An orb by any other name would circle as smoothly

Alan Stern, the principal investigator of the New Horizons Mission to Pluto is calling for a new definition for planets in order to return Pluto to, what he and other see as its former glory, the status of a planet. The so called demotion of Pluto caused the release of strong emotions amongst the distant planet’s fans and the stunning success of the New Horizons mission added fuel to the flames in the on going debate. Many of those participating seem to be somewhat unaware of the fact that the definition of what is a planet has changed down the centuries and I thought I would write a brief guide to the changing fortunes of the term planet since its inception in antiquity.

It should be made clear that I shall only be talking about European astronomy and not any other traditions such as Chinese, Indian, Mayan astronomies etc. European astronomy/astrology has its roots in ancient Babylon. The Babylonian tradition was most concerned with the Moon and the Sun but the Babylonians were aware of the planets Mercury, Venus, Mars, Jupiter and Saturn, which they like other ancient cultures regarded as divinities. They tracked their orbits over very long periods of time and developed algorithms to determine their appearances and disappearances for omen astrological purposes. They don’t appear to have been interested in the mechanism of the planetary orbits. I’m anything but an expert on Babylonian astronomy/astrology and I don’t know if they had a collective name for them.

The direct inheritors of the Babylonian celestial interests were the ancient Greeks and they were very much interested in orbital mechanics and they also coined the term planet. For the Greeks all illuminated objects in the heavens were stars (aster, astron), as I explained in an earlier post. The stars as we know them were the fixed stars because they appeared to remain in place relative to each other whilst the sphere of the fixed stars rotated about the celestial axis once every twenty-four hours. It was of course the Earth that rotated about its axis and not the stars but the Greeks were not aware of that. The illusion that the stars, visible to the naked-eye, are all equidistant to the Earth is easy to experience. Just go out into the countryside were there is no light pollution and look up at the night sky on a clear night. You will see the ‘sphere of the fixed stars’, as experienced by the ancient Greeks. Comets, much rarer and apparently random, were hairy stars, the word comet derives from the Greek aster kometes, literally long-haired star. The five planets known to the Babylonians and the Moon and the Sun were all present on a regular basis but unlike the fixed stars they appeared to wander around the heavens and so they became asteres planetai that is wandering stars, from planasthai to wander. The Greeks had seven wanderers Moon, Mercury, Venus, Sun, Mars, Jupiter and Saturn. The Earth was of course stationary at the middle of the whole system and so was not a planet.

16th-century representation of Ptolemy’s geocentric model in Peter Apian’s Cosmographia, 1524
Source: Wikimedia Commons

Subsequent European cultures and the Islamic Empire inherited the Greek model of the heavens with its seven wanderers and nothing of significance changed down the centuries until the Renaissance and the advent of Copernican heliocentrism in 1543. Copernicus’s new model was of course a major upheaval. The Sun became stationary and the Earth became a planet wandering through the heavens. The Moon acquired a strange new status, no longer orbiting the centre, now the Sun, but orbiting the Earth. Heliocentricity took more than one hundred years to become establish and Copernicus’ upheaval brought no immediate change of terminology.

The heliocentric order of the heavens from Copernicus’ De revolutionibus 1543

The first change came in 1610 with the telescopic discovery of the four largest moons of Jupiter by Galileo and Simon Marius. Here we have four new celestial bodies orbiting a planet, as with the Moon, and not the centre of the cosmos. At first Galileo referred to them as stars or planets, leading Kepler, who was at first not clear what the four new objects were, to panic and fear that Giordano Bruno was right and that all stars had planets. This conflicted with Kepler’s own finite universe cosmology. He was greatly relieved to discover that the new planets were in reality moons and coined the term satellite from the Latin satillitem meaning attendant, companion, courtier, accomplice or assistant. Kepler was very fond of creating new scientific terminology. The term was not adopted immediately but by the end of the seventeenth century astronomers differentiated between planets and satellites, around the same time as heliocentricity became firmly established and the Sun finally ceased to be a planet and the Earth finally became one. Around the same time astronomers became convinced that the Sun was actually one of the ‘fixed’ stars.

We entered the eighteenth century with six planets, Mercury, Venus, Earth, Mars, Jupiter and Saturn and so it remained until the musician and amateur astronomer William Herschel shocked the world with the discovery of a seventh one, Uranus on 13 March 1781. The first new planet discovered in about four thousand years of planetary astronomy.

William Herschel. Portrait by Lemuel Francis Abbott 1785, National Portrait Gallery, London
Source: Wikimedia Commons

In the middle of the eighteenth century Johann Elert Bode published what is now know as the Titus-Bode law in which the distance of the planets from the sun seemed to fit an arithmetical series with a gap in the series between Mars and Jupiter. Herschel’s discovery of Uranus beyond Saturn fit the Titus-Bode series, which led the German astronomer Baron Franz Xaver von Zach to organise a systematic search for that ‘missing planet’ between Mars and Jupiter. In fact the discovery was made by the Italian astronomer Giuseppe Piazzi, who was not part of Zach’s search team but discovered Ceres on 1 January 1801, exactly, where it should be according to the Titus-Bode law and then there were eight. Interestingly Piazzi lost Ceres and Carl Friedrich Gauss developed a new method of determining planetary orbits, which allowed astronomers to find it again. Very soon other astronomers discovered Pallas, Juno and Vesta and there were now eleven planets. It was not long before it became clear that the four new celestial bodies were somehow different to the other planets and Herschel coined the term ἀστεροειδής, or asteroeidēs, meaning ‘star-like, star-shaped’, in English asteroid. These smaller wanderers were also known as minor planets or planetoids although it was first in the later nineteenth century, by which time several more asteroids had been discovered that these terms became established and the number of planets was once again reduced, not to seven but to eight!

