Do not pay! This is long overdue anyways. I actually had this all written out in outline format a long time ago, it was just a matter of putting it into paragraphs.
As part of the S24 Charity Drive, I offered to auction off an “Astronomy Explainer”. This was the winning bid:
So, as many of you are aware, I hold the stance that Pluto is not a planet. There’s a lot of layers to this, all of it culminating in the International Astronomical Union (IAU) re-defining the then 9th planet as one of the first dwarf planets. This has been a very controversial stance, even among astronomers - and for good reason! So what I’d like to do today is go over how we got here, why the IAU decided what it did, and the implications of this decision.
The History of Pluto
Before Pluto
To get into the history of Pluto, we have to talk about the history of Neptune. Uranus had been discovered by William Herschel in 1781, and was finishing up its first orbit when astronomers noticed something strange. When studying its orbit and checking it with Newton’s Laws of Gravitation, both John Couch Adams and Urban Le Verrier saw that Uranus’ motion was being perturbed by some separate, massive object. Without knowing about the other, both predicted the existence of a planet beyond Uranus, and both calculated where it should be. However, Le Verrier made his prediction public, and so one month later on September 24, 1846, Johann Gottfried Galle and Heinrich Louis d’Arrest were able to observe Neptune within 1 degree of where Le Verrier predicted it would be.
Cartoon demonstrating how an outer object (purple) can alter the orbit of an interior planet (blue object). At point a, the outer planet gravitationally tugs on the inner planet, accelerating its motion. At point b, the outer planet now impedes the motion of the inner planet, decelerating it.
This discovery would stand as a huge victory for the predictive powers of Newton’s Laws and classical mechanics - but astronomers would not be satisfied there. Following observations of Neptune showed that it alone would not explain the perturbations in Uranus’ orbit, and in fact Neptune itself appeared to have some irregularities in its own orbit. Thus, the search for a “Planet X” began.
The Search for Planet X
Percival Lowell, professional rich man turned astronomer* founder of the Lowell Observatory in Flagstaff Arizona, became convinced that he could find this mysterious planet. Lowell conducted several searches for Planet X, concluding that it should have a mass of approximately 7 Earth masses and should be observable with the telescopes of the time.
*Much of Lowell’s inspiration for finding Pluto may have arisen from his incorrect prediction of canals on Mars that were built by an intelligent civilization. Like I said, this guy started off as a businessman and author, not an astronomer, and was itching to get recognition as a credible scientist.
William Pickering, one of the other founders of the Lowell Observatory and director of the Harvard Observatory, also claimed to find evidence of Planet X (or as he called it, Planet O because haha get it, O comes after N?) and even claimed there were several other trans-Neptunian planets past it. This prediction bombed hard.
Upon Lowell’s death in 1916, internal politics and a general lack of funds stopped any search for Pluto at Lowell Observatory. It wouldn’t be until 1929, after the building of a new, 13 inch telescope, that 22 year old Clyde Tombaugh would be given the job of searching for Planet X. Tombaugh would essentially compare images of the sky taken on separate days, and look to see if any objects had moved relative to the background field of stars. Comparing these images to the known positions of the planets (which too moved relative to the stars) would allow him to look for any new objects.
After a year of searching, Tombaugh finally noticed the needle in the haystack.
It’s hard to see in this image, but the arrows point to where Pluto was on two separate days in 1930. These are original images from Clyde Tombaugh’s discovery.
Things are not as they seem
It didn’t take very long for things to unravel for Pluto. Within two years of its discovery, its estimated mass was decreased from 7 Earth masses to 1 based on measurements of its very low albedo (it’s reflectiveness). Seventeen years later Gerard Kuiper estimated it to be 0.1 Earth masses, and we now know it to be about 0.00218 Earth masses. The original Planet X hypothesis - which required Pluto’s mass to alter the orbits of Uranus and Neptune - was falling apart at the seams. It turned out that Pluto also had a wildly elliptical orbit, and even spends a long portion of its orbit closer to the Sun than Neptune does. Furthermore, further observations put the entire Planet X hypothesis to bed: calculations using more accurate formulas showed that discrepancies in Uranus’ orbit could not be caused by a different planet, and Voyager 2’s observations of Neptune found that the blue planet’s mass was about 1 Mars mass smaller than expected.
