A Recipe for Returning Pluto to Full Planethood

A storm is brewing, a battle of words and a war of the worlds. The Earth is not at risk. It is mostly a civil dispute, but it has the potential to influence the path of careers. In 2014, a Harvard led debate was undertaken on the question: Is Pluto a planet. The impact of the definition of planet and everything else is far reaching – to the ends of the Universe.

It could mean a count of trillions of planets in our galaxy alone or it means leaving the planet Pluto out of the count – designation, just a dwarf planet. This is a question of how to classify non-stellar objects. What is a planet, asteroid, comet, planetoid or dwarf planet? Does our Solar System have 8 planets or some other number? Even the count of planets in our Milky Way galaxy is at stake.

"Dawn arising." The latest image of Ceres - February 12, 2015 -  by the Dawn spacecraft from 80,000 km. With icy deposits pock marking its surface, a possible reservoir of water below its surface, is Ceres a planet, dwarf planet, an asteroid or all three? (Credit: NASA/Dawn)
“Dawn arising.” The latest image of Ceres – February 12, 2015 – by the Dawn spacecraft from 80,000 km. With icy deposits pock marking its surface, a possible reservoir of water below its surface, is Ceres a planet, dwarf planet, an asteroid or all three? (Credit: NASA/Dawn)

Not to dwell on the Harvard debate, let it be known that if given their way, the debates outcome would reset the Solar System to nine planets. For over eight years, the solar system has had eight planets. During the period  1807 to 1845, our Solar System had eleven planets. Neptune was discovered in 1846 and astronomers began to discover many more asteroids. They were eliminated from the club. This is very similar to what is now happening to Pluto-like objects – Plutoids. So from 1846 to 1930, there were 8 planets – the ones as defined today.

The discoverer of Pluto - Clyde Tombaugh in the 1930s and again with homebuilt telescope in the 1990s that earned him an assignment at Lowell Observatory - discover Planet X. Cremated remains of Clyde are attached to the New Horizons space probe now approaching the dwarf planet Pluto.
The discoverer of Pluto – Clyde Tombaugh in the 1930s and again with homebuilt telescope in the 1990s that earned him an assignment at Lowell Observatory – discover Planet X. The cremated remains of Clyde are attached to the New Horizons space probe that is now approaching the dwarf planet Pluto.

In 1930, a Kansas farm boy, Clyde Tombaugh, hired by Lowell Observatory discovered Pluto and for 76 years there were 9 planets. In the year 2006, the International Astronomical Union (IAU) took up a debate using a “democratic process” to accept a new definition of planet, define a new type – dwarf planet and then set everything else as “Small Bodies.” If your head is spinning with planets, you are not alone.

All two body systems have a barycenter, the shared point in space around which they orbit. Pluto and Charon’s happens to be between both bodies due to their proximity and similar mass. (Credit: NASA/New Horizons)

Two NASA missions were launched immediately before and after the IAU announcement took affect. The Dawn mission suddenly was to be launched to an asteroid and a dwarf planet and the New Horizons had rather embarked on a nine year journey to a planet belittled to a dwarf planet – Pluto. Principal Investigator, Dr. Alan Stern was upset. Furthermore, from the discoveries of the Kuiper mission and other discoveries, we now know that there are hundreds of billions of planets in our Milky Way galaxy; possibly trillions. The present definition excludes hundreds of billions of bodies from planethood status.

The presently known largest trans-Neptunian objects (TSO) - are likely to be surpassed by future discoveries. Which of these trans-Neptunian objects (TSO) would you call planets and which "dwarf planets"? (Illustration Credit: Larry McNish, Data: M.Brown)
The presently known largest trans-Neptunian objects (TSO) – are likely to be surpassed by future discoveries. Which of these trans-Neptunian objects (TSO) would you call planets and which “dwarf planets”? (Illustration Credit: Larry McNish, Data: M.Brown)

There are two main camps with de facto leaders. One camp has Dr. Mike Brown of Caltech and the other, Dr. Stern of the Southwest Research Institute (SWRI) as leading figures. A primary focus of Dr. Brown’s research is the study of trans-Neptunian objects while Dr. Sterns’s activities are many but specifically, the New Horizons mission which is 6 months away from its flyby of Pluto. Consider first the IAU Resolution 5A that its members approved:

(1) A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.

(2) A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

This is our starting point – planet, dwarf planet, everything else. Consider “everything else”. This broad category includes meteoroids, asteroids, comets and planetesimals. Perhaps other small body types will arise as we look more closely at the Universe. Within the category, there is now a question of what is an asteroid and what is a comet. NASA’s flybys of comets and now ESA’s Rosetta at 67P/Churyumov–Gerasimenko are making the delineation between the two types difficult. The difference between a meteoroid and an asteroid is simply defined as less than or greater than one meter in size, respectively. So the Chelyabinsk event absolutely involved a small asteroid – about 20 meters in diameter. Planetesimals are small bodies in a solar nebula that are the building blocks of planets but they could lead to the creation of all the other types of small bodies.

