The Life and Times of Pluto

This week, three people have approached me wondering why Pluto isn’t a planet anymore. My brief explanation of “it doesn’t meet the characteristics of a planet” didn’t set too well; they grew up with Pluto, they want it back! They want their children to grow up with Pluto! I realized after these exchanges that although the fate of Pluto has been a news feature since it’s contestation began a little over a decade ago, not many people know the details behind the decision. Each of the people I spoke to this week suggested I explain the story in more complete detail on my blog. As so, by request: a brief history of the outer planets and the rise and fall of Pluto. (Left, Pluto.)

Mercury, Venus, Mars, Jupiter, and Saturn have been known for millennia. Bright and large enough to be seen from Earth with the naked eye, the Ancient Greeks recognized these planets every time they looked up to the night sky and saw wanderers among the fixed stars. In 1781, a seventh planet joined the solar system. Astronomer William Herschel observed the planet from his home garden in Bath, England using a telescope. By 1873, it was officially recognized by the Royal Society. Uranus became commonly accepted as a new planet. (Uranus as seen by the Hubble Space Telescope in 2007.)

In the early 1800s, astronomers plotting Uranus’ obit noticed inconsistencies; Uranus didn’t follow the orbit their mathematical models dictated it should. Without the technology to delve deeper into Uranus’ secrets, the only explanation was that some mass beyond the planet was exerting a gravitation pull strong enough to account for the observed deviations.

In 1845 and 1846, English astronomer John Couch Adams used Newton’s and Kepler’s laws to develop a mathematical model that predicted the orbit of a trans-Uranian planet. French astronomer Urbain Le Verrier independently predicted the same planet at the same time. Neither man, however, actually looked for their mystery planet. They each hired observatories to look for them.

Adams convinced Cambridge Observatory director James Challis to look for his predicted planet, but a lack of enthusiasm led Challis to miss the planet that turned up right in front of his face. Le Verrier had better luck. He convinced Berlin Observatory astronomer Johann Gottfried Galle to look for his planet. The very evening Galle received Le Verrier’s letter requesting his assistance, the astronomer found what is now Neptune in images previously taken at the Observatory. (Left, hurricanes on Neptune are clear in this image stitched together from tow pictures taken by Voyager 2.)

But astronomers weren’t entirely satisfied. Some still believed that Neptune alone didn’t explain Uranus’ orbit. The idea of a planet outside the orbit of Neptune – a trans-Neptunian planet – was a popular one. Using the same method of applying Newton’s and Kepler’s laws to predict the new planet, the hunt was on for Planet X.

In 1894, American astronomer Percival Lowell moved to Flagstaff, Arizona to set up an observatory – the high elevation of the small city offered clear air and the climate promised nearly year-round visibility. He had two goals – finding proof of intelligent life on Mars, and finding Planet X. Although he used models to predict Planet X’s location and search the right part of the sky, he never found it before his death in 1916. (Lowell at work at the observatory that still bears his name.)

The hunt resumed at the Lowell Observatory in 1929 when Illinois-born astronomer Clyde Tombaugh arrived. He used a blink comparator, a machine that alternated between two images of the night sky, to detect a moving object. Stars won’t move from night to night, but planets will. On February 18, 1930, Tombaugh found a tiny moving dot in two of his images. By May 1, his discovery of Planet X was accepted around the world and had been formally named Pluto.

The plates Tombaugh used to discover Pluto. The arrow points to the tiny white dot that moves between the two plates.

Originally, Pluto was thought to be roughly the same size as Earth with an orbit well outside of Neptune’s. But as astronomers applied better and better technology to their study of the new planet, a very different world emerged. Importantly, it was one that didn’t agree with the other eight planets in the solar system. It’s an anomaly in almost every way.

From Mercury to Neptune, all the planets trace slightly elliptical but regular orbits on the same plane – they are all on the ecliptic. Their orbits also never cross. Pluto on the other hand traces out a much more pronounced ellipse, so much so that it spends twenty-two of its 200 year orbit inside the orbit of Neptune. It is also highly inclined to the ecliptic. It sticks out like a sore thumb. (Left, Pluto’s orbit in purple compared to the other eight planets.)