The Demotion
Things were never clear for Pluto from the start. Not even a month after its discovery, many other astronomers were casting doubt on the importance of Pluto and whether it deserved to be a planet at all. In the New York Times, astronomer Armin O. Leuschner wrote:
Harlow Shapley, director of the Harvard Observatory, also wrote in the same article:
(This is called foreshadowing.)
Other “planets” have lost their planetary status before, most notably the giant asteroid Ceres. Ceres, the largest asteroid in the Asteroid Belt, was classified as a planet at its discovery in 1801. The next year, the asteroid Pallas was discovered in the same orbital area. Pretty soon things got out of hand - and astronomers quickly realized that Ceres, Pallas, and their friends were part of a different type of astronomical object. They reclassified the objects as asteroids and moved on.
Anyways, fast-forward past the discovery of Pluto to 2005 and the discovery of Eris, another high-eccentricity trans-Neptunian planetoid. Suddenly, astronomers realized that they were in the same predicament as their predecessors 200 years ago. The IAU quickly convened to try and re-define planets, coming up with this definition:
This was extremely unpopular, especially with Eris’ discoverer Mike Brown. The definition proposed by the IAU considered Eris, Pluto, Ceres, and even Pluto’s moon Charon as planets, upping the Solar System to 12 planets. Brown, however, said that this definition meant there were over 50 planets in the Solar System, and as such it was not a workable definition.
After some arguments, elections, and other shenanigans, the IAU came to a settled definition:
Pluto does not control its orbit - the most notable example invading Pluto’s space being the behemoth that is Neptune. Eris and Ceres also failed to clear their neighborhood, and so were disqualified from being called a planet.
These three, fulfilling only criteria 1 and 2, were to be named dwarf planets. Haumea and Makemake were later added as dwarf planets, and there are many more planet-like objects that are fulfil the criteria but are not “officially recognized”.
The Debate
This was a very unpopular decision for a variety of reasons. First of all, the definition proposed and accepted by the IAU still felt very vague. What does “clearing the neighborhood” really mean, after all? Sure, Jupiter thoroughly dominates its orbit mass-wise, but there are tons of asteroids hanging out in its orbit (the Trojans, for example). How do moons interact with this definition? Charon is about 12% the mass of Pluto, and so they revolve around a barycenter outside of Pluto. Does Charon get to be a dwarf planet? Finally, the IAU vote only made up about 5% of professional astronomers and astrophysicists. Who put these people in charge of what is a planet and what isn’t? Many professional astronomers also disagree with the definition, and they did not get a voice.
But we all know why people really get worked up about Pluto, and it’s not their vast knowledge of the intricacies of Solar System dynamics. It’s because it makes them FEEL angry. The emotional connection people made to Pluto enlarged the issue from just a silly scientific classification debate to a personal attack on the populace. After all, we all grew up learning there were 9 planets. Why should we stop? Why can’t we just add a 10th planet? People started to see Pluto in themselves - it was being kicked out of the club because it was small and did not fit in. No one likes it when that happens to them, and so many people humanized Pluto and felt that the action was somehow unfair to a cold, dead rock in the depths of the Solar System (and it doesn’t help that we grew up watching an animated dog with the same name). Many adults and school children began writing angry letters to astronomers who publicly stated they would strip Pluto of its planetary status. When the American Museum of Natural History removed Pluto from their list of planets, director Neil deGrasse Tyson recalled getting hate mail from several third graders.
The wildest thing I came across was the purely American reaction to this. Pluto was America’s only planet, being discovered by our American patriot Clyde Tombaugh. Suddenly scientists and other countries want to take that source of pride away? They won’t take our Hummers and they sure won’t take our Pluto. California - the state that literally employs Mike Brown - called the IAU’s decision a “scientific heresy”. Illinois, where Tombaugh was born, and New Mexico, where Tombaugh lived, said “screw you” and declared Pluto to be legally a planet whenever it passed over their skies. Americans took this personally, much more than any other country had.