Dr. Alan Stern, project scientist for New Horizons and Neil deGrasse Tyson discuss the New Horizons spacecraft in the mission operations center at JHU/APL. The interview was for a NOVA special (12/14/2011), the Pluto Files, about a Kansas farm boy, a missing planet and the 70 years of astronomical discoveries leading to the present day. (Credit: JHU/APL,PBS)
Dr. Alan Stern, project scientist for New Horizons and Neil deGrasse Tyson discuss the New Horizons spacecraft in the mission operations center at JHU/APL. The interview was for a NOVA special (12/14/2011), the Pluto Files, about a Kansas farm boy, a missing planet and the 70 years of astronomical discoveries leading to the present day. (Credit: JHU/APL,PBS)

Putting aside the question of “Small Bodies” and its sub-classes, what should be the definition of planet and dwarf planet? These are the two terms that demoted Pluto and raised Ceres to dwarf planet. It is also interesting to note how Resolution 5A is meant exclusively for our Solar System. In 2006, there were not thousands of exo-planets but just a few dozen extreme cases but nevertheless, the IAU did not choose to extend the definition to “stars” but rather just in reference to our pretty well known star, the Sun.

Recall Tim Allen’s movie, “The Santa Clause”. Clauses can cause a heap of trouble. The IAU has such a clause – Clause C which has caused much of the present controversy around the definition of planets. Clause (c) of Resolution 5A: “has cleared the neighborhood around its orbit.” This is the Pluto killer-clause which demoted it to dwarf planet status and reduced the number of planets in our solar system to eight. In a sense, the IAU chose to cauterize a wound, a weakness in the definitions, that if left unchanged, would have led to who knows how many planets in our Solar System.

The question of what is Pluto is open for public discussion so armed with enough knowledge to be dangerous, the following is my proposed alternative to the IAU’s that are arguably an improvement. The present challenge to Pluto’s status lies in the Kuiper Belt and Oort Cloud. Such belts or clouds are probably not uncommon throughout the galaxy. Plutoids are the 500 lb gorilla in the room.

Two spacecraft, Dawn and New Horizon will reach their final objectives in 2015 - Dwarf Planets - Ceres and Pluto. (Credit: NASA, Illustration - T.Reyes)
Two spacecraft, Dawn and New Horizon will reach their final objectives in 2015 – Dwarf Planets – Ceres and Pluto. (Credit: NASA, Illustration – T.Reyes)

This year, as touted by the likes of Planetary Society, Universe Today and elsewhere, is the year of the dwarf planet. How remarkable and surprising will the study of Ceres, Pluto and Charon by NASA spacecraft be? There is a strong possibility that after the celestial dust clears and data analysis is published, the IAU will take on the challenge again to better define what is a planet and everything else. It is impossible to imagine that the definitions can remain unchanged for long. Even now, there is sufficient information to independently assess the definitions and weigh in on the approaching debate. Anyone or any group – from grade schools to astronomical societies – can take on the challenge.

To encourage a debate and educate the public on the incredible universe that space probes and advanced telescopes are revealing, what follows is one proposed solution to what is a planet and everything else.

planet: is a celestial body that a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium – nearly round shape, b) has a differentiated interior as a result of its formation c) has insufficient mass to fuse hydrogen in its core, d) does not match the definition of a moon.

minor planet: is a planet with a mass less than one Pluto mass and does not match the definition of a moon.

inter-Stellar (minor) planet: is a (minor) planet that is not gravitationally bound to a stellar object.

binary (minor) planet: is a celestial body that is orbiting another (minor) planet for which the system’s barycenter resides above the surface of both bodies.

These definitions solve some hairy dilemmas. For one, planets orbit around the majority of most stars in the Universe, not just the Sun as Resolution 5A was only intended. Planets can also exist gravitationally not bound to a star –  the result of it own molecular cloud collapse without a star or expulsion from a stellar system. One could specify gravitational expulsion however, it is possible that explosive events occur that cause the disintegration of a star and its binding gravity or creates such an impulse that a planet is thrusted out of a stellar system. Having an atmosphere certainly doesn’t work. Astronomers are already anticipating Mars or Earth-sized objects deep in the Oort cloud that could have no atmosphere – frozen out and also despite their size, not be able to “clear their neighborhood.”

An animation (above) of Kepler mission planet candidates compiled by Jeff Thorpe. Kepler and other exoplanet projects are revealing that the properties of planets – orbits, size, temperature, makeup – are all extreme. Does Pluto represent one of those extremes – the smallest of planets? (Credit: NASA/Kepler, Jeff Thorp)


The need to create a lower-end limit to what is a planet reached a near fever pitch with the discovery of a Trans-Nepturnian Object (TNO) in 2005 that is bigger than Pluto – Eris.  Dr. Michael Brown of Caltech and his team led in the discovery of bright large KBOs. There was not just Eris but many of nearly the same size as Pluto. So without clause (c), one would be left with a definition for planet that could allow the count of planets in our Solar System to rise into the hundreds maybe even thousands. This would become a rather unmanageable problem; the number of planets rising year after year and never settled and with no means to make reasonable comparisons between planetary systems throughout our galaxy and even the Universe.