Pluto’s orbit wasn’t it’s only irregular feature. In 1978, better technology revealed previously unknown details about its size and composition. More detailed images revealed that it wasn’t as large as Earth at all. It had a moon, Charon, that looked like part of Pluto as seen by less sensitive instruments. Pluto was suddenly not only the smallest planet in the solar system, but smaller than seven moons. Jupiter’s Ganymede, Callisto, Io, and Europa; Saturn’s Titan; Neptune’s Triton; and our moon all dwarf Pluto. (Above, Hubble looks at Pluto in 2006. The dwarf planet is pictured with its moons Charon, Nix, and Hydra. It’s easy to see how Pluto and Charon could be misinterpreted as one planet.)

Pluto and Charon were revealed as an extreme case of tidal locking. Both bodies have one side that constantly face another. Many moons in the solar system are tidally locked to their planets – such as our moon – but Pluto is the only planet that is also locked to its moon.

A scale model of the solar system. Pluto does look like the odd planet out.

It’s characteristics also set it apart. The eight other planets all fall into two groups: the four inner planets are rocky bodies while the four outer planets are gas giants. Pluto is neither. It has a small rocky core, a layer of water ice, and a surface of frozen nitrogen. It is an ice ball. It does have an atmosphere, but a scant one made up largely of nitrogen, methane, ethane, and carbon monoxide. The tidal locking between Pluto and Charon also affects this atmosphere; the side facing Charon has more methane while the side facing away from Charon has more frozen nitrogen and carbon monoxide.

A further oddity around Pluto was recognized not long after its discovery: it doesn’t explain the observed abnormal motions of Uranus and Neptune. This didn’t sit right with astronomers. Was there something else with mass out there? The idea that nothing at all resided beyond Pluto was equally hard to accept. How could the space beyond Pluto just be empty? (It turns out that the deviations were not due to another planet but miscalculations in the gas giants’ masses. Data gathered from the Voyager probes solved that mystery.)

Seemingly empty areas in space had been filled long before the empty space after Pluto became a problem. In the 1770s, Prussian astronomer Johann Daniel Titus worked out a mathematical formula to account for the distance between orbits of the planets. His Titus-Bode formula (Johann Elert Bode made one of the first uses of the formula so is partially credited) predicted a gap between Mars and Jupiter, and he didn’t believe it could be empty. Observations backed up the math, and his hunch. The gap was there and it wasn’t empty. At the turn of the 19th century, Chair of Astronomy at the University of Palermo Giuseppe Piazzi found a tiny moving object in an orbit within the Titus-Bode space. He named it Ceres, and it remains one of the largest objects in what has since been discovered as a very busy asteroid belt. (Left, a bird’s eye view of our asteroid belt.)

So couldn’t the same apply to Pluto’s orbit? Couldn’t there be other things out there, further things that jus needed to be found?

The idea of a trans-Plutonian “asteroid belt” had occurred to Ernst Opik. In 1932 he hypothesized that comets originated somewhere in space and that there were a lot of them. Jan Hendrick Oort took up Opik’s idea in 1950 and proposed a region of small icy bodies far from the Sun. Comets had been observed to lose material as they traveled near the Sun, a store of icy bodies explained why comets kept coming. They orbited peacefully until got pulled towards the inner solar system. The region has since been accepted and named the Oort Cloud. (Right, a diagram showing the Oort Cloud.)

But it didn’t answer the question of planets beyond Pluto, and so the hunt for the new Planet X resumed in the 1980s. In the 1990s, the effort began to pay off. In 1992, David Jewitt and Jane Luu at the Manu Kea Observatory in Hawaii found an object beyond Neptune. Its orbit is beyond Pluto’s, is less inclined relative to the ecliptic, and it does not cross Neptune’s orbit. It was the first trans-Neptunian object to be classified in the Kuiper Belt (KY-per),  a region of small rocky-ice bodies that orbits close to but outside that of Pluto. In 2000, Robert MacMillan discovered Varuna, another object outside Neptune’s orbit that was also classified as a Kuiper Belt object.

