Pluto: An Alternate History

New Horizons at a Pluto that never was. Image credit: NASA
Astronomical errors led to the discovery of Pluto in 1930. If those errors had been avoided, then it is likely that no one would have gone looking for a trans-Neptunian planet, and Pluto probably would not have been spotted until the 1970s or 1980s. The result: we would never have called Pluto a planet.

I will defend this bold assertion shortly; before that, however, an overview of planet-hunting history since the 18th century is in order. This will provide the context we need to understand why Pluto was found so soon and why it became included in the Sun's family of planets.

The Solar System known to humans ended at Saturn until 1781, the year comet-hunter William Herschel stumbled upon Uranus. After a time, astronomers noted that the seventh planet did not move quite as expected. They speculated about the existence of an eighth planet massive enough to tug on Uranus with its gravity.

Twenty years after Herschel found Uranus, Giuseppe Piazzi found Ceres in the space between Mars and Jupiter. In short order, other astronomers found Pallas, Juno, and Vesta. Until the early 1850s, these worlds were considered to be planets, bringing the total known to 11.

There the planet population stood until 1845, when K. L. Hencke stumbled on Astraea and then, in 1847, Hebe. Astraea was the 12th planet discovered, but Hebe was the 14th, for the search for a planet beyond Uranus had paid off in 1846 with the discovery of Neptune.

Neptune's gravity accounted for the irregularities in the orbit of Uranus. However, it soon became clear that Neptune did not move exactly as expected. This led some to propose the existence of yet another large planet in the outermost reaches of the Solar System.

Meanwhile, the number of worlds known between Mars and Jupiter took off like a rocket. In addition to Hebe, 1847 saw the discovery of Iris and Flora. In 1848, Metis joined the list of planets. Hygeia was found in 1849, and Parthenope, Victoria, and Egeria in 1850. Irene and Eunomia joined the list in 1851, bringing the total number of planets orbiting the Sun to 23.

By then, most astronomers had decided that enough was enough. Clearly, Ceres and her sisters had much in common. It seemed that they were representatives of a new class of small Solar System bodies. By 1854, a term that Herschel had coined after the discovery of Ceres, Pallas, Juno, and Vesta had gained widespread acceptance. The worlds between Jupiter and Mars became known as "asteroids" and the Solar System planet count shrank to eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. (The terms asteroid, minor planet, and planetoid have, historically, been used interchangeably.)

There things stood until the early years of the 20th century, when wealthy and eccentric American amateur astronomer Percival Lowell got into the act. Lowell had founded an observatory in 1894 in Flagstaff, Arizona, to seek evidence of intelligent life on Mars. He wrote a series of books in which he argued that fine lines some astronomers glimpsed on the disk of Mars were strips of vegetation growing beside canals dug by an ancient, dying martian civilization.

Though a hit with the public, Lowell's vision was greeted with derision by professional astronomers. By 1906, even he had begun to lose faith, so he gave his observatory a new mission: Lowell Observatory would search for the undiscovered planet beyond Neptune. Lowell called it Planet X. His calculations gave it six times the mass of Earth. Other astronomers, such as William Pickering, sought a trans-Neptunian planet, so the search became a race.

Clyde Tombaugh found Planet X at Lowell Observatory in 1930, 14 years after Percival Lowell's death. It was soon named Pluto for the Roman god of the cold, dark underworld. There was much rejoicing — at first.

Pluto was an odd customer from the get-go. Its orbit crosses Neptune's and is tilted 17° relative to the plane of the Solar System. It was also mysteriously faint. A world large enough to tug on Neptune should have been relatively big, hence relatively bright. Weird Pluto didn't even show a planet-like disk. This led to much puzzlement and at least one imaginative theory (see "Pluto, Doorway to the Stars" in the More Information section below).

Pluto's peculiarities also fueled detractors who believed that it did not qualify to be grouped with Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. I conducted a very cursory search — paging through a science textbook from 1933, just three years after Pluto was discovered — and found that Pluto's peculiar orbit had already led at least two geologists to call it a planetoid, not a planet.