The book that tells the story of discovery - trans-Neptunian objects (TNO) that led to the downfall of Pluto from full planethood to that of a dwarf. The 2006 IAU decision was a pre-emptive strike to stave off a proliferation of planets in our system. It worked but "killed" Pluto. Did it have it coming? Dr. Brown also agrees that the present definition of planet is flawed and incomplete. (Photo Credits: Caltech/M.Brown)
The book that tells the story of discovery – trans-Neptunian objects (TNO) that led to the downfall of Pluto from full planethood to that of a dwarf. The 2006 IAU decision was a pre-emptive strike to stave off a proliferation of planets in our system. It worked but “killed” Pluto. Did it have it coming? Dr. Brown also agrees that the present definition of planet is flawed and incomplete. (Photo Credits: Caltech/M.Brown)

Two more celestial body types follow that are proposed to round out the set.

moon: is a celestial body that a) orbits a (minor) planet and b) for which the barycenter of its orbit is below the surface of its parent (minor) planet.

This creates the possibility of a planet-moon system such that its barycenter is above the surface of the larger body. Pluto and Charon are the most prominent case in our Solar System. In such cases, if one body meets the criteria of a (minor)planet, then the other body can also be assessed to determine if it is also a (minor) planet and the pair as binary (minor) planets. If the primary body was a minor planet, it is possible that the barycenter could be above its surface but the secondary body does not meet all the criteria of a minor planet, specifically “differentiated interior”.

The definition of moon is compounded by the existence of, for example, asteroids with moons. For such objects, the smaller object is defined as a satellite.

Satellite: is a celestial body that a) orbits another celestial body, b) whose parent body is not a (minor) planet.

Another permissible term is moonlet which could be used to describe both very small moons such as those found in the Jovian and Saturn systems or a small body orbiting an asteroid or comet. Moonlet could replace satellite.

The discriminator between planet and moon is not mass but simply whether the celestial body orbits a (minor) planet and the barycenter resides inside the larger body. The definition of moon excludes the possibility of a planet orbiting another planet except in the special case of binary (minor) planet.

Defining a lower size limit to “Planet” is necessary to compare stellar systems and classify. A limit based on the body’s average surface pressure and temperature or the surface gravity could define a limit. While they could, they are not practical because of the extremes and diverse combinations of conditions. Strange objects would fall through the cracks.

Removing clause (c) – “has cleared the neighborhood around its orbit” – will avoid a future conflict such as a very low mass star with a plutoid-sized object or smaller, in a close orbit that has cleared its neighborhood.

Additionally, choosing to declare that Pluto becomes the “standard weight” that differentiates minor planet from planet sets a precedent. In an era in which computers measure and tally the state of our existence, setting this limit to include Pluto and return it as the ninth planet of our Solar System, is, in a small but significant way, a re-declaration of our humanity. Soon we will be challenged by artificial intelligence greater than ours; we are already have. Where will we stand our ground?

Forget about Pluto for a moment. Should Eris be our tenth Planet? Like Pluto it has a prominent moon- Dysnomia. Human perception and conceptions of the Universe have shaped our view of the Solar System. The Ptolemaic system (Earth centered), Kepler's Harmonic Spheres, even the fact that ten digits reside on our hands impact our impression of the Solar System (Photo Credits:NASA/ESA and M. Brown / Caltech)
Forget about Pluto for a moment. Should Eris be our tenth planet? Like Pluto it has a prominent moon- Dysnomia. Human perception and conceptions of the Universe have shaped our view of the Solar System. The Ptolemaic system (Earth centered), Kepler’s Harmonic Spheres, even the fact that ten digits reside on our hands impact our impression of the Solar System (Photo Credits:NASA/ESA and M. Brown / Caltech)

The consequences of this proposed set of definitions, makes Ceres a minor planet and no longer an asteroid. Many trans-Neptunian objects discovered in this century become minor planets. Of the known TNOs only Pluto and Eris meets the criteria of planet.The dwarf planet Eris would become the tenth planet. Makemake, Sedna, Quaoar, Orcus, Haumea would be minor planets. By keeping Pluto a planet and defining it as the standard bearer, only one new planet must be declared. Surely, more will be found, very distant, in odd elliptical and tilted orbits. The count of planets in our solar system could rise by 10, 20 maybe 50 and perhaps this would make the definition untenable but maybe not. So be it. New Horizons will fly by a dwarf planet in July but this should mark the beginning of the end of the present set of definitions.

Three perspectives of a ten planet Solar System. No longer Earth centered, or with harmonic spheres but now with planets outside the ecliptic plane and growing. How many planets would be too many? (Credits: Wikimedia, T.Reyes)
Three perspectives of a ten planet Solar System. No longer Earth centered, or with harmonic spheres but now with planets outside the ecliptic plane and growing. How many planets would be too many? (Credits: Wikimedia, T.Reyes)

This set of definitions defines a set of celestial bodies that consistently covers the spectrum of known bodies. There is the potential of exotic celestial objects that are spawned from cataclysmic events or from the unique conditions during the early epochs of the Universe or from remnants of old or dying stellar objects. Their discovery will likely trigger new or revised definitions but these definitions are a good working set for the time being. Ultimately, it is the decision of the IAU but the sharing of knowledge and the democratic processes that we cherish permits anyone to question and evaluate such definitions or proclamations.To all that share an interest in Pluto as or as not a planet raise your hand and be heard.

A video from 2014 by Kurz Gesagt describing the Pluto-Charon system. Is this a binary planet system or one of the “dwarf” variety?