But the discovery of Kuiper Belt and Oort Cloud object didn’t deal Pluto the fatal blow. Two men are credited with – or are responsible for, depending on your view – killing Pluto: Neil deGrasse Tyson, and Mike Brown.

In 2000, the American Museum of Natural History in New York City opened its new Frederick Phineas and Sandra Priest Rose Centre for Earth and Space – simply the Rose Centre – featuring the newly renovated Hayden planetarium. When it came time to organize the display of solar bodies, the museum curators grouped like objects together according to the five major types: the terrestrial planets, the Asteroid Belt, the Jovian planets or Gas Gaints, the Kuiper Belt and the Oort Cloud. Pluto didn’t fit the bill as a terrestrial planet or a Jovian planet, but it did fit with the Kuiper objects so that’s where it ended up.  (Left, Neil deGrasse Tyson shows Pluto Pluto.)

A year later, visitors starting noticing something wrong with the giant scale model solar system hanging from the ceiling. Pluto was missing. The apparent discrepancy made the front page of the New York Times and a firestorm followed. As the centre’s director, Tyson took the hit as the man who demoted Pluto and became the target of the public response. People reacted strongly to the destruction of their middle-school astronomical memories. Mnemonics like “My Very Elegant Mother Just Served Us Nine Pizzas” became useless to a generation. Demonstrations were staged (right). Pluto’s status also became an emotional issue as the planet was anthropomorphized on t-shirts with slogans reading “Don’t worry, Pluto, I’m not a planet either.”

But Tyson didn’t change science books or grade school curriculum. His view of Pluto was safely in New York City. It was Mike Brown, an astronomer at the California Institute of Technology, who put the final nail in Pluto’s coffin.

Along with a small team of researchers, Brown found his first trans-Plutonian object in 2003 and named it Sedna. Observations suggested Sedna was about three-quarters less massive than Pluto with a highly elliptical orbit that it took about 12,000 years to complete. It was thought to be about three times as far from the Sun as Pluto, and it was an early contender for a tenth planet. But it wasn’t bigger than Pluto, so it joined the growing number of Kuiper objects. Browns continued to find more objects in the far reaches of the solar system, but none were bigger than Pluto so none qualified as a planet. (Left, an arrow points to Sedna in one of the images Brown used to discover the object.)

In 2005, Brown’s luck changed. He found an object larger than Pluto, orbiting well beyond Pluto’s orbit, larger than Pluto, and with its own moon. Temporarily named Xena, it was the first real contender for a tenth planet in our solar system.

It ultimately came down to the International Astronomical Union to decide if Xena was a planet or not. The problem was what this decision would do to the other large objects found in the solar system. If Xena was a planet, Sedna could be considered a planet as well. So could Varuna. So could Ceres for that matter, the large object in the Asteroid Belt. If the IAU decides that Xena wasn’t a planet, it would go down as another Kuiper Belt object and take Pluto with it. The two objects had too many similarities. What happened to one would likely happen to the other. (Right, Xena.)

The IAU’s decision was based on a members-only vote taken during the society’s 2006 General Assembly in Prague. The important resolution was on what made a planet a planet. With an agreed upon definition, classification of solar object would be much simpler and clearer. The IAU proposed solar bodies be classified as three types: planets, dwarf planets, and other, each with its own set of criteria.

Planets had to meet three criteria. A planet had to be big enough that its own gravity made it round, it had to orbit the sun and not be a satellite of another body, and it had to be massive enough to clear the area around it in space of any debris. Dwarf planets were also held to three criteria. A dwarf planet must orbit the sun, it must have sufficient mass such that it’s own gravity makes it nearly round, it must be too weak to clear debris from its surrounding area in space, and it must not be a satellite of another body. All other bodies in the solar system were held to a single criteria. As long as they orbited the sun and were neither planets nor dwarf planets, they would be known as “Small Solar System Bodies.” (Left, an artist’s concept of the view of the solar system from Sedna.)

This definitions of planet, dwarf planet, and “small solar system bodies” passed with a sweeping majority; the votes weren’t actually counted. The vote meant Xena was officially a dwarf planet, as it was subsequently renamed Eris; it’s moon, Dysnomia.