We know now that the calculations that pointed to a big planet beyond Neptune — a planet with enough gravity to account for the discrepancies in Neptune's orbit — were flawed. The astronomers had got Neptune's mass wrong. Put the correct mass into the equations and the Neptune discrepancies vanish.

By the time we worked out that we had no need of a planet beyond Neptune, we knew that Pluto was too small to be that planet anyway. After we found its moon Charon in 1978 we could accurately calculate Pluto's mass. Its mass is about 0.0022 that of Earth (Earth = 1). Put another way, Pluto is about one-fifth of 1% as massive as Earth.

What if somehow we'd computed Neptune's orbital motion properly and never set out to find Planet X? If Lowell and others hadn't raced to find a trans-Neptunian planet in the 1906-1930 period, then it's quite possible — even likely — that we would not have stumbled upon Pluto until the 1970s or 1980s.

Let's say arbitrarily that we discovered Pluto and Charon together in 1978. Just as in our timeline, we would have used Charon's orbital motion to compute Pluto's tiny mass. Small mass combined with Pluto's weird orbit around the Sun would have meant that we would not have rushed to call Pluto a planet.

We probably would instead have rushed to seek other bodies like Pluto, and it is likely that with 1980s and 1990s technology we would have found several. That would have been the clincher. Pluto, we would have decided, was the first body to be found in a new population of bodies. We would have cited Ceres and the Main Belt asteroids as a precedent.

Would we then have called Pluto an asteroid? I suspect so. We might have called the Asteroid Belt between Mars and Jupiter the Inner Asteroid Belt and the one containing Pluto the Outer Asteroid Belt. No doubt some would have dubbed Pluto "the Ceres of trans-Neptunian space."

Perhaps we would have adopted a different name for the Outer Asteroid Belt: the name most astronomers have in fact adopted. In our timeline, David Jewitt and Jane Luu discovered the first trans-Neptunian body (other than Pluto) in 1992. Called 1992 QB1, it was the first recognized member of the long-hypothesized Kuiper Belt.

In our 2015, we know of more than a thousand Kuiper Belt Objects (KBOs) out of a population that might number in the billions. Most, like 1992 QB1, are quite small; perhaps a couple of dozen are similar to Pluto and Charon in terms of size and mass (Pluto is about 2370 kilometers wide, or about two-thirds the diameter of Earth's moon; Charon, 1200 kilometers across).

Had we found Pluto in 1978, we would still have sought to explore it, for it remains the nearest large trans-Neptunian body. Quite probably a space mission much like New Horizons would have been launched to asteroid Pluto, just as Dawn was launched to asteroids Ceres and Vesta. (Dawn, however, was able to orbit both bodies; New Horizons was a fast flyby.)

How might the world have changed if Pluto had not been found until 1978?

The discovery of Pluto in 1930 helped to repair Lowell Observatory's battered reputation, permitting it to grow into the respected institution it is today. Had it not found Pluto, its greatest claim to fame, it might not have survived. Perhaps it would have closed its doors in the 1930s.

Without Lowell Observatory, its home city, Flagstaff, Arizona, would have developed a different character. It would not have passed the world's first dark-skies ordinance in 1958 nor become world's first International Dark-Sky City in 2001.

My late wife and I would have had to find a different place to get married. We were wed in 1998 on the Lowell Observatory grounds, near the bucket-shaped dome housing the 24-inch Clark refractor Percival Lowell used to map canals on Mars (a telescope I learned to operate in 2001 and used to observe Mars in 2003).

Without Lowell Observatory, Flagstaff would probably not have become home to an unusually large number of scientific institutions for its size. For example, the U.S. Naval Observatory, where Charon was discovered, probably would not have set up shop west of town in the 1950s.