Further Reading

Learn All About Pluto, The Most Famous Dwarf Planet, E. Howell, Universe Today, 1/17/2015

A synopsis of Pluto facts and figures at Universe Today, compendium of pages on Pluto

What is the Kuiper Belt?, video, Universe Today, 12/30/2013, Fraser Cain asks Mike Brown to explain the Kuiper Belt

Is The Moon A Planet?, E. Howell, Universe Today, 1/27/2015

It Looks Like These Are All the Bright Kuiper Belt Objects We’ll Ever FindUniverse Today, 1/12/2015

2015, NASA’s Year of the Dwarf Planet, Universe Today, 12/14/2014

A Serendipitous All Sky Survey For Bright Objects In The Outer Solar SystemCornell University Library, 1/5/2015

Ten Years of Eris, at Mike Brown’s Planets, 1/5/2015

My condolences to the friends and family of Tammy Plotner, the first regular contributing writer to Universe Today. Can’t we all relate to what drew Tammy to write about the Universe? She wrote outstanding articles for U.T.


55 Replies to “A Recipe for Returning Pluto to Full Planethood”

  1. How about:
    What would the atmoshperic composition be at Goldilocks distance.

    Although, I liked the kinetic mass balance equation a bit more.
    If you smashed the Moon into Earth gives different result than if you smashed Charon into Pluto.
    Earth2* would be left afterwards, but no Pluto2.

    *(Plus lots of rubble ofc)

  2. There are several resources that should be added to your list:

    The Case for Pluto, a book by Alan Boyle: http://www.amazon.com/The-Case-Pluto-Little-Difference/dp/0470505443

    Is Pluto A Planet? a book by Dr. David Weintraub: http://press.princeton.edu/titles/8247.html

    Pluto Confidential, a book by Laurence A. Marschall. who voted for the IAU decision, and Steve Maran, who opposes it: http://www.amazon.com/Pluto-Confidential-Insider-Account-Ongoing/dp/1933771801

    “Responding to the IAU: Pluto and the Developing Landscape of the Solar System,” Laurel’s Pluto Blog: http://laurelsplutoblog.blogspot.com/2013/02/responding-to-iau-pluto-and-developing.html

    I will raise several points in response to your discussion. Essentially, your planet definition is the same as that proposed by Dr. Ken Croswell, who argues for setting the minimum threshold for planethood at the mass and size of Pluto. http://www.kencroswell.com/NinthRockFromTheSun.html

    While removing the “requirement” that an object clear the neighborhood of its orbit is a smart and necessary choice, the schematic you and Croswell support still has the problem of placing two unlike types of objects in the “minor planet category.” Objects in hydrostatic equilibrium, even those smaller than Pluto, are complex worlds, often with geology, weather, and layering. These things make them very different from tiny, shapeless asteroids and comets, which are essentially rubble piles of rock and/or ice. This is why I recommend keeping the term “dwarf planet” but amending the definition to include dwarf planets as a subclass of planets. That will encompass Ceres, Haumea, Makemake, Eris, and any future spherical objects found in our solar system and others, noting they are intrinsically akin to the larger planets and different from asteroids.

    It is disappointing to once again read a statement that was proven erroneous more than four years ago–specifically, that Eris is bigger than Pluto. While this was initially thought to be so, in November 2010, a team of astronomers led by Dr. Bruno Sicardy obtained a more accurate measurement of Eris when it occulted a star, and found it to be marginally smaller than Pluto.

    There is also a little too much promotion of Mike Brown and his book in this article. What many do not know is that Brown was one of a team of three astronomers who discovered Eris in 2003, and his colleagues do not necessarily share his view about Pluto and planet definition. Dr. Chad Trujillo has made no public statement on the issue while Dr. David Rabinowitz actually signed a petition of 300 professional astronomers led by Stern rejecting the IAU decision.

    What is also troubling is the degree of legitimacy and authority you give to the IAU in this matter, neither of which they deserve. Saying our solar system has had only eight planets for the last eight years assumes the IAU is the sole arbiter of what a planet is and how many there are. The same is true of your statement, “Ultimately, it is the decision of the IAU…” when it comes to defining the term planet.

    Science is not done through decrees by authority. No one has a monopoly on the truth to the point that their statement should be considered unquestioned fact. Only four percent of the IAU even voted in 2006, and most are not planetary scientists but other types of astronomers. Their decision was immediately opposed by hundreds of professional astronomers in a formal petition led by Stern. We have an issue with two camps, and neither should be arbitrarily given sole legitimacy simply because they want it.

    The IAU leadership has been asked repeatedly to revisit this issue and has repeatedly refused. This led many planetary scientists to boycott the 2009 and 2012 General Assemblies. By choosing not to fix its mistake and even to make claims that the issue will never be reopened, the IAU has effectively abdicated responsibility in this area. This is why many in the field now advocate the creation of a separate planetary science organization to deal specifically with the burgeoning field of planetology. A definition of planet should be made by those who study planets, not by those who study quasars or galaxies, or any of the many other fields of astronomy. As Stern often quips, a podiatrist and a brain surgeon are both doctors, but one would not go to the podiatrist for treatment of a brain tumor.

    The planet definition that makes the most sense is the geophysical one, according to which a planet is any non-self-luminous spheroidal body orbiting a star, free floating in space, or even orbiting another planet. Spherical moons can be essentially considered secondary or satellite planets.

