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Is Pluto A Planet or Not?

Someone in my club recently posted to our mailing list a discussion of Pluto's demotion and some of the controversey surrounding it. I'd previously written a couple Quora articles on the subject and thought it might be worth doing a bit of an in-depth article. This is the fruit of that labor.

The controversy surrounding Pluto's "demotion" from planet status to dwarf planet is a bit of a mess. Like many, I believe the IAU definition needs work. On the other hand, I'm in the camp that doesn't think Pluto should be categorized as a planet.

It helps, I think, to look at the history.

People have been looking for more planets for centuries. The planets Mercury, Venus, Mars, Jupiter, and Saturn have been known since ancient times, and their orbits could be observed with the naked eye to astronomers and astrologers throughout history. No new planets were found, however, until the 18th century.

In 1781, William Herschel recorded observations of an object that he initially took as a comet, but turned out to be Uranus. He noted that at higher magnifications it increased in size, which shouldn't happen with "fixed stars." Since he was able to resolve the object as more than a point-source of light he knew it wasn't a star. At first, he thought it was a comet or "nebulous star" (I'm guessing that's a planetary nebula). When he presented it to the Royal Society, he still thought it was a comet. British Astronomer Royal Nevil Maskelyne corresponded with Herschel and noted that he hadn't observed a tail or coma and it appeared to be in a circular orbit. By 1783, it was pretty much accepted by the scientific community that it was a planet, which Herschel then decided to name Georgium Sidus (George's Star, named after King George III - the same King George that we declared our independence from in 1776). The name didn't catch on, but it took almost 70 years before the name Uranus was generally accepted.

Herschell wasn't actually the first to spot Uranus, but the first to really take an interest. It may have been seen by Hipparcos in 128 BCE. Flamsteed cataloged it as 34 Tauri in 1690, which is the earliest definitive sighting, but not as a planet.

After Uranus's discovery, of course, there were several astronomers who compiled further observations and attempted to refine the orbit. In 1821, Alexis Bourvard published astronomical tables of the orbit of Uranus, but observations began to deviate significantly, which lead to the belief that there may be another body exerting gravitational influence upon Uranus. This lead to both John Couch Adams and Urbain Le Verrier to refine the calculations. In 1846, Johann Gottfried Galle found the planet which would come to be named Neptune about 1 degree away from where Le Verrier's calculations put it, and about 12 degrees from those of Adams.

But even after the discovery of Neptune, the orbits didn't quite work out mathematically. As it turns out, those discrepancies can be chalked up to Einstein's theory of Special Relativity. But that wasn't known yet, so astronomers theorized that there must be something else still out there to find. And so the hunt was on.

For several decades, astronomers and mathematicians tried to calculate where such a ninth planet might be found, and followed up their calculations with observations. Of course, those went nowhere.

In 1894, Percival Lowell founded the Lowell Observatory in Arizona, and in 1906, he began the hunt for what he termed "Planet X." Unfortunately, he died in 1916 before he could find anything.

In 1929, the director of the Lowell Observatory, Vesto Slipher, hired a 22 year old kid from a farm in Kansas named Clyde Tombaugh. Tombaugh had made his own telescopes and conducted observations and made sketches. He submitted them to Slipher, who was impressed enough to give the kid a job. Upon arrival at the observatory in Flagstaff, he was handed the job of searching for Planet X. By that time, it obviously wasn't being taken too seriously, such that Slipher could just hand it off to a kid fresh from the farm.

Tombaugh went to work with the 13 inch wide-field scope. He began capturing survey images, and then would go back a couple weeks later over the same patch of sky. With two images of the same sky, he could use a blink comparator - a device that allowed him to flip back and forth between two images rapidly - to look for objects that would move between the two.

