Pluto has captured the hearts and minds of many people. Even now, nearly 15 years after it became a dwarf planet, people still ask why that had to happen and what it actually means.
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In 2006, the International Astronomical Union (the global organisation of astronomers who decide how astronomical objects should be named, among other things) was faced with a dilemma. Other planets similar to Pluto and around the same distance from the Sun as Pluto had been discovered, along with smaller objects, and it was going to get too messy to add all of these small planets to the list of planets in the Solar System.
To solve this problem, the International Astronomical Union came up with three rules that an object needs to follow to be classified as a planet.
The first is that a planet must orbit a star. This works for the planets in our Solar System since they orbit our Sun, and also works for planets in other solar systems that orbit their stars. This rule helps to remove moons from the list of potential planets because they orbit a planet and not a star. Pluto obeys this because it orbits our Sun.
The second rule is that the planet must be large enough to be spherical, due to its own gravity. This means it has to be heavy enough to form a ball and not be some kind of strange blobby potato shape. This rule removes almost all asteroids from being a planet, except for Ceres, which is spherical. Pluto also ticks this box, meaning that it passes the first two rules.
The third and final rule is that a planet must have cleared its orbit. This means that to be classified as a planet, the planet must be the biggest thing in its orbit. This is where Pluto and other objects like it run into trouble. Pluto's orbit crosses over Neptune's, which means that Pluto is not the biggest thing in its orbit. Pluto's orbit is also interesting because it is at a different angle to the rest of the planets in our Solar System.
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Since Pluto didn't pass the third rule, it was no longer classified as a planet, but it did have a whole new category made for it. Pluto is what we now call a 'dwarf planet', and it has quite a few friends in its new class, including Ceres, and some of those objects that were being found near Pluto's orbit, the culprits that led to this discussion.
Some people still think that the most important thing about Pluto is whether or not it should be a planet. However, thanks to the New Horizons spacecraft, we know that is not the case. New Horizons was a mission to investigate Pluto and other objects beyond the orbit of Neptune, and the images it returned of Pluto were stunning. Pluto is an active world with mountains, canyons, glaciers, a subsurface ocean and even a weather cycle, so don't let anyone try to convince you that dwarf planets aren't just as interesting as all the other planets! So instead of worrying about what we call Pluto, focus on the cool mysteries it holds.
- Eloise Birchall is a PhD student in Astrophysics at the Research School of Astronomy and Astrophysics at the Australian National University.