    There is absolutely no scientific basis for objecting to a large number of solar system planets, even if that runs into the hundreds or thousands. Does anyone object to there being billions of stars or billions of galaxies in the universe? Does anyone say Jupiter can have only four moons because kids won’t be able to memorize 67 names? Memorization is not important for learning; memorizing a list of planet names dates back to the days when little was known about the solar system’s planets beyond their names. Today, that is no longer true. Instead of having kids memorize a list of names, we should teach them the different subcategories of planets and their defining characteristics. More subcategories can be added upon the discovery of planets that do not fit the ones we have. So what if that means we have no set number of planets because more are continually being discovered? Why is that a problem? People of all ages are generally excited by the discovery and addition of new planets, which therefore presents new opportunities for astronomy outreach.

    No planet definition should ever be set in stone or never revisited. As we discover more and stranger objects in this solar system and others, the definitions we use will need to evolve to encompass the latest discoveries.

    1. Laurele. Thank you so much for your comment. Yours and the following comments reveal the complexity of the problem of defining planet and everything else. The fact that so many people consider this important shows that we really are beginning to step out into the Universe, beyond Earth. The references you list are important. Thanks. Calling most of everything a planet including, for example, Titan, is not the right choice, in my opinion. Titan and Earth-sized or larger “moons” of distant super Jupiters were creations of that planets gravity well and accretion process. They should be classified as just moons of a planet. If Jupiter and its moons were flung out of the Solar System, again, one would have an interstellar planet with moons. If Titan were expelled from Saturn by some gravitational influence but remain in orbit around the Sun, then I think it would be fair to reclassify Titan as a planet. Interesting, fun and great problem. Imagine how much better we will all understand the cosmos. The problem is certainly made more complicated by IAU execution and subsequent behavior on the matter. I would agree that maybe we need to consider some other organization for resolving the matter.

  3. I feel that Pluto is not a “proper” planet. Pluto-Charon is too much different from the “real” planets. And it doesn’t dominate.

    Then again, if an Earth-sized world was found orbiting Sun farther out, I would probably call it a planet, even if it hadn’t (yet) “cleared its neighborhood” and had a tilted non-circular orbit.

    1. What makes something a “proper planet?” Both Pluto and Charon have the same processes and structures as their larger counterparts. The only difference is they are smaller.

      Many giant exoplanets have orbits around their stars that are far more elliptical than Plutos. If an elliptical orbit precludes an object from being a planet, what then are these objects?

      Why should dominating its orbit be a requirement for planethood? Such a requirement is inherently biased against objects further from their parent stars, which have larger and larger orbits to clear. Planets should be defined by their intrinsic properties, not by their locations.

  4. So basically they want to claim anything as massive as Pluto is a planet? Yuck, what do you do with the countless objects that will be discovered where we simply do not know if an object has a diameter comparable to Pluto much-less that the object is more massive than Pluto? You are just trading one arbitrary solution for another. Leave well enough alone.

  5. I dunno. I admit to not reading the full article because this debate has never been interesting to me. The solar system is what it is and we should study it, but explain to me how this is any more than semantics? Who cares one way or the other?

    1. It is partly semantics but to treat it only as such is to dismiss the physics and fundamental nature of celestial bodies. The classification system that we discuss strikes at the nature of the universe. After so many thousands of forgotten generations of humans and our predecessors, we reach this point and are attempting to name, classify and compare the building blocks of the cosmos should be considered a great privilege and undertaken with a sense of awe. Yes, maybe we need a new organization to define these terms fairly and correctly.

  6. This is a battle which cannot be won. How long is a piece of string?

    I imagine how someone at that infamous IAU meeting asked: “What is a planet, really?” And he got the embarrassing answer: “We don’t know. Oops, let’s make something up and vote about it so that the public doesn’t get the impression that we don’t know what we are talking about!” And from there on.

    What a planet is, is culturally defined. The week has seven days, one for each of the planets. Including the Sun and the Moon, but not Earth because it wasn’t until the 17th century that it was discovered that the Earth is a planet. Voting or defining by some astronomer at some meeting doesn’t change this. They should keep to their astronomy instead of making fools of themselves. But it is a funny show!

    1. “Voting or defining by some astronomer at some meeting doesn’t change this. They should keep to their astronomy instead of making fools of themselves.”

      … Erm… I’m REALLY curious about how you think that Astronomers attempting to add a new and clearly necessary iteration to a measuring system specific to their discipline would be NOT sticking to their astronomy… This is clearly no different than when geologists chiseled the list of how the different sizes and types of rocks would be labeled, when physicists were breaking down their categorization of the particle world and biologists were dissecting the veritable zoo of cellular structures. The implication of your snide venom is obviously flawed… Maybe consider WHAT you’re saying more in your comments rather than how witty/clever/ironic you think they are?

      1. Do any exoplanets exist? Or are they maybe exodwarfplanets? Is a hot super-Jupiter which has not cleared its orbit a “dwarf planet”? By the way, how useful is a categorization of objects which includes both Mercury and Saturn? You could study Titan or Ceres up close and still have no idea how IAU classifies it.

        The IAU should not have messed with this. They unfortunately tried, and they failed miserably and became everyone’s laughing stock. Or do you have a definition of how long a piece of string is?