Piazzi’s book “Della scoperta del nuovo pianeta Cerere Ferdinandea” outlining the discovery of Ceres, dedicated the new “planet” to Ferdinand I of the Two Sicilies.
Source: Wikimedia Commons

It was eight because in the mean time both the English astronomer John Crouch Adams and the French astronomer Urbain Le Verrier had predicted the existence of an eighth planet based on gravitational anomalies in the orbit of Uranus and on 23 September 1846 the German observational astronomer discovered Neptune, the eighth planet, based on the predictions of Le Verrier.

Urbain Le Verrier
Source: Wikimedia Commons

In the late nineteenth century similar anomalies in the orbit of Neptune led Percival Lowell to predict the existence of a ninth planet and he set up his own observatory to search for it. In 1916 Lowell died without having found his predicted planet. However in 1929/30 the young Clyde Tombaugh discovered Pluto, the ninth planet.

From: O’Hara, Elva R. (2006). Clyde W. Tombaugh: Farm Boy Reached for the Stars. Borderlands 25.
Source: Wikimedia Commons

As with Ceres and the asteroids Pluto’s planetary status was challenged by the discovery of other orbiting objects in the Kuiper belt outside of the orbit of Neptune from the 1990s onward. The discovery of Eris in 2005 led to a serious reconsideration of Pluto’s planetary status and famously in 2006 the International Astronomical Union introduced a new formal definition of the term planet, which removed Pluto’s planetary status and according to Pluto’s fans demoted it to the status of a dwarf planet. At the moment there are five recognised dwarf planets Pluto, Ceres (the largest asteroid), Haumea, Makemake and Eris.

Eris (center) and Dysnomia (left of center), taken by the Hubble Space Telescope
Image NASA
Source: Wikimedia Commons

As I said at the beginning the Pluto fan club has not given up the fight and are now proposing a new definition of the term planet, which would not only return Pluto to its planetary status but also apparently the Moon. I hope I have shown that the term planet has gone through quite a lot of changes over the last two and a half thousand years or so since the ancient Greeks first coined it and we can, I think, assume that it will go through quite a few more in the future in particular with respect to the thousands of exoplanets that astronomers are busy discovering.



Filed under History of Astrology, History of Astronomy, History of science

44 responses to “An orb by any other name would circle as smoothly

  1. The “Pluto fan club” is not an emotion-based group of fanatics but people who genuinely favor a geophysical planet definition over a dynamical one, as a geophysical definition puts an object’s intrinsic properties before its location when classifying it. Nineteenth-century astronomers ended up being wrong in demoting Ceres, at least according to the geophysical view, because it is in hydrostatic equilibrium (rounded by its own gravity) and therefore a small planet.

    The IAU definition was never anything more than one view in an ongoing debate, adopted by just four percent of its members, most of whom were not planetary scientists but other types of astronomers. It was immediately opposed in a formal petition by an equal number of professional planetary scientists. Ironically, the four percent of the IAU who voted on this misused the term “dwarf planet,” which was first coined by Alan Stern. Stern intended it to designate a third class of planets in addition to terrestrials and jovians, not to designate non-planets. Just correcting this one point to establish dwarf planets as a subclass of planets would significantly reduce the controversy in this matter.

    To not incorporate New Horizons’ findings at Pluto and Dawn’s findings at Ceres into a new definition makes little sense. Ironically, these two small planets have remarkable similarities in that both have a history of cryovolcanism and both may have subsurface oceans that could harbor microbial life.

  2. According to Caltech’s Mike Brown (the miscreant who was part of the Pluto demotion) there is strong evidence for a large planet, 10x earth’s mass, far beyond the kuiper belt. He gives a great intro to the evidence here (in which case we’ll go back to 9. Or 15. Eeks):

    • As far as I am concerned, as I do not use the IAU definition, our solar system already has a minimum of 13 planets. The hypothetical undiscovered planet should be called Planet X, the traditional term for a suspected but undiscovered body. I am taking part in the Zooniverse search. There could be several planets out there or possibly a brown dwarf, which is not a planet at all, or even a second Kuiper Belt.

      • Totally agree. The categories are constructs of course, but the IAU came up with a really ugly kludge, specifically designed to demote Pluto). Defining an object like a planetary body by its effects on surrounding objects rather than its physical characteristics seems pretty daft to me.