The Astrogeology Branch of the U.S. Geological Survey used Lowell Observatory telescopes for Moon mapping starting in about 1960, then moved to Flagstaff in 1963. Had it not become based in Flagstaff, it would likely have been split between rival lunar geology groups in Menlo Park, California, and Washington, DC.

The Astrogeology Branch was largely responsible for astronaut geology training during Apollo. Much training took place near Flagstaff — at the Grand Canyon, on the Bonito Lava Flow and Cinder Lakes, in and around Meteor Crater. With no Astrogeology in Flagstaff, Apollo geology training would have followed a different course.

Those are mostly negative or neutral changes in the timeline. Would there have been any positive ones?

I suspect that, had we found Pluto in 1978, not 1930, we would have been spared the ego clashes and animosity generated when Pluto was "demoted" in 2006. No one could have exploited the hyped-up controversy over whether Pluto was a planet to gain fame and sell books and Internet content because there wouldn't have been any hyped-up controversy.

We also would have been spared the odd, unsatisfactory term "dwarf planet." A dwarf planet fails to "clear" its orbit but orbits the Sun and is round like a planet (or, to put it another way, it is in hydrostatic equilibrium — during formation its gravity was sufficient to pull the stuff it is made of into a spherical shape). Pluto orbits the Sun and is round, but has a resonating relationship with Neptune and has neighbors in similar orbits, so it has not cleared its orbit. Hence, Pluto is a dwarf planet. Ceres has earned the dwarf planet classification, too, as have three other bodies out past Pluto — including, oddly enough, Haumea, which is apparently oblong.

I should note here that asteroid Vesta would probably have been called a dwarf planet under the current definition if it hadn't had its south pole blown off by a collision with another, smaller asteroid after its gravity had finished pulling it into a spherical shape. If Haumea can be a dwarf planet, then why can't more nearly spherical Vesta?

The most embarrassing thing about the dwarf planet label is that bodies we call planets do not clear their orbits. Jupiter's Trojan asteroid swarms and Earth's Near-Earth Asteroid population attest to this. Even more bizarre, Neptune remains a planet even though the presence of Pluto means that it has not cleared its orbit. Its gravity has "managed" Pluto's orbit, but Pluto is still there. So, strictly speaking, most or all of the Solar System's planets are dwarf planets.

Note that the definitions say that planets and dwarf planets orbit the Sun. They thus manage to exclude the thousands of planets we have found orbiting other stars. Basically, they assume a Sun-centered universe. Those who proposed and supported the current definitions of planet and dwarf planet didn't have that in mind — in fact, according to at least one source, extrasolar planets were excluded because of concerns about accurately labeling planets and brown dwarfs. It's worth noting this peculiarity, however, because it points up the fact that the definitions need work.

It is possible that the non-discovery of Pluto in 1930 would have had other, unforeseeable effects outside the world of astronomy. In a world where a butterfly's flapping wings in New York City might produce a typhoon in Taiwan, anything seems possible. Perhaps the Cuban Missile Crisis would have gone hot — or not happened at all. Perhaps Steven Spielberg would have directed Star Wars. Perhaps Apple would have been named Radish. Who can say?


Historical Geology, R. Moore, McGraw-Hill Book Company, 1933, pp. 5-6, 651.

"The Asteroids: History, Surveys, Techniques, and Future Work," T. Gehrels; in Asteroids, T. Gehrels, editor, The University of Arizona Press, 1979, pp. 3-24.

Twitter correspondence with C. Lintott (, 16 May 2018.

More Information

Clyde Tombaugh's Vision of Mars (1959)

Pluto, Doorway to the Stars (1962)

New Horizons II (2004-2005)


  1. > Who can say?

    When asked for comment, Who muttered something about fixed points in time, before vanishing into a nearby Police Box...

  2. - Basically, the definition is too simple. It assumes a Sun-centered universe. (I love this comment. Sounds so pre-Copernican.)

    I think I'm emotionally stuck on nine planets. I'll admit it. I grew up knowing nine planets, it's burned into my hard coding. It's ROM. (The first step to recovery is admitting your issue, right?)