Pluto is not massive enough to clear its own area in its orbit. It failed the third criteria, making it a dwarf planet. But it had one last chance to stay in science textbooks with the other eight planets. A separate resolution was voted on to determine whether or not Pluto should stay a planet and be its own category: ice planet. (Right, an artist’s concept of Eris and Dysnomia.)

On the matter of Pluto officially designated as a dwarf planet because of its inability to meet the new “planet” criteria, the motion passed with 237 votes in favour, 157 against, and 17 abstentions. Just like that, the solar system was left with eight planets and a series of dwarf planets.

The case for Pluto’s demotion is hard to argue, but people still can’t let it go. In honour of Pluto’s discoverer Clyde Tombaugh (left), the state of Illinois passed a Resolution restoring its planetary status within the state. Popular culture has seized on the loss of a planet; t-shirts with a crying Pluto and cartoons depicting a dejected planet continue to circulate. But Pluto will never really go away. It’s story is a great one that will certainly make it stand out in middle school textbooks for generations to come.

An artist's concept of the view from Pluto. Charon appears as a sliver while the Sun is a dot far in the distance.

 Suggested Reading/Selected Sources

Mike Brown. How I Killed Pluto and Why It Had It Coming. Spiegel & Grau. 2010. I’ve synthesized and simplified Brown’s story; his book gives the full, dramatic details down to the fights between astronomers over who discovered Eris first.

Neil deGrasse Tyson. The Pluto Files. W. W. Norton & Company. 2009. The documentary of the same name is also worth watching.

Richard Corefield. Lives of the Planets. Basic Books. 2007.

International Astronomical Union, Results of the 2006 Resolution Votes. 

NASA’s Discovery site about Small Worlds in the solar system. 

New York Times article reporting Pluto no longer a planet (2001).

Neil deGrasse Tyson replies to NYT headlines about Pluto’s new status.

Illinois reinstates Pluto’s planetary status.


12 thoughts on “The Life and Times of Pluto

  1. Good summary of the situation. I enjoyed reading it. It’s worth mentioning that several of the largest asteroids and even Jupiter’s moons were called planets for a while, so Pluto isn’t alone in being reclassified. A couple of years ago when my son’s Grade 5 class (age about 11 here in Australia) were studying space I gave a session on what is a planet, precisely because of the fuss being made over Pluto. I gave every student a paper with various characteristics of a particular solar body on it but without the object’s name, all of which had been called planets at one time, and we went through the list moving to one side of the room or the other, trying to figure out what made a planet a planet. They all became very involved in whether they were a planet or not, and at the end I handed out name cards to each of them and told them what they were (as well as the major planets, some were asteroids, some were moons, some were trans-Neptune objects, but all had been called planets). Then we all agreed on the important criteria and looked at what the IAU had decided and why that meant Pluto didn’t fit with the rest anymore. His friends still talk about that session.

  2. You do readers a tremendous disservice by presenting only one side of an ongoing debate as fact when this is not the case. There is an equally legitimate scientific viewpoint that rejects the decision made by four percent of the IAU, most of whom are not planetary scientists. Hundreds of professional astronomers led by New Horizons Principal Investigator Dr. Alan Stern immediately signed a formal petition rejecting that decision, a viewpoint they maintain to this day. One of those signatories was Dr. David Rabinowitz, a co-discoverer of Eris. Your references should include books by scientists representing the other side: “Is Pluto A Planet?” by Dr. David Weintraub, “The Case for Pluto” by Alan Boyle, and “Pluto Confidential” by Laurence Marschall and Stephen Maran. The third book was written by two authors who represent opposing positions on this issue.

    According to the equally scientific geophysical planet definition, “clearing its orbit” is not a requirement for an object to be a planet. That criterion puts the focus solely on where an object is to the exclusion of what it is. If Earth were in Pluto’s orbit, it would not clear that orbit either. That means the definition could lead to the absurdity of the same object being a planet in one location and not in another.