  3. As I have pointed out elsewhere, the Moon’s orbit around the Sun is convex (the gravitational force of the Sun on the Moon is about twice the gravitational force of the Earth on the Moon), so it is more accurate to think of the Earth and Moon as twin planets. Unfortunately the lack of scale in Copernicus’ model is misleading. See:
    for a simple explanation

    • “As I have pointed out elsewhere, the Moon’s orbit around the Sun is convex (the gravitational force of the Sun on the Moon is about twice the gravitational force of the Earth on the Moon), so it is more accurate to think of the Earth and Moon as twin planets.”

      That’s why Isaac Asimov wrote a book about the Earth-Moon system called The Double Planet.

      On the other hand, the centre of mass is deep inside the Earth. One shouldn’t go too far the other way out of spite.

      If it were mine to decide, I would say that a planet is roughly spherical due to its own gravity and orbits a star (as opposed to a planet). Thus, Pluto would be a planet, and Ceres, but not the Moon nor any of the other large satellites (the Galilean satellites, Titan, etc).

      • I wasn’t making the comment out of spite, as you put it. If you check the relative gravitational forces from the Sun and the planet on the moons you quote, I think you will find that our Moon is the only case where the Sun’s gravitational force is greater. Incidentally, the barycentre of the Pluto-Charon system is outside the surface of Pluto, so on your definition, Charon is also a planet.

      • Perhaps the following: to be a planet, it has to be spherical, the orbit has to be convex relative to the star, and the barycentre of the orbit cannot be inside another body. 🙂

      • “Perhaps the following: to be a planet, it has to be spherical, the orbit has to be convex relative to the star, and the barycentre of the orbit cannot be inside another body.”

        I suggest checking that Pluto’s orbit (measured at the centre of the body) is always convex towards the Sun. If Pluto were to fail under this criterion, you would be shooting yourself in the foot.

  4. philtill777

    Sadly the tale you’ve told here is urban legend, and even many astronomers are repeating this incorrect tale rather than checking the facts. It’s easy to check facts nowadays on Google. You can check what I’m about to say on by searching these terms for planetary bodies. Use advanced search to specify the century you want to check.


    When Galileo discovered the moons of Jupiter the word he chose to call them was “planets” — the “Medicean Planets”. When Huygens discovered Titan orbiting Saturn the word he chose to name it was “planet”. He called all the moons “secondary planets” orbiting “primary planets.” When Herschel discovered the moons of Uranus in the LATE 18th century he also called them “secondary planets”! This was long, long after the Copernican revolution. Why did they keep calling moons “planets”? Because at the time of Galileo they realized the Moon has mountains so all these bodies are geophysical objects like Earth. The dynamical situation of these bodies was not as important as their intrinsic (geophysical) nature. That “primary-” and “secondary planet” terminology remained popular all the way onto the 20th century. Check it out on google scholar. This is true. It is also true that is became common to call them “satellites” and even later “moons” during this time, but the terms were all considered interchangeable. That’s because they were still considered to be planets in their geophysical nature — bodies like Earth — even though they were in a different dynamical subcategories. They were NEVER reclassified from planets to satellites.l at any time in history. “Satellite” was a dynamical subcategory, not a reclassification. What about later in the 20th century–did we finally stop calling them planets and go exclusively to “satellite” or “moon”? No. Last night I found 27 recent papers that call Titan a planet. I found more that call Europa a planet. And there are many that call our Moon a planet. There has literally NEVER been a time in history since the Greek word “planet” was invented that humans have not called the Moon a planet. Dynamically it is a satellite. Geophysically it is a moon. Let me recommend this book to you, available on Google books: “The Moon: considered as a Planet, a World, and a Satellite.” This was published in 1885 by two astronomers at tee Royal Observatory. The vast majority of the book (12 chapters) considers its geophysical characteristics and formation. That is the “planet” part. Then there is one chapter that calls it a “world” and describes what it would be like for creatures living on its surface. Finally, there is one chapter that calls it a “satellite” and describes its relationship with the Earth. Notice that it is first and mainly a planet. But being a planet does not stop it from also being a satellite. It is both. One is geophysical, the other is dynamical. (“World” is contextual for creatures on its surface.) This is a geoscientists have ALWAYS viewed these bodies from the time of Galileo until the present. This is why the 2006 IAU planet definition was such a disaster. It was a dynamical definition taking over a word that is historically geophysical, and it is an unworkable definition for doing comparative planetology.

    Please check this out and please publish a revised account of the true history of this word. There is a huge amount of data that you can check.

    For starters, here’s an example of Carl Sagan casually calling Titan a planet:

    Thompson, W. Reid, and Carl Sagan. “Titan: Far-infrared and microwave remote sensing of methane clouds and organic haze.” Icarus 60.2 (1984): 236-259.

    “…ray paths through the atmosphere, with atmospheric sphericity (and in fact emission from beyond the solid disk of the planet)…”

    Another author in 2004:

    Simakov, M. B. “Possible biogeochemical cycles on Titan.” Origins. Springer Netherlands, 2004. 645-665

    “The time of existence of the Titan’s juvenile ocean was enough for arising of the first protoliving objects. As the planet developed through time several energetic processes (irradiation, lightnings,meteoritic and comet impacts) could produce different forms of fixed nitrogen…”

    Another in 2007 (after the IAU definition, which geoscientists are forced to ignore:

    Snowden, D., et al. “Three‐dimensional multifluid simulation of the plasma interaction at Titan.” Journal of Geophysical Research: Space Physics 112.A12 (2007).