    Alternatively, your structure of what-if-we-discovered-Pluto-in-1978 makes a pile of sense. That said, the definitions have been this generation's to make. Who is to say the 2075 definition would be the same? Hence, the drama and controversy ring a little hollow to me because our science is so new (relatively) in this field. Also, who is to say that the ancient Arabs, Chinese, or Greeks in the science of their day did not have a better definition in a yet-undiscovered scroll somewhere? (Religion regularly impeded Copernicus and Galileo, I'm not the history expert, not sure what pressures Ptolemy, Chen Zhou, or Hipparchus faced.)

    All that said (sorry, verbose), as a pragmatist I see exploration steps of:
    1. Earth Orbit
    2. Beyond Earth Orbit
    3. Planetary Flyby/Orbit
    4. Planetary (or moon, any moon) landing
    5. Extra-solar exploration (Voyager 1/2 and beyond, planet discovery)

    as this generation's challenges. It's an EXPLOSIVE era of exploration, and it doesn't look like it's stopping any time soon (slowing a bit maybe, thank you US congress), so labeling Pluto as Planet/Dwarf Planet/Planetoid seems like small potatoes to me. I consider myself outrageously lucky to live in this time and the planet-defining drama of our era seems nearly as silly as religious controversy did to Galileo.

    1. Truth be told, I would have liked to have seen Pluto grandfathered in for purely emotional/cultural reasons. It might still be done. But that wouldn't be scientific, or so I hear any time I suggest it. Science is often not scientific, however. There's ego and bad manners and crankiness and back-stabbing and opportunism and holding on to old ideas far too long, just like in everything else.

      This started out as a discussion of the Pluto naming issue - basically, the Pluto team hasn't followed protocol and agreed to name themes and submitted name banks, and it's using informal names, not real ones, reason unknown. Actually, it's been suggested that all those unscientific factors I listed are probably involved. I decided, though, to look at what might have happened had Pluto not been discovered until technology made stumbling upon it fairly easy, however. That seemed like more fun.


  3. Tagline made me curious:
    "the best spaceflight history blog in the west"
    "West" (western hemisphere or coast?)
    Who are your prime competitors?

  4. Six months had gone by since Beyond Apollo's demise, so I felt I needed to replace the "the blog formerly known as Beyond Apollo" subtitle. I considered "the best space history blog in the universe," but decided that the claim might not be true (perhaps there is a really awesome cephalopoid space history blog in the Virgo Supercluster). I almost went with "known universe." But then one morning I was looking at the empty space for the subtitle and "the best space history blog in the west" just popped out. Could be west of the Pecos, could be the western US, could be the western hemisphere or the area where western culture rules, could be the western spiral arm of the galaxy. I'll leave that to the reader to decide. As for competitors, I don't compete, I just do my thing.

  5. Hi David, I agree on your criticism on the definition for a Planet by the IAU.

    But Pluto isn't a planet like Ceres or Sedna aren't too. Nevertheless these objects seem to be the most interesting bodies in our solar system.

    Clyde Tombaugh did a hard job by comparing pictures over and over. Just because he looked into the right direction he found Pluto. And that was much luck because Pluto is not located at the ecliptic like all the other planets are.

    The Hubble-Space-Telescope-Team did a great job for finding the next target for New Horizons. It's just 45km (30 miles) wide. That's fascinating, such a small object at that distance. But they looked only at a very small region, you can imagine what more must be out there.

    The target for the Pluto discovery by 1978 is too early in my opinion. No one would have known where they have to focus on. I would settle this into the 1990s and later, when earth based telescopes got a boost.

    1. "...comparing pictures over and over..."

      It's amazing the time they had in the 30s with no Kardashians on TV. :-)

      Seriously, to the extent that the JWST pictures "should be" (hopefully no mirror anomalies like Hubble at first!) top end, and robotics will be half a decade more advanced, we ought to be able to do slide comparative analysis on a geometric scale above Tombaugh for exoplanets. Promising times.

