what is a planet anyway? astronomers campaign to avenge pluto
Let’s travel back to the late 2000s, to carefree days when the biggest acrimony in the popular science world was Pluto’s demotion to a Kuiper Belt object. While the decision was made by an international scientific body, not everyone agreed with it, and over the years, numerous teams of astronomers and planetary scientists challenged the vote and official definition of what is a planet and whether that definition makes sense. And now, not only is yet another group ready to keep the debate going, they’re arguing that plenty of other objects in the solar system could, and should be considered planets as well, and a critical criterion for what constitutes a planet is poorly thought out and makes little scientific sense.
In the typical eye-catching way popular science sites cover similar thought experiments, the headlines scream that these scientists want to promote 150 other objects to planetary status, but that’s not at all what their massive paper actually wants to accomplish. Instead, it’s a deep dive into the history of astronomical taxonomy, how planets were defined over 600 years, and why this topic matters from a scientific standpoint. In their view, planets are important places where extremely important processes happen. Stars may create all the heavy elements in the universe, but planets are where those elements create geology, exotic chemistry, and even life, and understanding what object qualifies as a planet is fundamental to astronomy.
In fact, that number only appears in the study to say that astronomers used to classify asteroids as potential planets for a century and a half, and the idea of 150 planets actually comes from a completely separate statement by lead author Philip Metzger that using the original definition of a planet from the dawn of astronomy would yield close to that number of planets in the solar system because anything geologically active would qualify, including bodies that are plainly and obviously moons like Io, Titan, Triton, and Enceladus. Nowhere does he actually argue that we should return to that definition, just that the International Astronomical Union excluded Pluto without providing sound cause for the change in taxonomy.
Most scientists approve the idea that a planet should be massive enough that gravity allows is to achiever hydrostatic equilibrium — which is a very fancy way of saying it should be spherical in shape as dictated by physics — and orbit a star. What sticks in their craw is the notion that it should also be the dominant object in that orbit, which they say is meaningless and was pretty obviously tacked on to make sure Pluto, Sedna, Orcus, Ceres, and other small, spherical ice balls orbiting the sun weren’t treated as planets to keep the official list neat and tidy, so kids had to memorize just eight planets instead of 20 or more. This, researchers rightfully argue, is a really terrible reason to demote Pluto or any other stellar body.
So, with all that in mind, is there actually any consensus on what a planet should be and how it needs to be classified? Actually, yes. As we just outlined, it should be a mass large enough to be spherical and orbit the sun. We could then classify them by other aspects of their physics and chemistry, such as gas giants, terrestrial worlds, and ice or rocky dwarves, just like we classify stars based on their temperature and mass in the Hertzsprung–Russell diagram. What if kids end up having to memorize 20, or 30, or 50 planets? Well, that’s the thing, nature really does not care what kids may or may not have to memorize in school, and that’s a separate problem from correctly classifying solar system objects.
Space, physics, and chemistry don’t present us with neat little categories and boxes for how to label and study things, a lot of objects will sit on the fence between definitions we thought to be distinct, like Super-Earths and brown dwarves. Our job in exploring the cosmos is to roll with the punches and adjust our bureaucracy to reality rather than try to cram reality into what we’d prefer to be the case. This is what’s at the heart of the scientific debate about Pluto’s demotion and why it refuses to die down. If the IAU committed the mortal sin of ignoring science for the sake of tidy nomenclature, it needs to be repented as quickly as possible, with Pluto restored to its rightful place and similar worlds elevated right along with it.
See: Metzger, P., et al., (2022) Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science, Icarus, DOI: 2021.114768