        (I politely ignore your OT trolling attacks on me personally.)

  7. Good to see someone take a more comprehensive “look” at this issue.. While the author tries to address “all” issues.. the article could have been more “fair and balanced”..
    #1: The IAU’s “democratic” voting should have been more elaborated.. ie: just 4% of the membership voting on the last hour is sorta ramrodding a “special interest” viewpoint.. Well documented..
    #2: The author should not only mention Brown’s book but also mention Alan Boyle’s book.. The pictorial display of Brown’s book can be construed as an ad for it.. the more fair balanced approach is just mention both books.. or pictorially display Boyle’s book too.
    #3: Asking for a public debate between Neil DeGrasse & Alan Stern would have been more “forwarding” this topic than just a pic of them both..
    #4: Counting and remembering names beyond 10 planets is not hard.. we US citizens quite easily remember 50 States.. !!! .. its the proper definition that is key..
    #5: The definition of a minor planet “below a mass” of a certain planet cries the need for a new definition which is not “name” specific. A Chihuahua and St. Bernard are both dogs..
    #6: Would have been nice.. if the author had mentioned about the “Geo Physical” definition of a planet and in some details.. as that’s the contra IAU definition viewpoint..

    This planetary definition should be revisited after all the science data is in from Ceres & Pluto missions.. and then too one needs revisit the faith in the IAU and its ability to do fair job with this “definition”.

    Due to better and more sophisticated star gazing techniques, the rate of discoveries have become more rapid w/scientists being able to view data w/o even being at the observatories (unlike how Tombaugh did so painstakingly).. maybe Tombaugh would have discovered more objects w/the currently available techniques..
    This whole class of Kuiper and Oort Cloud objects along w/all the Exo-Planet discoveries being reported.. calls for (maybe) a new Organization of just Planetary Scientists responsible for “crafting” this new definition.. But then, this time.. let the public review and comment on it too.. rather than be subject to this last hour ramrodding of a definition..

    Let Science take over NOW.. and lets enjoy these 2 wonderful missions (Dawn & New Horizons) and text books it will write.. Rarely has one felt such ground swell of excitement and anticipation of the unknown.. this time w/young kids rooting and hoping these missions will prevail.. I am sure the next “Carl Sagans” will come from those kids.. We need more such discovery missions..

    1. Astronfan, as with Laurele and everyone, thanks for your comments. What a great problem to resolve and how great it is to be in the era that will reach some first set of definitions, a foundation at least upon which sound revisions can be made. I would not put Science on that high pedestal. What you are saying is ‘let scientists take over’ and that just is a return to demagoguery. The IAU’s misteps is an opportunity to improve processes hopefully one in which many can partake in the discourse. However, I would agree – that we should not become so shrill that we lose site of the incredible journey and exploration of two new worlds that is happening now. Let’s hope that our two machines do not fail us and breakdown at inopportune moments.

    2. “…we US citizens quite easily remember 50 States”

      Unless you’re over the age of 65 or a geek, I’m afraid that more Americans than not aren’t even aware of which is North and which, South… between Canada and Mexico, save all 50 states names. This has been tested.

      “The definition of a minor planet “below a mass” of a certain planet cries the need for a new definition which is not “name” specific. A Chihuahua and St. Bernard are both dogs..”

      It’s simply a case of a science suddenly discovering many more of a certain type of thing (in our Solar System and others), which forces the need to add classification terms to make organizing the data easier. House keeping. All the sciences do this for convenience of communication within their ranks about a subject. INDEXING, for Zod’s sake. This debate about the debate is simply based on uninformed opinions and who can make more noise. It’s silly. Stop and let the people do their work.

  8. Of course they are all planets and there is nothing wrong with finding more of them. Beyond silly ..
    I don’t think using Pluto’s size as a standard planetary margin is such a great idea though. It should be something from basic physics and not based on one example.

    1. Did you see the infographic in this article representing the largest of the even more populous list of known Dwarf Planets? There alone are ten, not one, not even counting those objects’ satellites. And one is even larger than Pluto. That’s the whole point. More and more of these objects, let alone planet types per extra solars, are being discovered every day and because of this, Astronomy’s data classification system is on the verge of getting VERY cluttered and disorganized. It’s like why there are “silt”, “sand” “pebbles”, “rocks”, “boulders”, etc. and all relegated to specific size ranges, in geology. All the sciences do this to make it easier to be specific when they communicate within their ranks about their datasets. It’s not, “silly” at all. It’s necessary and been happening as long as there’s been science.

      1. This story is more about Ceres than Pluto and about organizing things based on actual knowledge and methods.
        It’s just that it seems that public sentiment, mine included for a change, which defines how words are used, puts more emphasis on planet being round and orbiting a star, than it’s housekeeping manners.
        Or in other words, they are all planets : dwarves, proto, giants, gaseous, rocky : if it’s round and orbits a star – planet, simple.
        If you feel you need more precise definitions, figure out which gas is most prone to gravitational collapse, and calculate it’s mass when it gets round with no rotation.
        That should work anywhere.

      2. I dig the public sentiment thing and as far as that goes, whatever anyone wants to call the things in common conversation is all well and good. I was commenting much more on the comments about why the scientists are debating this than on the story itself. What I said was the actual reason Astronomers are going through this process as has been asked and wondered about and commented on in this thread.