    Pluto is NOT an iceball; it is estimated at 70 percent rock. And it does fit with the other solar system planets if one considers that we have THREE, not two classes of planets–terrestrials, jovians, and dwarf planets. That was the intention of Dr. Stern when he coined the term “dwarf planet”–to designate a third category of planets that are large enough to be rounded by their own gravity but not large enough to gravitationally dominate their orbits. He never intended for dwarf planets to not be considered planets at all.

    Elliptical orbits do not preclude an object from being a planet. We have now discovered many multi-planetary exoplanet systems in which all the planets–giant planets–orbit in different planes, in orbits far more elliptical than of Pluto. Some of these cross one another’s orbits. Yet according to the IAU definition, none of these would be considered planets.

    The IAU made a premature decision based on erroneous information, specifically that Eris is larger than Pluto when this is not the case. The decision was based on a non-scientific notion that our solar system cannot have too many planets. The resolution was adopted in violation of the IAU’s own bylaws, which require any resolutions put to the floor of the General Assembly first be vetted by the proper IAU committee. This did not happen. Instead, the General Assembly rejected the recommendation of its own committee and approved a hastily thrown together, poorly worded, and confusing resolution.

    Please be fair to those who ask you this question, and tell them it remains an open debate. The so-called “reclassification” is wrong. Brown has very unscientifically dubbed himself the “plutokiller” and made comments denigrating Clyde Tombaugh as not having been a “real astronomer” and saying Pluto would never be a planet again as long as he (Brown) lives. That is not science.

    As an astronomer myself, I support an alternative planet definition: A planet is any non-self-luminous spheroidal body in orbit around a star (or having once orbited a star). Anyone who wants to hear both sides of the debate should listen to the transcripts of the Great Planet Debate, held in August 2008 at the Johns Hopkins University Applied Physics Lab in Laurel, MD, directly in response to the IAU decision. Those transcripts can be found here:

    I would be happy to answer any questions presenting the other side of this issue. Visit my Pluto Blog at . I am also working on a book about Pluto, which has experienced multiple delays, but is still in the works.

  3. Laurel, your initial accusation is inaccurate and insulting. Amy wrote a blog post explaining why Pluto was reclassified by the IAU. She did not write a post about the ensuing highly emotive and at times completely irrational argument that this reclassification provoked so your accusation of one-sidedness is disingenuous and unwarranted. It is of course open to you to draw attention in the comments, as you have done, to this ensuing debate.

    You keep using the phrase scientifically legitimate which is a term not really applicable to the process of classification, an arbitrary process utilized in order to have a clearer view of data or phenomena under consideration. One can classify a given group of objects literally in an infinite number of ways and the choice of which classification to use is a pragmatic and not a scientific one.

    When it became obvious that Pluto was only one of potentially hundreds if not thousands of celestial objects orbiting the sun in the Kuiper Belt the IAU chose for pragmatic reasons no longer to call those objects planets, mirroring a similar somewhat less formal decision no longer to call Ceres, Pallas, Vesta and Juno planets at the beginning of the 19th century.

    You use the expression so-called “reclassification” and thereby display a double ignorance of language use. Both so-called and the quotation marks call into question the legitimacy of the use of the term reclassification a clear example of tautology. However much worse is the fact that the term reclassification is perfectly correct in this context and can not be called into question. That you chose not to accept the reclassification does not stop it being one.

    A planet is any non-self-luminous spheroidal body in orbit around a star (or having once orbited a star).

    This would mean that all asteroids and all periodic comets are reclassified as planets a legitimate classification but one that is pragmatically next to useless.

  4. I find controversy in science is always fascinating, it adds the human element.
    In this case its more about classification being the problem than any contentious scientific theory.

    ‘Labeling’ is an important part of science but it may often veer off into areas of opinion and bias,
    where extensive debate is needed and often on going.

    I certainly don’t think this article does its “readers a tremendous disservice”, that is a biased statement and suggests that the writer has in someway harmed us. If the aim of this post, as I understand it, was simply to explain why Pluto is no longer classed as a planet, a timeline that chronicles the main events leading to Pluto’s current status? Then the goal was achieved.

    You can only criticise an article based on its aims, not everything it was unable to include or what you personally wished it contained.

