Is Pluto a Planet (Script) (Patreon)

You know what a planet is, right? A big round thing that orbits a star. Uh, not so fast. The surprisingly vicious debate over the planetary status of Pluto has given us a fascinating glimpse into what a scientific definition really is. And perhaps the word planet is too vague to be used as a scientific definition at all.

We love to classify things. Labels help us keep stuff organized in our heads. In science, categorization provides a fast and easy way to know the properties of a member of the group just by knowing what group it belongs to. Chemists group elements on the periodic table, those groups exhibit similar chemical behavior that reflect outer-shell electron number. Biologists group organisms by similar physical characteristics, and this taxonomy reflects genetic relationships. Astronomers are all about space taxonomy. We classify galaxies based on their shape, black holes based on how they feed, stars based on their colour and brightness, and planets by… well, by a set of criteria that has caused more tension and heartbreak than any made-up grouping scheme really should. Because a change in that scheme demoted Pluto from planet to not-planet. Today we’re going to settle whether this was reasonable, and whether we should keep the word “planet” at all.

The definition of “planet” has changed a lot. If you were an ancient astronomer like say, Ptolemy, the planets were the asteres planetai, the wandering stars. These included Mercury, Venus, Mars, Jupiter, and Saturn, but also the sun and the moon - basically anything that moved relative to the background stars. Note that this does NOT include the Earth. This definition of “planet” was the most sensible classification for thousands of years based on our observations and understanding of the universe. But understanding improves.

In 1543 Nicolaus Copernicus published “On the Revolutions of the Heavenly Spheres” which cast the Sun, not the Earth, as the center of the universe and the Earth in its proper place among the planets. This picture was cleaned up by the observations of Galileo and Brahe and the mathematical models of Kepler and Newton. Newton’s Laws of Gravity led to consistent predictability of the motion of the heavenly bodies. The solar system finally made observational and theoretical sense: there were now 6 planets orbiting the sun in perfect mathematical harmony. But new discoveries were about to mess up this beautifully simple picture.

See that’s the thing about science. No so-called fact is ever perfectly safe. Everything is subject to revision if evidence turns against it. That’s the beauty of the scientific process. Keep this in mind for when we get to Pluto.

New wandering stars were discovered in the centuries following Newton. Uranus had been spotted many times throughout history, but was only identified as a planet after William Herschel recorded its movement in 1781. That same motion almost perfectly reflected the clockwork predictions of Newtonian mechanics. "Almost". Slight deviations in Uranus's orbit betrayed the existence of Neptune, which was discovered first in the mathematics and then with a telescope in 1846. Between these two discoveries, four other bodies joined the planets. At the beginning of the 1800s Vesta, Juno, Ceres and Pallas were all spotted between the orbits of Mars and Jupiter and classified as planets. But astronomers kept finding more and more objects in that vicinity, and eventually realized that a new group was needed in our taxonomy. So the class of asteroids came into being.

You might argue that Ceres, Juno, Pallas, and Vesta were unfairly demoted from planet. After all, Ceres is 20% the diameter of Mercury, so it’s in the planetary ballpark. But where do you draw the line? How many asteroids should we then classify as planets? Or do we drop Mercury, which is, after all, only 50% larger than our own moon and smaller than Saturn’s moon Titan. Our taxonomy had to evolve as our understanding of the solar system grew, and so the term planet was reserved for the now-familiar 8 - Mercury through Neptune, although the definition of the word was still vague.

This is where Pluto finally comes into the story. Even with the discovery of Neptune, the orbit of Uranus still appeared a bit off - at least in some calculations. So began a feverish search for yet another planet to resolve this discrepancy. The search for the so-called Planet X became the grail quest of Percival Lowell, businessman turned astronomer, and he built the Lowell Observatory in Arizona with that singular goal. In 1930, 14 years after Lowell’s death, but still powered by his observatory and his family’s fortune, Planet X was finally discovered. Or so we thought. Clyde Tombaugh, a 22 year old Kansas farm boy, spotted a moving speck of light in a series of photographic plates taken under the guidance of Vesto Slipher, Lowell observatory’s director. Orbital calculations put the object beyond the orbit of Neptune, and it appeared within 6 degrees of one of Slipher’s predictions for the location of the mysterious planet X.

Astronomer’s were expecting a planet - and one roughly in the location of the new discovery - so perhaps it’s not surprising that Pluto was hailed as a planet without due scientific process. That’s not to say everyone agreed. The orbit of this new object was far more elliptical - stretched out - than any other planet. It also seemed too faint to possibly have the mass required to explain Uranus’s orbital discrepancies. 

By 1931, astronomers had figured out that there didn’t need to be a ninth planet to account for Uranus’s orbital discrepancies. But because Pluto was the only such object yet discovered at that distance, it kept its classification as a planet. New astronomy textbooks included distant Pluto and generations of students memorized 9 rather than 8 planets.