  6. I like what you say about Pluto, Ceres, and Sedna not being planets, yet they are interesting all the same. A body does not have to be called a planet to be important to our understanding of the Solar System. That should be obvious by now.

  7. David,
    If you don't mind me asking, how (not what) was compensation generally structured within Wired during your contributions there? Do they count the number of hits, and you would get a tiered payment? I picked up Beyond Apollo through Wired, but as they changed the format, Wired online overall lost it's attractiveness. Been reading them since the mid-90s and I really feel their articles have moved from edgy and bold to more mainstream, from technical to more like "USA Today". Just a gut feel, no real good examples. It might be I just read more now.

  8. Raddish... I like your sense of humour.
    Never realized before the Pluto - Apollo geology connection. Even centralized in Flagstaff training test pilots for geology was already hard enough - Alan Shepard was notably hard to deal with. The Apollo 15 was more cooperative.

    1. I maintain the historical archive for USGS Astrogeology in Flagstaff. Shepard was an especially bad case. Some astronauts were eager to learn geology. Don Wilhelms' TO A ROCKY MOON (my favorite Apollo history) captures that well. He is forthright about how Shepard's attitude made Apollo 14 a lost opportunity for scientific exploration.

  9. You should have a look here. There are some amazing alternate space histories.

    There's a related forum here

  10. Thank you for this article. I thoroughly enjoy your historical articles, but you have an amazing grasp of the hypothetical reality that could have easily existed but for a slight twist in history. Please continue to produce these sage observations.

  11. Why, thank you. That's very kind of you to say.

  12. Nice article, David. I will read it more closely later. I have to make some money now.


    Pluto and Ceres are planets. We do not know if Sedna is in hydrostatic equilibrium. If two of the world's leading planetary scientists, Dr. Alan Stern and Dr. David Rabinowitz, one of the 3 co-discoverers of Eris, say Pluto is a planet, I will take their word for it over some Internet guy who doesn't even have a name. No offense. And if you don't know why the case for Pluto and Ceres is stronger than that for Sedna, that also raises an eyebrow.

    1. I wanted to add that, one doesn't have to be famous to have a brain and use it. Appeal to authority is a logical fallacy - just because people who impress you say something doesn't mean it's accurate or even sane. What any given authority says is just one more data point. Myself, I like to think things through. Then, if it's worth putting on the blog, I throw what I think out for people to debate.


  13. Your response is puzzling to me, I think in part because I work in the planetary science world. I think you are seeing this issue and the people involved in a popular press sort of way. The popular press seeks out conflict (and invents it if it cannot find it) and heroes. The Pluto classification issue is about the process of science, which occurs all the time across many fields and is often messy and confusing. Personalities enter into it - usually to its detriment - but that kind of thing tends to fall away over time, enabling us to answer real questions. I doubt that anyone will remember Alan Stern in 20 or 30 years - he's not Galileo - but the science will remain.

    I hope that you will indeed read what I wrote carefully and with an open mind.

    Two questions, if you don't mind - you did not mention the other accepted dwarf planets (Makemake, Haumea, Eris). Do you contend that those are planets, too? Also, there are about 20 candidate dwarf planets. Why did you emphasize Sedna?


  14. Clyde Tombaugh made several other important discoveries in his Pluto search, including the Perseus-Pisces Supercluster.

  15. @David You doubt that anyone will remember Alan Stern in 20 or 30 years??? The New Horizons mission and its findings, along with its principal investigator, will likely go into the history books and be remembered for centuries! You seem to have a personal bias against Alan Stern, who is a leading planetary scientist and just one of many who rejects the IAU requirement that an object must gravitationally dominate its orbit to be a planet.