    2. The first definition – a planet :

      “.. is a celestial body that a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium – nearly round shape, b) has a differentiated interior as a result of its formation c) has insufficient mass to fuse hydrogen in its core, d) does not match the definition of a moon.”

      Is absolutely spot on right to me.

      There are many types of planet ranging from gas giants like Jupiter though to planets like Earth and also planets like Pluto.

      A dwarf star is still a star and a dwarf planet is still a planet just as a dwarf person is still a person – wherever they may live and however small they may be!

      I think the IAU made a terrible mistake when they demoted Pluto,Eris and Ceres (among others) and I think its long overdue that that error on their part was acknowledged and corrected.

  9. Saying dwarf planet seems to be very subjective. It is still a planet as the Inclusion of the name planet is still in the name dwarf planet. To me that means Pluto is a planet! Just as a person referred to as dwarf does not mean that they are not a person. We love to label and categorize so much. If such a small world was in the goldilocks zone and had intelligent life how could we justify saying it is not a “real” planet. Given enough atmosphere with the right elements any decent moon with a comfortable temperature could support life. Are we not speculating life on the moons of Jupiter. Somewhere that supports life would be a world. A world is better than a planet no matter how big.

  10. Back in 2006, right after the reclassification of Pluto as a dwarf planet, I was buying gasoline at a local station. The female attendant noted my “Pluto is the Coolest Planet” tee shirt, expressed her fierce disapproval of what had transpired with Pluto, and then growled “There will be consequences for this!”

    She was right. People love Pluto. Don’t mess with it!

  11. Sorry, but this proposal is a piece of cr*p. Why the heck would anyone choose “one Pluto mass” as the yardstick for anything, except for misguided sentimental reasons???

    And why stop there? Why exclude Charon? After all, why should Pluto be a planet, but Charon not? What about the other KBOs? And why not include Ceres and Vesta as planets? Why not include our Moon and the Sun? I’m sure there are people out there still rejecting the heliocentric worldview, should we next hear what they have to say? And why not include planets existing only in the imagination of some people?

    I’m all for hearing a reasonable attempt to redefine what is and isn’t a planet, but sorry, “one Pluto mass” is just arbitrary sentimentalism.

    1. @ ^ Tony Mach, Um, didn’t you read the proposed planet definition provided in the article there?

      It isn’t just Pluto and it is far superior to the current rather absurd IAU definition. The definition there would add the other ice dwarfs incl. Ceres, Eris , Haumea, Sedna etc .. and I and other supporters of that definition have no problem with that at all.

      1. No, the definition suggested in the article would not add the other current dwarf planets- the author clearly states it would only elevate Eris. Making Pluto’s mass the dividing line is an arbitrary and misguided choice.

  12. Come on IAU, Pluto is no comet, no asteroid and very much a planet worthy of that title and recognition as such. Pluto has for instance its own weather, more moons than the whole four inner solar system rock dwarfs combined and maybe rings and more as well.

    Its time to recognise reality and acknowledge that all dwarf planets (whether icy or rocky – remember Earth couldn’t clear its orbit at Pluto’s distance either!) are planets too every bit as much as their bigger brethren!

  13. “Planet” should be recognized to be an unscientific term but rather a historical term. This should then be understood to include only the nine planets.

    Then you either choose another term for the other stuff or use modifiers. So exoplanets, dwarf planets, Oort planets, etc. should be OK.

    The Harvard outcome makes more sense than does what a non-quorum of the IAU set up.

    1. Historically a planet was any object that moved against the background of fixed stars and thus included the Sun and Moon.

  14. Elephant in the room: Haumea.

    While it’s not actually spherical, it is low mass, so if you do a bit of hand-waving, it would be spherical , if it was a bit more massive than it actually is. So, Bob’s your uncle, it’s a dwarf planet too.


    1. The issue isn’t whether an object is exactly spherical; it’s whether it is massive enough to be shaped by its own gravity rather than by its chemical bonds. Haumea is shaped by its gravity and is therefore a small planet.

      1. Laurele,

        Thanks for your response. I agree with what you say, but don’t agree the IAU have adequately constrained their definition of hydrostatic equilibrium. This is a product of mass AND the composition of matter to be compressed/shaped – e.g. an icy body would be shaped more easily than a rocky body.

        Also the IAU offers no constraint around the extent to which a body must be shaped by its gravity to achieve dwarf planethood. Most reasonably sized asteroids and comets are shaped by gravity to some degree even though they remain potato-shaped. So, where should we draw the line? 50% shaped, 75%, 90%?

        I don’t think a dwarf planet need be exactly spherical, but I object to the fact that Haumea is not even a bit spherical, it’s ovoid.

  15. What is a moon? (What about Trojan asteroids?)
    What is an asteroid? (With a tail? Or a moon? Or a ring?)
    What is a star? (How to strictly define a brown dwarf ex-star object?)
    Can an asteroid become a moon, or even a planet? If two brown dwarfs merge, do they then become a star?

    This list can go on and on and on. The interesting thing with astronomy is that everything gets more unique the closer one looks at it. Categorization is neither interesting nor useful.