    That aside, I agree with Laurels questioning of the IAU’s decision.
    It seems to me that we don’t yet know enough about other solar systems to be able to correctly understand how to classify our own. Thus it is likely to be constantly changing as we learn and compare with other systems.

    Perhaps Astronomers could learn a lot from the Biological and Evolutionary sciences and how they go about the constant problem of naming species and subspecies and understand that disagreements may be common because of differing philosophies and approaches that mean that, unfortunately, not everyone can agree?

  5. Mike Harmon wrote: It seems to me that we don’t yet know enough about other solar systems to be able to correctly understand how to classify our own. Thus it is likely to be constantly changing as we learn and compare with other systems.

    That is the nature of classification and reclassification in science, as we learn more we adjust and change our categories and the elements that constitute their memberships. Historically this has already happened several times with planets and will almost certainly happen again in the future. what I very strongly doubt is that Pluto will ever again be placed in the same category as the Earth or Jupiter.

    Historically planets were stars that wandered as opposed to those that made up the sphere of the fixed stars, the Greek word literally meaning wanderer. This of course included the Sun and the Moon as well as the then known five planets. Later as Marius and Galileo discovered the first four moons of Jupiter they were originally also called planets until Kepler coins the word satellite, meaning companion, and as the heliocentric model of the solar system became accepted our moon lost its planetary status and also became a satellite.

    When the astronomers originally discovered Ceres the first of the asteroids it too was originally categorised as a planet until it became obvious that there were many such objects mostly comparatively small, when they were given their own category, asteroids. Somewhat of a misnomer as it means star like.

    Somewhere down the line the Sun was finally recognised as a star and it too lost its planetary status.

    Pluto discovered in the search for a hypothetical tenth planet was also originally given planetary status, which as I have already stated above was removed when it was realised that it was one of many Kuiper Belt objects with their own distinctive characteristics.

    Scientific categorisation and classification is never chiseled in stone and the list of what are and what are not planets was not handed down to Moses along with the Ten Commandments. These things have changed and evolved down the centuries and will continue to do so into the future.

  6. I apologize if I sounded too harsh or critical in my comment. It results from frustration with the notion that the planet definition debate is resolved when the reality is it is not.

    This issue is not about emotion; it is about the fact that readers deserve to know that to many planetary scientists, Pluto is still a planet, as are all dwarf planets. If the author quotes scientists who clearly represent one side of this debate, such as Brown, she should also seek out scientists who represent the other side, such as Stern, and list all books on the topic as references so readers can read them and draw their own conclusions.

    Yes, the IAU’s “reclassification” process was arbitrary; it was done via a flawed process in which a hastily-thrown together resolution was adopted by a tiny minority of the group and subsequently opposed by an equal number of professional astronomers. My point is that the IAU statement is not some sort of “gospel truth,” and the opposing view has no less scientific merit.

    As far as we know, Pluto is not one of hundreds or thousands of similar objects in the Kuiper Belt. Pluto, Haumea, Makemake, and Eris are significantly different from the majority of KBOs and comets because unlike those, they are large enough and massive enough to be rounded by their own gravity, a state known as hydrostatic equilibrium. This makes them complex objects that are geologically differentiated into core, mantle, and crust, just like Earth, objects with atmospheres and weather. Pluto may even have a subsurface ocean that could host microbial life. It is estimated at 70 percent rock, and Eris, being more dense, likely has a higher percentage of rock. Therefore, these objects are far more akin to the bigger planets than they are to the tiny iceballs in the Kuiper Belt and shapeless asteroids in the asteroid belt, all of which are shaped only by their chemical bonds.

    Even if there are 1,000 large objects in the Kuiper Belt, all of which are in hydrostatic equilibrium, it makes no sense to say we have to limit the number of planets for the sake of convenience or “pragmatic reasons.” The solar system has whatever number of planets it has. We don’t say Jupiter can have only 4 moons because 63 is too many. We don’t say the universe cannot have billions of stars and billions of galaxies.