Fast forward several decades. With the advent of giant telescopes and digital cameras, we began to find more and more objects that muddied the definition of “planet”. In the late 80s the first brown dwarf was discovered. These giant orbs of gas aren’t massive enough to ignite nuclear fusion in their cores like a true star, but still seemed too massive to be called planets. And yet some brown dwarfs orbit other, more massive stars just like planets do. Our overly vague definition of “planet” left these brown dwarfs in taxonomical limbo.

Also through the 1990s more and more moving specks were discovered within our solar system, beyond Neptune’s orbit. They were all much smaller than Pluto, but appeared to form a belt of countless objects - what we now call the Kuiper belt - that encompasses Pluto’s orbit. Pluto became to the Kuiper belt what Ceres was to the asteroid belt. The biggest fish in the pond. Big enough to cling to its title of planet. Or so we thought.

Telescopes got bigger and our mapping of the Kuiper belt became more thorough, and in the early 2000s a number of objects in the Kuiper belt and beyond were found to be similar in size to Pluto. Quaoar, Makemake, Orcus, Sedna, Eris, and more. Eris was the last straw. It’s 28% more massive than Pluto, which spurred NASA to initially hail it as the tenth planet. But theoretical predictions suggested that we’d only seen the tip of the Kuiper belt iceberg - there must be hundreds more objects in the mass range of Pluto - and perhaps up to a couple of thousand. If we classed them all as planets, schoolchildren would need a novella-length mnemonic to remember them all.

And so, despite the anger of schoolchildren everywhere, it was in their own interests that astronomers decided to act. In 2006, the International Astronomical Union, the governing body of all astronomy names, designations, and definitions, met to finally define what it meant to be a planet in our solar system. A number of definitions were debated. They eventually voted and agreed to the following. A planet must:

1. be in its own orbit around the Sun, not around another planet like a moon
2. Have sufficient mass to assume hydrostatic equilibrium, meaning be roughly spherical
3. Have "cleared the neighborhood" around its orbit.

While Pluto satisfies the first two, it doesn’t meet the third. “Planet” got redefined to something Pluto just isn’t. In this same process the IAU created an entire new class of object - dwarf planet - an object which has its own orbit and is spherical-ish, but is not massive enough to clear its orbit. Pluto, Eris, Ceres, Haumea, and Makemake were all promoted to dwarf planet, and many similar objects will no doubt follow.

While this definition was meant to settle the debate, some still argue that Pluto should be grandfathered in as a planet. Even the current NASA Administrator, Jim Bridenstine, is declaring Pluto a planet once more saying “it’s the way I learned it.” Well, political appointees declare lots of things and not everything we learn as kids is true. Fortunately the actual scientific process is more rigorous than that. Scientific definitions require careful thought, precision, and broad, expert consensus, and are subject to revision. Sure, we could add a fourth requirement to the definition of planet, like: 4: ignore all of the above if changing things makes people sad. But because we are all curious and open-minded scientists here, perhaps it would be more fun to think about WHY this reclassification is so contentious. Why all the sentimentality?

The reclassification from planet to dwarf planet DOES seem like a demotion. Look, Pluto - I’m afraid you haven’t cleared your orbit of debris this quarter. We’re going to need to reevaluate your role in this taxonomy. Don’t think of this as a demotion - it’s more of a ... reincentivizing horizontal pivot.

That conversation would make anyone cry. But let me assure you - the 13 sextillion kg ball of rock and ice that is Pluto doesn’t have strong opinions about its own taxonomic status among a few classification-crazy bipeds several planetoids away. We perceive this reassignment as a demotion because we anthropomorphize everything. But with such powerful imaginations, can’t we just reframe this? As we’ve peered deeper into our universe, we’ve realized that it’s full of weird, beautiful, and important worlds, some we now call planets, some not. For example, the moons of Jupiter and Saturn are active worlds that may prove to be the only other homes for life within our solar system. And Pluto itself proved far more interesting than “just another planet”. We already knew about planets - but Pluto’s discovery revealed the existence of the Kuiper belt, which is of fundamental importance to our understanding of how the solar system formed. Pluto went from being the least of the planets to one of the greatest of a new class of object.

And what about the term planet? Despite our attempts at improving the rigor and usefulness of the term, there’s still some ambiguity. The inner, rocky planets are very different from the outer gaseous planets. Should they receive different names? There’s no single perfectly-correct way to classify groups of objects. Taxonomy is, by nature, a somewhat arbitrary art. And the new IAU definition of planet, while being more precise, was somewhat tuned to a pre-determined desire - to eject Pluto and retain the 8 other planets. A touch arbitrary perhaps, but the new definition is scientifically useful. The eight solar system bodies currently defined as planets certainly share plenty enough in common - similarities in the way they formed and the way they behave make it *useful* to categorize them under a single label. And like any aspect of the scientific process it’s subject to revision. Maybe at some point in the future, as we learn more about how different worlds form, astronomers will change the definition of planet again. The language we use to describe the universe becomes more precise as we learn about its nature. And anyway, the word “world” still applies to Pluto - a rather more poetic label for one of the greatest dwarf planets in known spacetime.