    “When the plasma flows near Titan the plasma that encounters Titan’s ionosphere is slowed and the frozen-in field lines are forced to drape around the planet.”

    • Sorry, your comment got caught in the spam filter

    • _The large satellites of our solar system, including Earth’s Moon, have always been called planets._

      So has the sun, for ages. Now what?

      • philtill777

        “So has the sun, for ages. Now what?”

        Verb tense: so WAS the sun, but now it hasn’t been since we switched from a merely phenomenological view (lights in the sky) to a geophysical one, which happened about the time of Galileo when he saw mountains on the Moon and reinterpreted “planet” to be a body like Earth. Remember this article was about how the definition changed over time. It did change that one time. Since then some scientists have drifted to a dynamical concept, but that was only recently and not universal since many of us have continued to use the geophysical concept that has existed for centuries.

  5. Jeb

    One for big block of cheese day.

  6. That article is in parts incorrect. The term “satellite” was in existence but did not yet prevail against the then popular “secondary planet” (vs. “primary planets”, those that orbit the sun) until the discovery of Phobos & Deimos in the late 19th century. Even then, it was still found in literature till the 1920s!

    And Pluto’s status was already challenged in Tombaugh’s own time, in the 1930s. Maybe that’s why some pretended it was as large as Earth. 😉

    So, next come the Vesta aficionados and want to upgrade protoplanets, too? Wait … Vesta was not discovered by an American. So that’s that!

    • philtill777

      That’s right. When Herschel discovered the moons of Uranus he called them “secondary planets” around their “primary planet”. That was late 1700s. He also called them satellites. That’s because dynamically they are satellites but geophysically they are planets. Even today, planetary science publications will call them moons or satellites when they want to make dynamical distinctions, but they call them “planets” in regard to their geophysical traits. I just counted dozens of recent papers calling Titan and Europa “planets.” These authors had no problem ALSO calling them moons or satellites in order to distinguish them from their primary planet when necessary.

      When you thoroughly examine the evidence (a rare thing in this subject) then you see that the meaning of “planet” has shifted only once in history, contrary to what this article says. That happened when Galileo discovered that the Moon has mountains. He then shifted from a phemomenological definition of planet from “lights in the sky that wander against the background stars” to “bodies like Earth”. He did not consider orbiting the sun to be part of that definition since he still called the Moon a planet. Then when he found the four largest moons of Jupiter, he figured they are like Earth’s moon so they are also geophysically planets like Earth’s Moon, so he called them the Medicean Planets (honoring the Medici family who supported him). Likewise, when Huygens discovered Titan orbiting Saturn, he called it a planet for the same geophysical reason and he coined the term “secondary planet” to make a dynamical distinction between types of planets. Even to this day planetary scientists call the Moon a terrestrial planet in many books and peer reviewed publications. Ever since Galileo we have ALWAYS used a geophysical definition for “planet”. Always. The definition has never changed.

      Even asteroids were not demoted from being planets as this article says. Herschel, who discovered Uranus, was disparaging then because they were small (whereas Uranus that he discovered was glorious). These small bodies had no round, visible disks like Mars or Jupiter. They looked instead like points of light, like stars, through the telescopes of that day. Therefore he said they aren’t as important as planets and should be called “star-like objects” (that’s the meaning of “asteroid”). If you think about it, he was making a geophysical definition of planet, basing it on size. He was NOT basing their less-than-planet status on their being in a belt (as the unhistorical urban legends will tell you, as this article repeated) but rather on their insufficient size. And of course Herschel would never have suggested this urban legend because (remember) he is the guy that called the moons of Uranus “planets”, so obviously he never had it in his mind that to be a planet a body had to reign in its orbit. I’ve never seen anybody ever produce evidence that the scientists of that day thought being in a Belt made them non-planets. So we should stop repeating this myth about the asteroids. Even to this day they are often called “minor planets” and there is a Minor Planet Center that studies them. I just read a paper published a half century after Herschel that had “minor planets” in the title but then throughout the entire paper it called them simply “planets”, dropping the adjective. This was after dozens of asteroids were known, and it shows that the author considered minor planets to be a type of planet, not a separate category.

      Another paper (by Kirkwood) published 71 years after Herschel’s death says this, “Our solar system as them known consisted of twenty-three planets, primary and secondary; it now contains over three hundred and eighty; of which more than three hundred and fifty are asteroids.” This shows that asteroids were still considered to be planets long after they were named asteroids. It was only in 2006 with the infamous Planet definition that the IAU said that we should stop using the term “minor planet”. So let’s dispense with the urban legend. The asteroids were NOT downgraded from being planets because they were in belt. There was NO historical precedence for doing that to Pluto and the other dwarf planets in the Kuiper Belt. Dwarf planets are and always have been planets, since planetary scientists have never, ever based the definition of “planet” on dynamical considerations, either because they orbit another planet or because they are in a belt.