To circle back to technical details, though, let’s look into the details as to why Pluto should or shouldn’t be a planet.
Why should Pluto be a planet?
Let me start by saying this is probably an unhelpful way of framing the question. Really - we should be asking why is the definition of planet so narrow?
The Trojan asteroids exist, Earth, Mars, and Neptune also have Lagrange point asteroids. Do we consider the existence of those asteroids as violation of criterion 3?
In my opinion - No! Those asteroids are only there because the gravitational relationship between the planet and the Sun means there’s a nice equilibrium point where they are.
How about moons? Criterion 1 clearly states it has to orbit the Sun - BUT this is not a robust definition in my opinion. Pluto actually also orbits a common center of mass with its largest moon Charon. If we disallow moons from being planets/dwarf planets because of this, why does Pluto even get to be a dwarf planet? Also our Moon is freaking huge. Jupiter’s moon Ganymede is LARGER THAN MERCURY. Callisto and Io are also larger and more massive than Pluto! Where is the consistency? Why should terrestrial planets even be considered planets anyways?
But besides the definition being stinky, why should Pluto be a planet?
Why shouldn’t Pluto be a planet?
Okay, so those are fairly compelling reasons to keep Pluto as a planet. However, I still think the points going for its demotion are much more scientifically rigorous.
Concluding Remarks
There are a few key points to go over here.
First of all, I don’t believe that anyone (even those who supported the demotion) likes the IAU definition. It is vague, not very rigorous, and has possibly left us more confused than before.
However, Pluto is very obviously distinct from the other 8 planets - the question is how do we classify it and other objects like it. Do we work from the ground up and create new classification systems? The terrestrial planets and gas giants are significantly different from each other anyways. Neil deGrasse Tyson commented:
Regardless, a lot of the debate really comes from the general population thinking Pluto is being slighted. I think this is silly - it’s a rock in space, it has no feelings (heart or not). Science should not care about how people think the anthropomorphized versions of objects feel, and even if we did care, it fits in a lot better with its other large TNO friends. Pluto would practically be the king of the TNOs.
The demotion also does not mean that we are less scientifically interested in Pluto and its friends, which was proven by the excitement over the New Horizons data release. If anything, it makes them more interesting because we now understand them as a different type of object, and it will be important to Solar System history to understand how they form with respect to gas giants and other objects further out. How does the hypothesized Planet Nine play into this? We really don’t know.
Ultimately it doesn’t actually matter! It’s a silly fight, just like how biologists fight over whether to classify viruses as “life” or not - they still exist and keep on kicking regardless of what we call them. Our understanding of Pluto is not drastically different if we call it a planet or a dwarf planet.
That being said, Pluto is not a planet.
As part of the S24 Charity Drive, I offered to auction off an “Astronomy Explainer”. This was the winning bid:
woelkers Wrote:I want to hear about the debate surrounding its planetary status, hear if there's any sensible arguments for why it still should be a planet, and understand the implications of the decision both ways.
So, as many of you are aware, I hold the stance that Pluto is not a planet. There’s a lot of layers to this, all of it culminating in the International Astronomical Union (IAU) re-defining the then 9th planet as one of the first dwarf planets. This has been a very controversial stance, even among astronomers - and for good reason! So what I’d like to do today is go over how we got here, why the IAU decided what it did, and the implications of this decision.
The History of Pluto
Before Pluto
To get into the history of Pluto, we have to talk about the history of Neptune. Uranus had been discovered by William Herschel in 1781, and was finishing up its first orbit when astronomers noticed something strange. When studying its orbit and checking it with Newton’s Laws of Gravitation, both John Couch Adams and Urban Le Verrier saw that Uranus’ motion was being perturbed by some separate, massive object. Without knowing about the other, both predicted the existence of a planet beyond Uranus, and both calculated where it should be. However, Le Verrier made his prediction public, and so one month later on September 24, 1846, Johann Gottfried Galle and Heinrich Louis d’Arrest were able to observe Neptune within 1 degree of where Le Verrier predicted it would be.