    Neither Pluto nor Ceres are asteroids because that term refers to objects not large enough to be in hydrostatic equilibrium--squeezed into a round or nearly round shape by their own gravity. That threshold, at least according to the geophysical planet definition, marks the distinction between asteroids and planets. Objects in hydrostatic equilibrium have complex processes and structures that make them smaller versions of the terrestrial planets. If Sedna is in hydrostatic equilibrium, it too is a planet according to the geophysical planet definition. Without question, Pluto is well beyond the threshold for being in hydrostatic equilibrium, so by the geophysical definition, it is certainly not too small to be a planet. There is no need for grandfathering Pluto based on emotional or cultural reasons because it already is one based on its intrinsic properties.

    Why should an object have to orbit on the ecliptic to be a planet? The ecliptic is simply the path the Earth takes around the Sun. Mercury is not on the ecliptic either. Many exoplanet systems have multiple giant planets all orbiting their star in different planes. Would you say these objects are not planets? Requiring an object to orbit on the same plane as Earth to be a planet amounts to a violation of the Copernican principle because it gives Earth an unjustified privileged position.

    The debate over Pluto's planet status did begin back in 1930 because astronomers could not resolve Pluto into a disk. Some thought it was a moon of a yet-to-be-discovered giant planet. The controversy has really never abated since Pluto's discovery. That is not a negative thing. Debate in science is good; it stimulates thought and motivates scientists to look closer by sending missions like New Horizons.

    What happened in 1930 was not the discovery of a non-planet but the discovery of a new class of planet. Finding something different from what one was searching for occurs frequently in science. Had Pluto been discovered in 1978, the same debate would have occurred; it just would have started later. Planetary science has really come into its own as a field only in the last 60 years. We are making revolutionary discoveries both in our solar system and others that will fundamentally change our concept of planet many times over the next few decades and centuries.

    What is wrong with the term dwarf planet? Do you object to the classifications of dwarf stars or dwarf galaxies? Establishing dwarf planets as a subclass of planets is consistent with other uses of the term "dwarf" in astronomy.

    Personalities will always enter into scientific discussions (look at the conflicts over the discovery of Neptune), but that is not the issue here. The science is the issue, and our understanding of Pluto's science for the next few decades will be based on the data and images sent back by the New Horizons mission. That data is revealing a complex, geologically active world one would be hard pressed to describe as anything but a planet.

    What we should really be focusing on is this new class of planets with subsurface oceans that could possibly host microbial life. This includes both Ceres and Pluto as well as several spherical moons of the gas giants, which could be considered satellite planets--Europa, Enceladus, Titan, and Triton.

    1. I have nothing against Alan Stern. I think his importance is inflated by the current excitement over the New Horizons Pluto flyby. Can you name the Viking or Voyager PIs (without looking them up)? How about the Apollo geology PIs? People get excited about what is here-and-now and forget celebrities very quickly.

      You aren't telling me anything I don't know already and seem to be repeating many of the points I make, but putting your own spin on them.

      The point of my alternate history is to indicate what might have happened had we not found Pluto so soon - so soon that we couldn't judge its context. As in fact actually occurred, thank you Percy Lowell. Had we not been looking for a planet to account for Neptune's orbital motion, had it not been found until 1978, and had it been joined by a population of bodies in trans-Neptunian orbits in the 1980s and 1990s, it's hard to see why anyone would have ever considered Pluto to be a planet. It just never would have occurred to anyone. Regardless of its shape, it would have been seen as an asteroid and probably called a KBO (and then given a yet-more specialized designation, since the KB has multiple populations). Asteroids are called all kinds of things along with being called asteroids. We have subclasses of planets, moons, comets, etc.

      I see nothing wrong with grandfathering in Pluto as a planet. That is my position in this debate. That's not scientific, it's cultural, but that's OK. It might well have happened had folks like Neil DeGrasse Tyson, Mike Brown, etc., not sought to exploit the hyped-up controversy. Scientists would, of course, see it as a KBO regardless. Specialists see lots of things in ways that are different from the way non-specialists see them.