Luckily for Tombaugh, Pluto was in a relatively sparse part of the sky in 1930, when he started imaging in Gemini in January of 1930. On February 18th, 1930, he compared two plates, one taken on January 23, and the other on January 29. In the span of 6 days between the two, a single small "star" had moved. A previous image taken on January 21 was then checked, and showed the same object and the same rate of motion. Further reviews of images taken going back to March of 1915 further confirmed the object. Planet X had been found.

Right from the get-go some astronomers questioned the nature of the object, but the public was quickly consumed by Planet X fever, making it difficult, if not impossible, to downgrade the planet. It was so popular that Walt Disney even named Mickey Mouse's sidekick, Pluto the dog.

Then, on January 5, 2005, Mike Brown at CalTech in Pasadena discovered an object far out past Pluto. Provisionally named 2003UB. Preliminary calculations suggested it could be larger than Pluto. Brown and his team started calling the object Xena, and announced it to the public on July 29th along with another object, 2005 FY9. They had planned to hold off further, but that changed when a team of Spanish astronomers announced the discovery of 2003EL61, later named Haumea, which Brown's team had been observing and preparing to announce. Brown soon after found out that the Spanish team had accessed his observation logs of Haumea on the day before the announcement, bringing their "discovery" of the object into question. In the end, the credit is given to both teams for the discovery, though the name proposed by Brown's team is the one that was adopted.

Back to 2003UB, aka Xena, and later receiving the official name Eris. Being potentially larger than Pluto (as of the New Horizons flyby of Pluto, it's been determined that it is not, in fact, larger... but just a hair smaller, than Pluto), the question arose: what kind of object is this? Brown initially told his wife that he'd discovered a planet. But then the discoveries of several others before and after started to bring the designation of planet into question.

Eris is nearly the same size as Pluto, and has at least one substantial moon (Dysnomia). Meanwhile, there's Haumea, Makemake, Gonggong, Quaoar, Sedna, Orcus, Salacia, and more. Are these all planets? If not, which aren't and why? What makes an object a planet?

The definition adopted by the IAU - to much controversy - is that a planet is a body that:

  1. is in orbit around the Sun
  2. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
  3. has cleared the neighbourhood around its orbit

The first element is not a problem: all of these bodies are orbiting the sun (this is in reference only to our own solar system, exoplanets are not subject to this definition).

The second one becomes a bit problematic. Beyond the 8 classical planets, we have several bodies that are rounded by gravity. The afore-listed objects are all rounded. And then there's Ceres, which until this point was classified as an asteroid, but it is roughly spherical. Why doesn't it get counted as a planet, or should it be?

The third element is the one that is most widely criticized. What does it mean to have cleared out the neighborhood?

Clearing out the neighborhood around its orbit refers to the body being gravitationally dominant in its orbit, with no other objects of significant mass co-orbiting with it. This is, of course, problematic as there are plenty of objects sharing an orbit with most planets. Jupiter has its Trojan asteroids. Saturn has its Centaurs. Earth has its Apollo asteroids. We haven’t completely cleaned out our own orbits. Though the argument here is that we are the obviously dominant mass in our orbit. Still, this definition is considered dubious by many.

But if we say everything that’s rounded by gravity and orbiting the sun is a planet, and we suddenly have 10 or so more, most of which are very small and very far away, largely not orbiting within the general plane of the traditional 8 (while they aren’t entirely co-planar, they are all fairly close to the same plane), and the orbits tend to be much more eccentric than those of the traditional 8 (while they’re all elliptical, the traditional 8 planets are fairly close to circular when compared to the remaining candidate worlds).

If we are to discard Eris, Sedna, Orcus, Quaoar, Makemake, and more, why should we keep Pluto? It is far more like the other dwarf planets than it is with the traditional 8 planets. What definition would one propose that includes Pluto, but rejects the rest?