Cartoon demonstrating how an outer object (purple) can alter the orbit of an interior planet (blue object). At point a, the outer planet gravitationally tugs on the inner planet, accelerating its motion. At point b, the outer planet now impedes the motion of the inner planet, decelerating it.
This discovery would stand as a huge victory for the predictive powers of Newton’s Laws and classical mechanics - but astronomers would not be satisfied there. Following observations of Neptune showed that it alone would not explain the perturbations in Uranus’ orbit, and in fact Neptune itself appeared to have some irregularities in its own orbit. Thus, the search for a “Planet X” began.
The Search for Planet X
Percival Lowell, professional rich man turned astronomer* founder of the Lowell Observatory in Flagstaff Arizona, became convinced that he could find this mysterious planet. Lowell conducted several searches for Planet X, concluding that it should have a mass of approximately 7 Earth masses and should be observable with the telescopes of the time.
*Much of Lowell’s inspiration for finding Pluto may have arisen from his incorrect prediction of canals on Mars that were built by an intelligent civilization. Like I said, this guy started off as a businessman and author, not an astronomer, and was itching to get recognition as a credible scientist.
William Pickering, one of the other founders of the Lowell Observatory and director of the Harvard Observatory, also claimed to find evidence of Planet X (or as he called it, Planet O because haha get it, O comes after N?) and even claimed there were several other trans-Neptunian planets past it. This prediction bombed hard.
Upon Lowell’s death in 1916, internal politics and a general lack of funds stopped any search for Pluto at Lowell Observatory. It wouldn’t be until 1929, after the building of a new, 13 inch telescope, that 22 year old Clyde Tombaugh would be given the job of searching for Planet X. Tombaugh would essentially compare images of the sky taken on separate days, and look to see if any objects had moved relative to the background field of stars. Comparing these images to the known positions of the planets (which too moved relative to the stars) would allow him to look for any new objects.
After a year of searching, Tombaugh finally noticed the needle in the haystack.
It’s hard to see in this image, but the arrows point to where Pluto was on two separate days in 1930. These are original images from Clyde Tombaugh’s discovery.
Things are not as they seem
It didn’t take very long for things to unravel for Pluto. Within two years of its discovery, its estimated mass was decreased from 7 Earth masses to 1 based on measurements of its very low albedo (it’s reflectiveness). Seventeen years later Gerard Kuiper estimated it to be 0.1 Earth masses, and we now know it to be about 0.00218 Earth masses. The original Planet X hypothesis - which required Pluto’s mass to alter the orbits of Uranus and Neptune - was falling apart at the seams. It turned out that Pluto also had a wildly elliptical orbit, and even spends a long portion of its orbit closer to the Sun than Neptune does. Furthermore, further observations put the entire Planet X hypothesis to bed: calculations using more accurate formulas showed that discrepancies in Uranus’ orbit could not be caused by a different planet, and Voyager 2’s observations of Neptune found that the blue planet’s mass was about 1 Mars mass smaller than expected.
The Demotion
Things were never clear for Pluto from the start. Not even a month after its discovery, many other astronomers were casting doubt on the importance of Pluto and whether it deserved to be a planet at all. In the New York Times, astronomer Armin O. Leuschner wrote:
Armin O. Leuschner Wrote:"The Lowell result confirms the possible high eccentricity announced by us on April 5. Among the possibilities are a large asteroid greatly disturbed in its orbit by close approach to a major planet such as Jupiter, or it may be one of many long-period planetary objects yet to be discovered, or a bright cometary object."
Harlow Shapley, director of the Harvard Observatory, also wrote in the same article:
Harlow Shapley Wrote:"[Pluto is a] member of the Solar System not comparable with known asteroids and comets, and perhaps of greater importance to cosmogony than would be another major planet beyond Neptune."
(This is called foreshadowing.)
Other “planets” have lost their planetary status before, most notably the giant asteroid Ceres. Ceres, the largest asteroid in the Asteroid Belt, was classified as a planet at its discovery in 1801. The next year, the asteroid Pallas was discovered in the same orbital area. Pretty soon things got out of hand - and astronomers quickly realized that Ceres, Pallas, and their friends were part of a different type of astronomical object. They reclassified the objects as asteroids and moved on.
