did, so we’re updating our words”.
And here’s the part I like best. Losing the title didn’t make Pluto less interesting. In 2015, a spacecraft called New Horizons, after nine and a half years of travel, flew past it and sent back pictures nobody expected: mountains of water ice as high as the Rockies, a vast, smooth, heart-shaped plain of frozen nitrogen, a thin blue haze. A dwarf planet, it turns out, can be one of the most surprising worlds we have ever seen.
So, no, Pluto isn’t sad. It’s doing fine. It’s the best-known dwarf planet in the solar system, with five moons and a heart on its surface. If anyone should be upset, it’s Ceres, which went through all of this a hundred and fifty years earlier, and never got a single T-shirt.