      Ultimately it comes down to what Bob Millis, director of Lowell Observatory, told me in 2001, when I asked him what would happen to Pluto if a body of its size or larger were discovered. He said, "It doesn't matter. Pluto will always be a fascinating object no matter what we call it."


  16. "It has only 0.02% of Earth's mass" - typo here, it should be 0.2%

    1. Actually, you had it right and I can't read (or calculate). It's very sad. You would do me a favor if you looked at how I have expressed it now and told me it was correct (or not).


    2. It looks right now. Another way of putting it is 1/458 of Earth's mass.

  17. According to the geophysical planet definition, neither Pluto nor Ceres could ever be considered asteroids, as asteroid are objects not large enough or massive enough to be in hydrostatic equilibrium. That, not size, is the important threshold. Both Ceres and Pluto are in hydrostatic equilibrium and have been revealed by the Dawn and New Horizons missions respectively to be geologically active and have the structure, composition, and processes the larger planets do. That makes them very different from asteroids. Most planetary scientists prefer the geophysical planet definition, so it is fair to say that if Pluto had not been discovered until 1978 (when its large moon Charon was first found), it still would have been designated a planet but also part of a binary planet system with Charon since their barycenter is outside Pluto and between both objects.

    1. Laurel:

      this is part two of a two-part comment - part one immediately follows

      I'm not sure when the planetary dynamicists would have gotten involved, with their theories about Neptune and Uranus migration. Perhaps awareness of the Pluto/Neptune relationship would have meant more progress in that field (which is really quite fascinating - the dynamic models, if accurate, reveal "fossil" Solar Systems that account for things like the Late Heavy Bombardment, a big planetary science mystery since the early 1970s, when I was a starry-eyed youngster).

      To tie this off, I'll note that *when* something is discovered matters at least as much as the qualities of what is discovered. What is the historical context of the discovery? In 1930, we were hunting for Planet X, and when we found something we immediately leapt on it because it fit our narrative and we lacked the ability to learn much about it. We didn't know much about comets, we only knew a thousand or so asteroids, and we knew of only about 10% of the moons now known to exist. Over the decades, we learned more, changing the context and enabling us to better understand new discoveries.

      Ultimately, the definitions we kick around today won't matter very much - new discoveries will create new context. The debate over whether Pluto is a planet will become a footnote, as will the 2006 IAU definitions of planet and dwarf planet. I predict that Pluto will take its place as the King of the Kuiper Belt, just as Ceres is the Queen of the Asteroid Belt (though there's some who think it originated farther out - that's a whole different kettle of fish!). The dwarf planet definition will go away, and the debates will shift to the planets of other stars.

      Actually, I forgot that I also wanted to clarify my position in the debate. I believe that Pluto should be grandfathered into the family of planets for cultural reasons. The definition of "planet" has evolved over the millennia - Earth was not originally considered a planet (it didn't wander in the sky). To me that suggests some flexibility. Call Pluto an honorary planet because it was considered a planet for a long time. I like that because it makes it a monument to a time before we understood the true overwhelming extent of the Solar System. For more than 60 years we had a strong clue that there was more to our Solar System, but we lacked the capability to confirm that fact. Pluto was the pioneer, the thing that kept us looking beyond Neptune. We should honor that identity.


  18. Laurel:

    I have divide this into two parts because it is too long. Oh, well.

    I'm glad that you took a few moments to look at my attempt to explore the role of contingency in planetary science/astronomy history. This is a history piece, so current definitions don't enter into it.

    It is really hard to argue credibly that astronomers working in 1978 - the date I postulate for the discovery of Pluto and Charon - would have had enough information to determine that Pluto is geologically active, that it is in hydrostatic equilibrium, and that the Pluto-Charon barycenter is outside Pluto. If you look at the Charon discovery image, you'll note that it is a barely obvious bump on a Pluto that spans a few pixels. Lowell Observatory missed it, but was able to confirm it on image plates it had on file going back years after the Naval Observatory found it.