This discussion of Pluto was sparked by a member of my astronomy club that shared a few pages of Alan Stern’s book Chasing new horizons: Inside the epic first mission to Pluto. Stern was the Principle Investigator for the New Horizons mission and seems to be somewhat biased concerning the nature of Pluto. In a section discussing the re-designation of Pluto from Planet to Dwarf Planet, he appears to take a somewhat critical, almost hostile, view of what happened. His bias seems particularly evident in the following sentence:

Then, in 2005, the discovery was reported of a Kuiper Belt planet later named Eris that was thought by its discoverer, Caltech scientist Mike Brown, to be slightly larger than Pluto (later this turned out to be wrong). (Stern & Grinspoon, 2019, p. 171, bold added)

The tone of this statement seems to be almost accusatory, almost as if Brown was way off the mark in suggesting Eris was larger than Pluto, which was essentially the final blow.

In fact, the diameter of Pluto is 2,377 kilometers (+/-3) while that of Eris is 2,326 km (+/- 12). When you’re measuring an object of that size several billion kilometers away, I think it’s fair to say that an error of a percent or two is hard to criticize. Further, the mass of Eris is 16.47x10^21 (+/- 0.09) while that of Pluto is only 13.03x10^21 (+/- 0.03). While it’s possible these measurements are slightly off, it’s still likely Eris has more mass than Pluto, regardless of diameter.

And it was only with the New Horizons mission’s flyby of Pluto that they were able to verify its actual diameter and show that its smallest possible diameter was still a tad larger than that of Eris. And this flyby happened over 10 years after the discovery of Eris.

Clearly the distinctions between Pluto and Eris with regards to just what kind of solar system body they are are fairly minor.

Stern goes on to excoriate the IAU and British Astronomer Brian Marsden for the demotion of Pluto. While there is a fair argument to be made concerning the flaws of this definition, Stern’s tirade comes off as somewhat puerile and churlish.

But all of this misses a major point: the classification of objects is really something of an exercise in semantics.

Pluto doesn’t care if it’s a planet, nor does Sedna. Nor does Jupiter, Saturn, Mars, or the remains of Alderaan. Whether or not it’s a planet, dwarf planet, or some other form of solar system body, it’s still an object of mass orbiting Sol and worthy of some amount of study. Fighting over the title makes no difference in that.

And for that matter, Pluto is the largest and most prominent member of a newly-defined (well, as of 2006) class: Dwarf Planets. These are bodies that are rounded by gravity, but haven’t cleared out the neighborhood. And as of last count there are at least 10 of them, and likely more. Brown himself suggested there may be bodies in the outer solar system larger than Mars. Whether or not that’s the case, such bodies will have much less to do with the inner solar system than the majority of the dwarf planets that lie in the Kuiper belt.

So… should Pluto be restored to planethood? Personally, I say no. But , again, it really doesn’t matter. It’s there, whatever you want to call it.

I would suggest that the whole reason Pluto was designated a planet from the start was largely due to a few factors: the first of which being that it was discovered by someone specifically looking for a planet. Tombaugh was in his early 20's when he discovered it. If he'd gone the traditional route, he'd still have several more years of school to go before getting a job at an observatory. He was a young man who was apparently eager to do well in his first assignment. And so he did... he discovered a planet... or at least something. Had it been discovered by a more traditionally trained and established astronomer, that person may well not have considered it a planet at all. I think that this could well be a case where someone went looking for something and found something that could be wedged into the right shape to fit the box they were trying to fill. While British astronomer Brian Marsden may have been a bit of a jerk in the way he handled it, he was probably right all along that Pluto shouldn't be seen a planet, but something else entirely.

I've taken the extra step of creating a reference list. I think it might be helpful for people to have access to the sources I've used.

One of the sources I cite regularly is the Wikipedia. Of course, the Wikipedia is not always looked upon as a reliable source. However, here I use it for basic reference information, such as lists and basic descriptions of objects, historical overviews, and such details as the diameter and mass of Pluto and Eris. Most of those are then cited in the references at the bottom of the Wikipedia pages, so, in a sense, I'm being lazy and not turning to the primary sources. But I'm not writing a college paper here, just a blog post. I think we can let it slide.

References:

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