Should Pluto Still Be a Planet? | Unveiled

Should Pluto Still Be a Planet?

Despite ideas on astronomy being around since at least Ancient Greece, we’ve only settled on an even close to firm definition of “what a planet is” quite recently - in 2006. It was then that Pluto, once thought of as “the ninth planet”, was stripped of its status and demoted to dwarf planet. But was the change warranted?

This is Unveiled and today we’re answering the extraordinary question; Should Pluto still be a planet?

Pluto was discovered in 1930 and was named after the Classical ruler of the underworld. At the time of its discovery, scientists labelled it the ninth planet in our solar system, and that title suck for seventy-five years. That Pluto was a planet in line with Earth, Mars, Jupiter and the rest wasn’t even called into question until 1992, when astronomers found out that there were other massive bodies around the same size as Pluto in the distant Kuiper Belt. The discovery of KBOs in general wasn’t yet enough to reclassify Pluto… but, in 2005, scientists discovered Eris, and Pluto’s fate was swiftly sealed. Eris is one of many Trans-Neptunian Objects, this time found in the Scattered Disc region of our solar system, but most importantly… it’s big. In fact, Eris is twenty-seven percent more massive than Pluto is (although Pluto looks slightly larger, because it has a bigger volume). Regardless, Eris complicated our ranking of Pluto as a planet. And, though initially there were whispers that Eris itself could assume planet status as “number ten”, the International Astronomical Union eventually decided to formally define what a planet was, which ultimately led to Pluto’s demotion. Cue the debate.

Since 2006, when the IAU’s definition of a planet was reached, it’s been a controversial topic… and not everyone agrees that the Union got it right. As it stands, the IAU lays out three primary criteria that an object must meet to classify as a planet. First, it has to orbit a star. Second, it has to be large enough for its own gravity to shape it into a sphere. And third, it has to be big enough to clear its local neighbourhood of any other objects or debris. The third criteria is what did for Pluto, because it resides in the Kuiper Belt and there are too many other icy objects around it, that it isn’t able to “clear”. The same can be said for other potential planet candidates like Eris and also Ceres which, though large, resides in amongst the asteroids of the asteroid belt.

There are various arguments against the IAU’s reclassifying of Pluto… One of the simplest being that it’s still a big object. It’s the ninth-largest object in the solar system and the tenth most massive. It’s the largest Trans-Neptunian object by volume and it holds five moons of its own - one of which is about half the size of Pluto itself. In terms of distance, it is further than the other planets from the sun by far, but at certain points in its unusual orbit Pluto is actually closer than the eighth planet is. Around twenty years of Pluto’s 248-year orbit are spent closer to us than Neptune.

But the criticism of the IAU’s definition - and its subsequent treatment of Pluto - goes deeper. For some astronomers, the rules set out in 2006 make it difficult for anything to count as a planet. For one, the first criteria - that a planet must orbit a star - seemingly discounts all rogue planets that aren’t in (or formed outside of) a star system. It works, to a point, for inside the solar system, but not for space in general. For, those like planetary scientist Alan Stern, though, the main issues rest with the third criteria - the one that Pluto itself fails on. That’s because, technically, no object even in just our solar system has managed to completely clear its local area of other debris. For Stern, the current definition also unfairly favours objects closer to the sun - since those that are further out, where there’s more debris in general, automatically have to be much larger to clear their path. If Earth were instead at Neptune’s distance, for example, even it wouldn’t qualify as a planet anymore. There’s a feeling in some circles that the IAU’s definition was devised more to limit the number of planets there could be rather than to recognise other objects with planetary status. It completely changed how we classify objects in the sky.

In that vein, arguably the main reason that Pluto was demoted in the first place was based on an error during the discovery of Eris. Eris was initially found to be larger than the one-time ninth planet, not just in terms of mass but also in terms of volume, which was what motivated scientists to reclassify it. Since those initial observations, however, we now know that Pluto actually is larger than Eris - if only very slightly. Had scientists known this from the outset, then the questions over Pluto’s credentials may never have been raised… and we might have instead been arguing for the induction of a tenth planet in Eris, and then a possible eleventh in Ceres. By now, the number of planets in the solar system could’ve been increased rather than decreased.

All of that said, it’s also true that not all astronomers disagree with the IAU ruling. For all those against, many are in favour of it. Although Pluto is a large object, it’s still tiny compared to even the next smallest planet, as Mercury is more than double its size. In fact, Pluto is so small that it’s not even as large as Earth’s moon. It’s also made mostly of ice (rather than rock, like Mercury, Venus, Earth and Mars) and has a much more elliptical orbit than other traditional planets. For those not against Pluto’s demotion to dwarf planet, it could be argued that our classifying it as a planet in the first place was done simply because we didn’t know any better at the time. Back when Pluto was discovered, we didn’t even know about the existence of the Kuiper Belt - so we couldn’t consider this new object as just one part of something bigger. In Ancient Greece, when the term “planet” was first devised, they didn’t even count Earth as one, seeing as they thought that it was at the centre of everything. Way back then, they also gave planet-status to both the sun and the moon. So, clearly, the way we categorise the solar system can, and has, changed.

And it’s arguably not all bad for Pluto as it is. While it may not be a planet according to the IAU, it is still a dwarf planet - which is a much rarer title. It only shares it with four other objects in the solar system: Eris, Ceres and two other KBOs; Haumea and Makemake. According to Neil deGrasse Tyson, the term “planet” may have far outlived its usefulness, anyway. Speaking to space.com, he said it’s much more informative to call celestial objects by more specific names - like a terrestrial planet, a gas giant, an asteroid, or a Kuiper Belt Object - as these titles convey more important details. So, although Pluto is no longer officially categorized as such, perhaps “being a planet” doesn’t matter quite as much anymore.

Regardless, the debate rages on - and Pluto will always, at least, be part of “the original nine”. Some still disagree with its demotion, but others are convinced it was the right call. In the meantime, as we continually learn and understand more about our particular place in space, we could well see the terms we use to describe the solar system change again and again, in the future. And that’s why Pluto might yet still be considered a planet.