Anyways, fast-forward past the discovery of Pluto to 2005 and the discovery of Eris, another high-eccentricity trans-Neptunian planetoid. Suddenly, astronomers realized that they were in the same predicament as their predecessors 200 years ago. The IAU quickly convened to try and re-define planets, coming up with this definition:
IAU Wrote:"A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."
This was extremely unpopular, especially with Eris’ discoverer Mike Brown. The definition proposed by the IAU considered Eris, Pluto, Ceres, and even Pluto’s moon Charon as planets, upping the Solar System to 12 planets. Brown, however, said that this definition meant there were over 50 planets in the Solar System, and as such it was not a workable definition.
After some arguments, elections, and other shenanigans, the IAU came to a settled definition:
IAU Wrote:
- The object must be in orbit around the Sun.
- The object must be massive enough to be rounded by its own gravity. More specifically, its own gravity should pull it into a shape defined by hydrostatic equilibrium.
- It must have cleared the neighborhood around its orbit.
Pluto does not control its orbit - the most notable example invading Pluto’s space being the behemoth that is Neptune. Eris and Ceres also failed to clear their neighborhood, and so were disqualified from being called a planet.
These three, fulfilling only criteria 1 and 2, were to be named dwarf planets. Haumea and Makemake were later added as dwarf planets, and there are many more planet-like objects that are fulfil the criteria but are not “officially recognized”.
The Debate
This was a very unpopular decision for a variety of reasons. First of all, the definition proposed and accepted by the IAU still felt very vague. What does “clearing the neighborhood” really mean, after all? Sure, Jupiter thoroughly dominates its orbit mass-wise, but there are tons of asteroids hanging out in its orbit (the Trojans, for example). How do moons interact with this definition? Charon is about 12% the mass of Pluto, and so they revolve around a barycenter outside of Pluto. Does Charon get to be a dwarf planet? Finally, the IAU vote only made up about 5% of professional astronomers and astrophysicists. Who put these people in charge of what is a planet and what isn’t? Many professional astronomers also disagree with the definition, and they did not get a voice.
But we all know why people really get worked up about Pluto, and it’s not their vast knowledge of the intricacies of Solar System dynamics. It’s because it makes them FEEL angry. The emotional connection people made to Pluto enlarged the issue from just a silly scientific classification debate to a personal attack on the populace. After all, we all grew up learning there were 9 planets. Why should we stop? Why can’t we just add a 10th planet? People started to see Pluto in themselves - it was being kicked out of the club because it was small and did not fit in. No one likes it when that happens to them, and so many people humanized Pluto and felt that the action was somehow unfair to a cold, dead rock in the depths of the Solar System (and it doesn’t help that we grew up watching an animated dog with the same name). Many adults and school children began writing angry letters to astronomers who publicly stated they would strip Pluto of its planetary status. When the American Museum of Natural History removed Pluto from their list of planets, director Neil deGrasse Tyson recalled getting hate mail from several third graders.
The wildest thing I came across was the purely American reaction to this. Pluto was America’s only planet, being discovered by our American patriot Clyde Tombaugh. Suddenly scientists and other countries want to take that source of pride away? They won’t take our Hummers and they sure won’t take our Pluto. California - the state that literally employs Mike Brown - called the IAU’s decision a “scientific heresy”. Illinois, where Tombaugh was born, and New Mexico, where Tombaugh lived, said “screw you” and declared Pluto to be legally a planet whenever it passed over their skies. Americans took this personally, much more than any other country had.
To circle back to technical details, though, let’s look into the details as to why Pluto should or shouldn’t be a planet.
Why should Pluto be a planet?
Let me start by saying this is probably an unhelpful way of framing the question. Really - we should be asking why is the definition of planet so narrow?
The Trojan asteroids exist, Earth, Mars, and Neptune also have Lagrange point asteroids. Do we consider the existence of those asteroids as violation of criterion 3?
In my opinion - No! Those asteroids are only there because the gravitational relationship between the planet and the Sun means there’s a nice equilibrium point where they are.