    In fact, the 19th century demotion of Ceres turned out to be an error. Nineteenth-century astronomers’ telescopes were not large enough to resolve Ceres into a disk, so no one knew it is spherical, meaning rounded by its own gravity, and therefore, a small planet. Today, we do know this, as we know that Ceres too is a complex object that also might harbor a subsurface ocean. Vesta and Pallas are borderline, significantly different from asteroids, as they are nearly in hydrostatic equilibrium, each appearing to have had a portion lobbed off in an impact. Vesta is known to be geologically differentitated. This is why some astronomers have proposed a new classification for Vesta and Pallas as “protoplanets” or “sub-dwarf planets.”

    Too many people act as though the IAU decision was some sort of gospel truth decided once for all eternity that automatically has to be accepted by everyone when this is not the case. It is a highly problematic classification system for many reasons, and one could argue it causes more confusion than it resolves. It certainly is not the only accurate scientific way of understanding our solar system; it is just one of many.

  7. Laurel,

    I’m not totally clear about what you are arguing.

    “Too many people act as though the IAU decision was some sort of gospel truth decided once for all eternity that automatically has to be accepted by everyone.” You seem to be taking the opinion that the IAU’s decision is gospel truth, dooming Pluto to something (I’m not quite sure what that something is, though). The IAU’s definition, flawed as it may be, doesn’t make Pluto not a planet. It reclassifies it as a dwarf planet. It’s still a planet. As you said, “And it does fit with the other solar system planets if one considers that we have THREE, not two classes of planets–terrestrials, jovians, and dwarf planets.” It’s a dwarf planet, it’s in the solar system. I don’t see what the problem is.

    “If Earth were in Pluto’s orbit, it would not clear that orbit either. That means the definition could lead to the absurdity of the same object being a planet in one location and not in another.” That’s not absurdity; that’s precisely how taxonomies work. If that were the case, then the earth would be a dwarf planet.

    If you were to actually read Brown’s book (though you say you don’t intend to) you’d find that he goes into a very long discussion of the problems of defining the word “planet.” He also talks a lot about his excitement about having discovered Eris and his hope that it would be a tenth planet. But he knew the decision carried a lot of weight because Eris’ fate would decide Pluto’s. He never set out to get Pluto – he’s not the evil and malicious astronomer you’re making him out to be. He found a planet, the IAU had to decide what it was, his planet was determined as a dwarf planet and took similar object, including Pluto, with it. They’re still planets. “How I Killed Pluto and Why It Had It Coming” is a much better title than “How I Found a Body Further Away than Pluto That’s Also Bigger than Pluto, and When the IAU Decided if I Had or Had Not Found a New Planet, a New Class of Planets Was Established, and Now My Planet and Pluto are Dwarf Planets.”

    By your definition of a planet, “any non-self-luminous spheroidal body in orbit around a star (or having once orbited a star)” basketballs should be planets. It’s a sphere, it’s on the Earth so in orbit around the sun, and it doesn’t produce its own light source.

    What is there to gain from Pluto being called a “planet” instead of a “dwarf planet”? Are kids really going to be worse off having eight planets to memorize instead of twelve, or fifty, or millions if we take your definition? Is “My Very Elegant Mother Just Served Us Nachos” really an awful mnemonic?

    Nothing you or the IAU say can change Pluto. You sound as though you think by changing Pluto’s status it’s going to be flung into the far reaches of deep space, lost forever, or that no one will ever again talk about it. It’s still out there, doing what it’s been doing since it’s been around. It doesn’t become more or less interesting because it’s a dwarf planet. New Horizons is still going to research it and send back a fantastic trove of information that will help us understand the universe a little more.

    This is how science works. New discoveries shed new light and things change. If it were static there would be no progress. Adding more classifications of planets doesn’t do anything but acknowledge that there is a greater variety of objects in our solar system than scientists previously knew about.

    If there are thousands of round bodies in the Kuiper Belt that are similar to Pluto, great. There will be a huge, rich field of study for all the future astronomers who are fascinated by dwarf planets.

  8. Terry, the problem is that the IAU voted specifically that dwarf planets are NOT planets at all. This was done by the rejection of resolution 5b, which would have encompassed both classical planets and dwarf planets under the broader umbrella of planets. That was Dr. Stern’s original intent in creating the term “dwarf planet.” If the resolution were amended to make this minor change, we would not have a potential situation where the same object would be a planet in one location and not a planet in another location. It (for example, Earth) would just be a different type of planet if it were in Pluto’s location. Currently, the IAU definition is inherently biased against planets further from their parent stars, which have larger and larger orbits to “clear.”

