But what happened back in 2006 anyway? Why after more than three quarters of a century did scientists suddenly decide that Pluto should be kicked out of the club? It’s an interesting story that has as much to do with Pluto itself as it does with our understanding of what words mean and what cool stuff is floating around in our solar system. In the end, the more we learn, the harder it is to sort out all the information.
Pluto was discovered by American astronomer Clyde W. Tombaugh on Feb 18, 1930 and it was immediately evident that something weird was going on. For a long time astronomers has been searching the sky for an elusive ninth planet, but Pluto wasn’t at all what they were expecting. The thing about the solar system is, before 1930, it kind of made sense. At the heart of the show was the sun: the massive fiery ball of gas that we all know and love. Moving outwards from the centre we had a quartet of small, rocky planets (Mercury, Venus, Earth, and Mars); followed by a rocky interlude known as the asteroid belt; then four ginormous planets made mostly of gas (Jupiter, Saturn, Uranus, and Neptune). There was a nice symmetry to things and scientists were fairly confident that any other planets they found in the outer reaches of the solar system would also be ginormous and gasy.
Pluto is neither of those things. First, it is small. So small that even spotting it was kind of a miracle. Pluto is 2,370 km (1,473 miles) in diameter, making it about two thirds the size of the moon (3,476 km) and less than half the size of the next smallest planet, Mercury (4,878 km). Second, Pluto is not gaseous. It is made of rock, ice and snow. On top of that though, there is lots of other weird things about Pluto. While the rest of the planets orbit the sun on a relatively flat plane, Pluto’s orbit is rakishly tilted 17-degrees, like a 20's-era mobster’s hat. Its orbit also crosses Neptune’s. Fortunately, due to the effects of their gravity on one another, they will never collide.
As small and weird as it may be, Pluto was happily admitted as the solar systems ninth planet. And so it remained until we had some things to compare it to. Actually, that isn’t entirely true. We have long known about another object in the solar system that poses problems for Pluto’s planet status. The object is Ceres, and though it doesn’t show up on any classroom diagrams, it sits in between Mars and Jupiter, happily going about its business. But Ceres is only 950 km (590 miles) in diameter, so even after discovering Pluto we were happy to label it as just the largest asteroid in the belt.
Pluto’s problems really began on January 5, 2005. Leading up to that date astronomers had been discovering objects that were similar to but smaller than Pluto out in the far reaches of the solar system. But is wasn’t until one astronomer named Mike Brown, author of the surprisingly thrilling and wonderfully titled How I Killed Pluto and Why It Had It Coming, discovered an object called Eris that the world had to stop and ask “Okay, so what they hell is Pluto?”
The problem with Eris is that it appeared to be bigger than Pluto. We now know that Eris is a hair smaller at 2,340 km (1,454) across, but it is still basically Pluto’s twin. It orbits outside of Pluto, at a rakish angle to the rest of the solar system and generally infringes on everything that once made Pluto special. The trouble was, if we called them both planets, there would be very little stopping us from having to include the hundreds or thousands of other objects in the outer solar system (the Kuiper Belt) as planets.
That simply isn’t practical, so astronomers sat down and finally defined what the word “planet” meant. Here’s what they came up with:
“A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood [sic] around its orbit.”
The upshot is that since both Pluto and Ceres cross the orbits of other objects, they can’t be called planets. Instead they are “Dwarf Planets”:
“A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood [sic] around its orbit, and (d) is not a satellite.”
There are five officially recognized dwarf planets in the solar system: Pluto, Eris, Ceres (upgraded from ‘asteroid’), Haumea, and Makemake. So although not being able to call Pluto a planet anymore seems to make the solar system a slightly lonelier place, in reality, we’ve gained a lot more company.