How about moons? Criterion 1 clearly states it has to orbit the Sun - BUT this is not a robust definition in my opinion. Pluto actually also orbits a common center of mass with its largest moon Charon. If we disallow moons from being planets/dwarf planets because of this, why does Pluto even get to be a dwarf planet? Also our Moon is freaking huge. Jupiter’s moon Ganymede is LARGER THAN MERCURY. Callisto and Io are also larger and more massive than Pluto! Where is the consistency? Why should terrestrial planets even be considered planets anyways?
But besides the definition being stinky, why should Pluto be a planet?
- Pluto is still a large object in an orbit that is (mostly) unique. Other TNOs have much more distant orbits that make them harder to justify as planets - for example, Eris is twice as far from the Sun as Pluto.
- Despite the fact that Pluto is small, it still has FIVE MOONS. That’s more than the Earth, Mars, Mercury, and Venus combined. Granted, 3 of them are super tiny, but they are still real moons!
- Pluto is extremely scientifically interesting. New Horizons revealed a ton of information about it (even if the same MURICA #1 politicians above thought sending a probe to Pluto was a waste of money), such as it having geologic activity, possibly having liquid water, and it’s iconic heart being the biggest glacier in the solar system.
Why shouldn’t Pluto be a planet?
Okay, so those are fairly compelling reasons to keep Pluto as a planet. However, I still think the points going for its demotion are much more scientifically rigorous.
- Pluto is tiny. It is so much smaller than any of the other 8 planets, and so much less massive. It is less massive than several moons, and it isn’t even massive enough to control its own moon!
- The orbit is distinctly different from the other 8 planets - it’s off the plane of most of the orbits and is much more eccentric than the others - cutting into Neptune’s orbit. While an elliptical object shouldn’t necessarily preclude Pluto from being a planet, it is clear that it doesn’t quite fit in with the rest.
- Pluto fits in much more nicely as a trans-Neptunian Object (TNO) than it does as a planet. And it isn’t even the biggest TNO - Eris is. If we include Pluto then we have to include TNOs such as Eris, Sedna, and Makemake. Depending on the definition we choose, we could have to include dozens of spherical objects! Do we want to include TNOs as their own planetary category? The IAU has given these objects the name “plutoid” as a seeming compromise with those who want to keep Pluto a planet.
Concluding Remarks
There are a few key points to go over here.
First of all, I don’t believe that anyone (even those who supported the demotion) likes the IAU definition. It is vague, not very rigorous, and has possibly left us more confused than before.
However, Pluto is very obviously distinct from the other 8 planets - the question is how do we classify it and other objects like it. Do we work from the ground up and create new classification systems? The terrestrial planets and gas giants are significantly different from each other anyways. Neil deGrasse Tyson commented:
Neil deGrasse Tyson Wrote:"The word 'planet' has surely outlived its usefulness. The time has come for us to create a fresh and sensible classification scheme from the ground up -- one that applies to all objects of our own solar system, yet is flexible enough to embrace newly discovered objects elsewhere in the galaxy. Other fields, such as biology, and even subfields of astrophysics that study stars and galaxies, have strong needs to classify objects and have solved this problem long ago. It's time for the community of planetary scientists to do the same. We should not 'agree to disagree, we should 'agree to converge'."
Regardless, a lot of the debate really comes from the general population thinking Pluto is being slighted. I think this is silly - it’s a rock in space, it has no feelings (heart or not). Science should not care about how people think the anthropomorphized versions of objects feel, and even if we did care, it fits in a lot better with its other large TNO friends. Pluto would practically be the king of the TNOs.
The demotion also does not mean that we are less scientifically interested in Pluto and its friends, which was proven by the excitement over the New Horizons data release. If anything, it makes them more interesting because we now understand them as a different type of object, and it will be important to Solar System history to understand how they form with respect to gas giants and other objects further out. How does the hypothesized Planet Nine play into this? We really don’t know.
Ultimately it doesn’t actually matter! It’s a silly fight, just like how biologists fight over whether to classify viruses as “life” or not - they still exist and keep on kicking regardless of what we call them. Our understanding of Pluto is not drastically different if we call it a planet or a dwarf planet.
That being said, Pluto is not a planet.