    The point of this piece is that we call Pluto a planet as a result of contingency - historical accident, if you will. Forget what we thought Pluto was like in 1930 and what we know it is like in 2017. In 1978, we would have seen a pair of tiny objects, unlike any known planets, in a comet-like or asteroid-like eccentric orbit tilted substantially relative to the eight known planets and in a resonating relationship with Neptune. Astronomers - not planetary scientists, at that point - were aware of and looking for the hypothesized Kuiper Belt. The debate would not have been about whether Pluto-Charon was a planet and a moon or a binary planet, it would have been about whether the Kuiper Belt had been found.

    It's possible Pluto-Charon might have ended up in the "giant comet" bin for a while, after the atmosphere enveloping both bodies was discovered. In that case, it's possible that some would have interpreted it as a cloud of material, not a pair of solid bodies. The Whipple "dirty snowball" nucleus model of comets was winning at that time, but there were still astronomers around who defended the older model. It would have taken a few years to sort that out.

    Finding Pluto-Charon would, in the meantime, have triggered a search for other bodies associated with the Kuiper Belt. These would have been found slowly, much as asteroids were found slowly at first, because of technology limitations. By the 1990s, however, we would have known of at least a few dozen objects, including more objects in a resonating relationship with Neptune. Probably we would have spotted some of the other larger KBOs, including, perhaps, other binaries.

    By that point, Pluto-Charon would have begun to be celebrated as the Ceres of the Kuiper Belt - not as an odd planetary pair. The first sign of a vast new realm (mostly) beyond the planets.

    end part 1


  19. Another oddball hypothesis to explain why Pluto looked so much smaller and dimmer than expected: it was proposed that Pluto was so smooth and shiny that what we were seeing was actually a reflection of the Sun!

    1. Michael:

      How large would Pluto have been in that case? It seems if it were highly reflective it could have been quite small. When astronomers discover a Solar System body, they have to work out whether it is small and light-colored or large and dark-colored.


  20. Historical definition: Middle English planete, from Anglo-French, from Late Latin planeta, modification of Greek planēt-, planēs, literally, wanderer, from planasthai to wander.

    But I'm the sort of person who would be cool with us living in a solar system with 140 planetes. I suspect a lot of what is driving the discussion is based on that realization, because who wants to learn 140 names? So a modern definition had to be devised, and the trouble with rules is that there are always edge case.

    Pluto has always been on the edge, and so the arguments begin.

    I'm old enough that I doubt I can remember to use the scientifically correct redefinition of Pluto as a dwarf, a word which also has connotations beyond what the word denotes. And so we come to the crux of the problem, language versus mathematics. One is emotional, the other is not.

    1. Ashley:

      This is an astute comment.

      The difficulty, as I see it, is non-recognition of Pluto's true significance - that it is the first body found belonging to the Kuiper Belt. That's a scientific reality, not an opinion or a value judgement. As such, it should be easy for scientists to accept. Most scientists with an interest in such things do, in fact, accept that Pluto is a member of the Kuiper Belt. There's no reason for it to belong to multiple categories - there's no scientific cause for that. There's no need to invent a new "dwarf planet" category. There are planets and there are KBOs and they are distinct.

      What will be really interesting is if we find a planet revolving around the Sun in the Kuiper Belt. That is, one that formed in the inner disk with the other planets and was ejected. Will we call it a KBO? I think the brown dwarf/planet distinction, which speaks to formation processes, might serve as a useful precedent.


  21. M. Portree, you have a terrific sense of humor. The last lines got me laughing like an idiot (Apple / Radish ROTFL).
    More seriously - lots of interesting points. About the Pluto controversy and its origins. Plus its impact on Arizona. And Apollo.

  22. Glad you like it! There's really no controversy. Science isn't supposed to be like that. But you mix in "people motivations" (as always happens) and magnify them using mass media, it gets sort of wacky, which can be entertaining! In the end, the KBOs, including Pluto, are fascinating bodies with many tales to tell, and that's the important thing.



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