      Herschel’s idea of basing planet-status on size is exactly what Alan Stern and co-authors are doing today with the Geophysical Planet Definition. According to this definition, most asteroids are not planets. Now we have better telescopes plus spacecraft and we see that one of the asteroids — Ceres — actually has enough size to be a planet. Herschel didn’t know that, but he was mostly correct.

      So from the time of Galileo until now we have ALWAYS had a geophysical definition for “planet”. This has never changed. The confusion came because we developed the alternative category names “satellites” and “asteroids” for those two dynamical subcategories of planets, but the major planets were never given a special subcategory name so they were just called “planets”. The non-geoscientists at the IAU got confused by that and took it to mean that those dynamical subcategories aren’t planets. What then would they call the top level category in a dynamical classification system that also includes and satellites and dwarf planets? Somebody came up with the term “planetary bodies”. Everybody agrees that “planetary bodies” include the major planets plus the moons plus dwarf planets. If you think about it, that term is actually acknowledging them all as planets. “Planetary” means that the bodies are geophysically planets (otherwise they would be “non-planetary bodies”) and “bodies” is dynamically nonspecific so it can include moons or dwarf planets in addition to “planets” meaning the major ones. So this dynamical catch-all term “planetary bodies” has become popular in recent years, but even the language it uses acknowledges the existence of a separate geophysical definition where all the bodies are “planetary”, i.e., having the intrinsic properties of a planet. Note also that the geoscientists who study planetary bodies are called “planetary scientists” and we are doing “comparative planetology”. These terms, too, acknowledge that these bodies are all planets from a geophysical perspective.

      • Are you sure that Huygens coined the “secondary planets” term? For he named Titan “Saturni Luna” throughout. Galileo, in any case, called his moons “Sideri Medici”, which is Medicean Stars, not planets.

        I admit that the term is tempting. Last year, when I contributed to a science book on Titan, I had to actively force myself not to refer to it as a planet. Geophysically and chemically, it has much more in common with Pluto than with, say, Phobos and Deimos.

        As for the asteroids, it was Olbers who popularized Herschel’s term because he developed the theory that Ceres, Pallas and Juno were fragments of a destroyed large planet, not planetary bodies in their own right.

      • philtill777

        Codex Regina: I will look up your Huygens question, but first Galileo. He called them both. In the subtitle of Sidereus Nuncius he called them “Medicea Sidera” and he repeated that once or twice in the text. However, he often called them “planets” in the text as well, and after all his diagrams he wrote this:

        “Hê sunt observationes quatuor Mediceorum Planetarum, recens ac primo a me repertorum, ex quibus, quamvis illorum periodos numeris colligere nondum detur, licet saltem quêdam animadversione digna pronunciare.”

        which translates

        “These are my observations upon the four Medicean planets, recently discovered for the rst time by me; and although it is not yet permitted to me to deduce by calculation from these observa­tions the periods of these bodies, yet I may be allowed to make some statements, based upon them, well worthy of attention.”

        Many times he calls them planets such as here:

        “There remains the matter, which seems to me to deserve to be considered the most important in this work, namely, that I should disclose and publish to the world the occasion of discovering and observing four PLANETS, never seen from the very beginning of the world up to our own times, their positions, and the observations made during the last two months about their movements and their changes of magni­tude… ”

        I’ll reply about the others when I get a chance to chase them down.

      • I am still trying to sort that one out. The term “satellites” was definitely coined by Kepler, though I believe he actually wrote “satellite planets”. I am not sure yet who was first to use the expression “secondary planets”. It was not Huygens, for the term was already known to Kepler, as we can see here (in “Epitome astronomiae Copernicanae”): But I could not find it in Galilei’s “Sidereus Nuncius”. So maybe it was Kepler after all.

    • philtill777

      Well maybe it was Kepler after all. I thought it was Huygens only because I had read a book that attributed it to him — at least that is what I remember (maybe I remember wrongly). I was going to look for an original source but I could not find a translation of his works and sorting through the Latin would take a long time. In any case, the term was used from early times until recent times. I recently saw “primarily planet” in regard to Jupiter’s relationship with Europa in this 2009 paper:

      Collins, Geoffrey C., et al. “Tectonics of the outer planet satellites.” Planetary Tectonics11 (2009): 264.

  7. Raj Pillai

    While I think the main contents of the story is a good read from a historical European contribution to astronomy perspective. The inference of the 1st para is flawed.. While Alan Stern has championed the Pluto mission for decades leading to a successful mission from which mankind has benefited. His recent proposal for a Geo Physical based definition is NOT an attempt to reinstate Pluto’s status but to clean up the mess the IAU has left us with.. Thats Alan’s bias.. Lets put science to this definition..
    Unlike a Kavili award winner who has claimed that “he would not ALLOW Pluto to regain its planetary status AS LONG AS he is alive”.. Here is a short video proving the point (see around 1:01 min).. Thats the unscientific, personal (maybe commercial) opinions/bias planetary scientist should not have..

  8. John Roth

    As far as the controversy goes, I’d like to point out one thing: the public has a large stake in the list of planets. Let’s not forget that. If the list goes to much beyond what it is now, a lot of relatively ordinary people are not going to be happy. Try to get high school students to memorize a list of dozens of planets. Won’t happen.