    I do have a problem with the notion that something becomes a fact because a person or group that appointed themselves as an “authority” decrees it so. That isn’t science; it’s dogma. As Dr. Stern says, the IAU can decree the sky is green; that doesn’t make it any less blue. Unfortunately, there were a lot of political and personal motivations that drove the 2006 decision, and this is important because the process of how we arrive at decisions is just as important as those decisions themselves. In fact, many astronomers believe this should never have been subject to an IAU vote. Time and new data tell us what reality is, not a decree from “on high.”

    I am troubled by the fact that the author failed to mention that there is a significant number of planetary scientists who formally opposed the IAU decision and continue to do so. To not tell readers that there are two competing views and that this is an ongoing debate rather than a settled matter equates to leaving out a very important aspect of this issue.

    I don’t believe in memorization at all. It is not an effective method of teaching. I would rather see kids taught the different classes of planets and their unique characteristics. Any attempts to artificially keep the number of planets small simply for the sake of memorization has no scientific basis whatsoever. So yes, I think your mnemonic is poor because it imposes this artificial cutoff. A better way of teaching the planets can be found at the end of Alan Boyle’s book “The Case for Pluto,” where he specifically discusses how to talk to kids about planets. His description is that our solar system has four plus four plus more, meaning four terrestrials, four jovians, and many more dwarf planets. I highly recommend you read his book, just to gain another perspective.

    While I have read books by dynamicists and supporters of the IAU decision, I do not plan to read Brown’s book. For one thing, I have absolutely no interest in his personal life, and he spends about a third of the book on that. Additionally, I still believe the entire “plutokiller” thing is misleading and overly sensational rather than scientific. I do not believe he is evil, but I don’t like some of the statements he has made, specifically the demeaning of Tombaugh and his statement that, “as long as I am alive, Pluto will never again be a planet.” He has also made personal attacks against me on the Internet, claiming I am not a “real astronomer,” and that I am “too old” to go into the field, displaying an inability to tolerate dissenting opinions. As far as I am concerned, he did discover a planet. In fact, he was for Pluto and Eris being planets before he was against it, something that to this day I do not understand. He had plenty of potential titles to choose from, and I think he should have chosen one that highlights his discovery of Eris. As someone who does astronomy outreach, I believe a huge opportunity was lost here. A lot of people to this day never heard of Eris because all the focus was on “killing” Pluto. That has actually led some younger kids to think Pluto has “disappeared” and has taken the focus away from what should have been the exciting discovery of a new planet.

    Basketballs would not be planets according to the definition I advocate, and here is why. The concept of “spherical” doesn’t just mean any round object and certainly not an object that is already part of another planet. It refers to a physical condition, specifically to an object having attained the size and mass to be squeezed into a round shape by its own gravity as opposed to being shaped only by chemical bonds. This state is known as hydrostatic equilibrium; when an object attains hydrostatic equilibrium, important processes begin occurring such as geological differentiation into core, mantle, and crust, geological activity, and weather. An object can be in hydrostatic equilibrium and be an oblate spheroid rather than completely round. The main point is the object is shaped by its own gravity.

    New discoveries do shed light and lead to changes, but these changes should be done thoughtfully and deliberately rather than hastily and for the wrong reasons. Just the notion of creating a planet definition without including exoplanets is already problematic. And knowing New Horizons was on its way to Pluto, why couldn’t the IAU wait until we get the new data from the flyby? As is now known, even the premise for their decision, that Eris was bigger than Pluto, was not correct.

    I plan on being one of those future astronomers who studies these bodies in the outer solar system, and as such, I want to see the public given access to all legitimate scientific perspectives on this topic, not just one.

  9. Minor point; Galle didn’t image Neptune to cinch its discovery from Berlin observatory; it was done the old fashioned pre-photographic way, with a star chart an observer and an eyepiece. Great post, keep up the good work!

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