    As far as a formal definition goes, I’ve got an opinion. If you’re going to consider the dynamics, go all the way. How important is the body to the dynamics of the system? How much improvement in your dynamical model would you get by adding it, how much degradation would you get by removing it? This is something that the average person can understand.

    • That would considerably simplify the Solar system as it would reduce it to two planets: Jupiter and Saturn. I remember back in the early 1970s when I was a member of the BAA and calculating cometary orbits using Merton’s method (this was in the days before electronic calculators), they were the only two planets used in the perturbation calculations. That may be due to the comets being relatively short-period, so they never got close to Uranus or Neptune.

      • John Roth

        You got good results ignoring the Sun? Egads!

        Seriously, if you’re calculating cometary orbits, that makes perfect sense. The gas giants are the only two planets that have a significant effect through the entire system – unless you’re trying to slingshot a space probe.

        I remember a text that showed how to calculate future positions using a scientific calculator. It got decent results with a plain Kepler ellipse, and added a simplified Jupiter pertubation for extra precision.

        The inner planets don’t have significant effects much past the main belt asteroids; the ice giants don’t have significant effects on the inner planets. However, to get decent asteroid ephemerides, you have to add in the four largest asteroids. There are probably some similar issues with KBOs.

      • @John Roth

        I assume you know the difference between a star and a planet and that perturbations are from the ideal elliptical orbit, but the problem with your wholly dynamical definition is that it doesn’t work in all cases as your own examples show. The purely geophysical definition of a planet is the only one that you can guarantee will work everywhere (not just in our Solar system). OK, it then adds a number of bodies that we do not consider planets now, but if Earth was a Trojan asteroid 60 degrees away from a warm Jupiter at Earth’s current distance from the Sun, would we consider it a planet or not?

        This is a very real issue because the number of exo-planets being discovered means that it is quite likely that all possible planetary configurations will occur somewhere.

    • Denise Rothstein

      We have to learn fifty states and their capitals, so a few more planets is NOT a problem.

      • John Roth

        And how many of those students in fact remembered them past the next test? I know people who couldn’t tell you what the capital of their state is. I know more than one person who got hassled by a cop for being Mexican when he was from New Mexico – and cops ought to know better.

    • An overwhelming majority of formal and informal public surveys over the last decade have shown people prefer an inclusive definition that increases the number of solar system planets to the confusing IAU definition that limits it. As the writers of the new proposal point out, memorization is not important for learning. There is no reason kids should have to memorize a list of names in studying the solar system. That is an archaic teaching method dating back to a time when little else was known about the planets other than their names.

      We don’t ask kids to memorize all the elements in the Periodic Table or all the rivers and mountains on Earth or the names of all Jupiter’s 67 moons. Similarly, we can teach kids the different types of planets and their characteristics along with the architecture of the solar system. We can also compare that architecture to that of other solar systems now that so many have been discovered.

      I’m not sure what you mean by saying, “If you’re going to consider the dynamics, go all the way.” No one is saying to ignore dynamics, just not to put dynamics ahead of an object’s intrinsic properties in defining that object. Making the simple change of recognizing dwarf planets as a subclass of planets does this by acknowledging that these objects do not gravitationally dominate their orbits while at the same time recognizing that geophysically, they are planets.

  9. Denise Rothstein

    And, Pluto should regain it’s planet status because it does revolve around a sun and no one cares about it’s size. It’s a planet period!

    • So do millions of asteroids. Do you want to upgrade them all to planets?

      • The point of the geophysical definition is to distinguish those objects that are in hydrostatic equilibrium from those that are not, as the two are very different. Asteroids are loosely held together rubble piles while planets are complex, geologically layered worlds that experience the same processes as the larger terrestrial planets. Blurring the distinction between these two types of objects makes little scientific sense–as does artificially limiting the number of solar system planets to a “convenient” number that does not reflect the diversity of what our solar system actually has.

      • ” Asteroids are loosely held together rubble piles…”
        Oh dear, if you can’t think of a reasonable answer just bullshit.

  10. You’re right, Thony, my bad. While many asteroids are rubble piles, many are not. However, even the ones that are not (I’m excluding Ceres, Vesta, and Pallas here, which are dwarf planet and protoplanets respectively, not asteroids) are remnants of planetary formation, are irregularly shaped, and do not experience the same complexity of processes as objects in hydrostatic equilibrium.

  11. Jeb

    I know nothing about Astronomy. But I would read the conclusion simply. In regard to terminology in Astronomy it demonstrates a historical tendancy towards conservatisim, in a slow moving but adaptive form.

    I can’t work out if I am wrong but it leaves you wondering if arguments here relating to terminology here, indicate that its something else that is yet to be settled and agreed on thats the issue rather than the descriptive language itself.

    It would also suggest if it is the case, the subject can role with the blows here and move on when it chooses not to mistake an argument over terminology for one about thought.

    • John Roth

      You’ve pretty much hit the issue on the head: the term “Planet” has a lot of emotion invested in it. “Planet” just seems to be more important than “star” or “satellite” or “dwarf planet” or any of the other names.

      That was the basis of my suggestion: if it’s importance you want, use something that rates its importance to the system as a whole. Not everything that’s in hydrostatic equilibrium has significant effects on the system as a whole: several of the moons of Jupiter and Saturn come to mind. Not everything that has significant effects on major parts of the system is large enough to be in hydrostatic equilibrium: three of the four largest asteroids in the main belt, for example.

      • laurel2000

        But again, your primary criterion is whether an object has effects on other objects or on the system as a whole. Advocates of the geophysical definition put primacy on objects’ intrinsic properties, not their effects on other objects. We are talking about two different ways of understanding solar system objects.

        Furthermore, according to the geophysical definition,three of the “first four asteroids” between Mars and Jupiter are not asteroids at all. Ceres is a small (dwarf) planet while Vesta and Pallas are protoplanets. This is because their complexity and closeness to hydrostatic equilibrium make them very different from the smaller asteroids.

      • _“Planet” just seems to be more important than “star” or “satellite” or “dwarf planet” or any of the other names._

        Which would seem like a sound argument for making the moon a planet, wouldn’t it? Yet somehow it feels ominous to watch a planet rising at the horizon.

      • philtill777

        The large satellites of our solar system, including Earth’s Moon, have always been called planets. When Galileo argued that the sun, not the Earth, is the center, he still called the Moon a planet. When he discovered the satellites of Jupiter he called them planets. When Huygens discovered Titan he called it a planet. When Herschel discovered Uranus’ satellites he called them planets. When Cassini discovered more satellites of Saturn he called them planets. Even to this day we call these bodies planets. Here are some examples of planetary science papers that call Earth’s moon a planet:

        1. Wanke, H., and T. Gold. “Constitution of terrestrial planets [and discussion].” Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences303.1477 (1981): 287-302.

        2. Phillips, Roger J., and Kurt Lambeck. “Gravity fields of the terrestrial planets: Long‐wavelength anomalies and tectonics.” Reviews of Geophysics 18.1 (1980): 27-7

        “We present a review of the long-wavelength gravity fields of the terrestrial planets, Earth, Moon, Mars, and Venus…”

        3. Watters, Thomas R. “Wrinkle ridge assemblages on the terrestrial planets.” Journal of Geophysical Research: Solid Earth 93.B9 (1988): 10236-10254.

        “Wrinkle ridge assemblages on the terrestrial planets…Moon, Mars, and Mercury.”

        4. Solomon, Sean C. “Formation, history and energetics of cores in the terrestrial planets.” Physics of the Earth and Planetary Interiors 19.2 (1979): 168-182.

        “The cores of the terrestrial planets Earth, Moon, Mercury, Venus and Mars differ…”

        5. Murray, Bruce, Michael C. Malin, and Ronald Greeley. “Earthlike Planets: Surfaces of Mercury, Venus, Earth, Moon, Mars.” Research supported by the John Simon Guggenheim Memorial Foundation and California Institute of Technology. San Francisco, WH Freeman and Co., 1981. 402 p. 1 (1981).

        “Earthlike planets: Surfaces of Mercury, Venus, earth, moon, Mars”

        6. Wieczorek, Mark A. “Gravity and topography of the terrestrial planets.” Treatise on Geophysics.–2007.–10 5 (2007): 165-206.

        “Gravity and Topography of the Terrestrial Planets…Earth, Venus, Mars, the Moon”

        7. Schubert, Gerald. “Subsolidus convection in the mantles of terrestrial planets.” Annual review of earth and planetary sciences 7.1 (1979): 289-342

        “Each of the terrestrial planets, Mercury, Venus, Earth, Moon and Mars…”

        I could give you many more for Europa, Ganymede, Titan, Triton, etc. planetary scientists have literall ALWAYS used a geophysical definition for planets. We have also used a dynamical definition to distinguish subcategories of planets. A satellite is a dynamical subcategory.

      • Jeb

        “You’ve pretty much hit the issue on the head”

        I suspect that just results in a headache. Danger of confirmation bias? Throatclearing. Its the answer I would give in regard to this issue in anthropology. But I may just be noting that in a means of indtifying my own existing bias and getting it out of the way as I attempt to learn something new.

        I have a bad habit of thinking out loud when I get a pattern match. Useful for me but not anyone else I suspect. Little more than a dog hears bell response while dealing with the discovery of new horrifying complexity I think.

        I have a similar issue looking at terms that track movement and specifically the terms used by experts to track movement in a different space. I have just started to review the neurobiology (well outside of my knowldge base).

        The issues looks rather complicated but that just may be down to my lack of understanding.

        It may also have a comparative value here, but again I can’t evaluate that.

  12. _Here are some examples of planetary science papers that call Earth’s moon a planet:_

    I am tempted to call that sloppy writing. I noticed how easy it is to fall into this trap when I contributed to that book on Titan. I am sure that out there, there are many papers that unintentionally refer even to Phobos and Deimos as planets.

    • philtill777

      Let’s hope you resist that temptation. i know many other planetary scientists who consider these bodies to be planets. Some have made it explicitly clear that they intend to call these bodies planets. Consider these two:

      1. Mccord, T. B., and C. Sotin. “The small planet Ceres: Predictions of current state.” EGS-AGU-EUG Joint Assembly. 2003.

      “Ceres orbits the sun and is large enough (1000 km diameter) to have experienced many of the processes normally associated with planetary evolution. Therefore, it should be called a planet even thought it orbits in the middle of the asteroid belt.”

      2. McCord, T. “Ceres: Dawn visits a warm wet planet.” European Planetary Science Congress 2014, EPSC Abstracts, Vol. 9, id. EPSC2014-96. Vol. 9. 2014.

      “Ceres likely contains considerable water, has differentiated, possesses a silicate core and water mantle, and has experienced major dimensional, thermal and chemical changes over its history, making it more a planet than an asteroid.

      And as I mentioned before, there is a long history of calling them planets. “Secondary planets” was popular until this century. We still call asteroids “minor planets”. We call ourselves “planetary scientists” when we study asteroids and moons, and our field includes “comparative planetology” for these bodies. This trend in calling them “planets” did not appear from thin air as if we were sloppy writers. We have definite ideas of what planets are. Even when we deferentially call Titan a “planetary body” to avoid the word “planet” (as in “major planet”), we are calling it a planet via the geophysical definition. The adjective “planetary” means “having the intrinsic properties of a planet” whereas “body” allows non-specificity in its dynamical state. If that weren’t so, if Titan were geophysically a non-planet, then we would have to call it a “Non-planetary body.” So even avoidance of the word “planet” involves a reference to the geophysical definition.

      Seriously, the issue here is that since Galileo there has always been a geophysical definition, while in just this past century a dynamical definition has started to become popular (partly fed by urban legends reinterpreting why the asteroids were reclassified), but the geophysical definition has always been here and has always been used and it makes much more sense for geoscientists doing comparative planetology. Even if you want to besmirch these scientists for sloppy writing, if they did it then it’s because they found the word useful, and really that’s the whole reason to invent definitions. The dynamical definition has no scientific value. The concept is valid, but why would scientists need this word defined as “clearing orbits” for their scI.e. To fix community to function better? They don’t. The whole IAU definition thing was motivated by a turf war over which IAU committe gets to name the
      That doesn’t count as a real need to improve science. The dynamical definition is also feeding on an outdated cultural desire (by some) to have a small number of “god-like” bodies ruling the solar system. The concept behind that is not that important to reach the public. A more important concept is that the solar system is dynamic and evolving and that bodies get shoved around, so even giant planetsthat once cleared their orbits later get ejected. This current orbit-clearing definition does the public a disservice by maintaining an old view of the solar system. Nobody even in the field of dynamics needs a special category-word that includes only these 8 bodies, since we all know what they are anyway.

      • It’s maybe a language bias. In German, we apply the term “celestial body” (Himmelskörper) to cover both planets and satellites, and that’s what I used in the case of Titan. I agree with regard to Ceres (my current book project), though, particularly considering the evidence that it might be a stray KBO and as such, deserved to be called a plutoid. 🙂

        However, I don’t think anybody is going to rewrite history and declare Neill Armstrong the first man who stepped on another planet. That privilege will belong to whoever steps first on Mars. That’s why I think Alan Stern’s proposal doesn’t stand much of a chance, as much sense as it makes from a strictly physical/chemical point of view.

        I don’t think that god-like bodies are the issue, despite astrology. It is rather the fear that American pupils may be intrinsically incapable of remembering more than nine names in a row. 😉 Anyway, the dwarf planet definition is a major issue only in America while people shrug it away with an amused laughter over here in Europe (according to my experience from giving public lectures). That’s another reason why I have doubts that Alan Stern’s proposal has chances: in the rest of the world, NOBODY CARES!

      • laurel2000

        Actually, a lot of people DO care and not just Americans. People all over the world, including Europe, don’t like the IAU definition, and many are ignoring its claim that dwarf planets are not planets and instead choosing to treat them as a subclass of planets. I’ve done public lectures too, as have many professional planetary scientists, and people hardly “shrug off” the issue. They always raise it and not with amusement but with confusion.

        We’ve already established there is NO need to memorize a list of planet names! American pupils will not have to memorize a list of 110 names because that is an archaic method of teaching that belongs to another time. Therefore, the claim that our solar system cannot have “too many planets” because it would pose memorization problems is essentially a straw man.

        The geophysical definition proposal, which is not solely Alan Stern’s proposal, absolutely stands a great chance of at least being used side-by-side with the IAU definition. You are assuming they plan to bring it before the IAU, which they do not, as they inherently reject the notion that only the IAU has the right to decide this. Again, the majority of IAU members are not even planetary scientists! And the IAU leadership has done nothing but dig in its heels and express determination to never again reopen this issue regardless of new discoveries and data. That is hardly a rational position.

        Instead, planetary scientists, who absolutely do care about planet definition, are likely to adopt this by consensus, essentially resulting in a more formal situation of what we have now–two separate definitions being in legitimate use, one by astronomers and the other by planetary scientists. That is not necessarily a bad thing.

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