  16. It’s not a planet. Get over it and move on. There is nothing gained scientifically in calling it a planet. The “make Pluto a planet again” crowd is acting out of emotion and not any interests in advancing science.

    Another thing: it is also only American’s making all the noise about it. The international astronomical community is happy with things the way they are.

    Get over yourselves. Move on.

    1. Wrong on multiple counts. Both the claim that support for Pluto’s planet status is driven by sentiment and the claim that it is limited to only Americans are false. These are straw men arguments designed to discredit those who oppose the IAU decision. Those who view Pluto as a planet do so because it is clearly spherical and orbits the Sun. It is obviously very different from and much more complex than the thousands of tiny KBOs in the Kuiper Belt. Many who oppose the IAU definition do so because they prefer a geophysical planet definition that first and foremost takes into account an object’s intrinsic properties rather than its location. Furthermore, support for the planet status of Pluto and all dwarf planets is not limited to Americans. Professional astronomers from all over the world signed Stern’s petition rejecting the IAU decision. And people ranging from amateur astronomers to members of the public to professional planetary scientists worldwide have publicly criticized and even rejected the IAU definition. The planetary science community is not happy with what you call “the way things are,” especially since most of the 424 who made the 2006 decision were not even planetary scientists. If everyone was so happy with it, you would not still see so much debate eight years later. There is no reason anyone should blindly accept a bad decision and “get over it” simply because of the passage of time.

  17. The term minor planet is already in use (much earlier than the term dwarf planet). Maybe you should just use the term dwarf planet, something like what Laurel described above, though I understand your differences with her

  18. Maybe we’re looking at this from the wrong angle. Maybe Pluto, Ceres and their kind being dwarf planets isn’t the problem- maybe it’s Jupiter, Earth and the rest of the big 8 just being planets is the problem. Call them major planets, classical planets, or choose some other qualifier. Then major planets can be planets, dwarf planets can be planets, and everyone can enjoy the coming years and decades of discoveries. They’re interesting worlds regardless of what we call them.

    1. Except that the “Classical Planets” do not include Neptune and likely not even Uranus. These are latecomers.

      ONLY 6 planets were known in “Classical” Greek and Roman times (Mercury, Venus, Earth, Mars, Jupiter, Saturn) – Prehistoric time for Jupiter, well before 2000 B.C. for Mars, before 1836 B.C. for Saturn, before 1581 B.C for Venus and before 1000 B.C. for Mercury. These are the “Classical Planets”

      Uranus may have been observed as early as Hipparchos in 128 BC and he included it his star catalog as a STAR. It was only in 1781 that Herschel presented his discovery as a comet or planet to the Royal Society and where it was accepted as a planet. In 1789, a new element was named after the new planet’s name suggested by Bode in 1783 (Uranus) and named Uranium. By 1850, the name Uranus for the planet was universally adopted as the name for the new planet. Uranus is the latinized version of the Greek God of the sky Ouranus or the father of Saturn.

      Neptune was observed as early as 1612 A.D. by Galileo but he did not note or understand that it was a planet. Uranus was not definitely identified or “discovered” as a planet until 1846 A.D.

      That would leave us with ONLY 6 “Classical Planets” and the rest …

  19. I like the addition of a differentiated interior structure. But your definition of a “planet” would include a brown dwarf. It can’t fuse hydrogen, but can fuse lithium. I’d make that can’t sustain any nuclear fusion processes.

  20. Lets address the double planet definition. The Barycenter theory is questionable. At some point Earth will be a double planet with the Moon as the moon continues to move away from the Earth. But let’s ignore that for now. I prefer the case made by Isaac Asimov in book “The Double Planet”. (http://en.wikipedia.org/wiki/Double_planet#Tug-of-war_definition). In that definition, a Planet always “falls” towards the sun. If two or more bodies orbit each other such that both always “fall” toward the sun then those bodies are a Double Planet, Triple Planet ….

    When considering the definition of a planet, one MUST consider orbital resonances and the effect that they have on “clearing the orbit”. For instance, the overwhelming gravitational effect of Neptune causes Pluto (and other Transneptunian objects) to be in orbital resonance with Neptune.

    When it comes to item C) of the planet definition “and must have cleared the orbit” one MUST consider the effects of Orbital Resonance. For instance, NEPTUNE makes all orbits of bodies within it’s gravitational influence unstable if said bodies are NOT in an orbital resonance with NEPTUNE. Pluto & Orcus (Plutoids 2:3). Other possible but unproven orbital resonances may be Eris (5:17), Makemake (6:11) and Haumea (7:12). These high resonances probably have not permitted NEPTUNE to have cleared the area of the paths of these bodies (Eris, Makemake, Haumea).

    For the known PLUTINO 2:3 resonances NEPTUNE assures that Pluto and Orcus will never meet and it makes all bodies not in a strong orbital resonance within the paths of Pluto or Orcus are unstable so, in essence, the paths that Pluto and Orcus take (kind of like a flower, not a static circle or ellipse like Earth) are definitely cleared of debris. Based on orbital resonances, the orbit of Pluto and Orcus have been cleared by NEPTUNE because of the overwhelming gravitational influence of NEPTUNE. I would not be surpised if NEPTUNE were found responsible for the gravitational forcing of Charon (and the other Pluto moons) into the proximity of Pluto and the eventual capture of same by Pluto. I could make the case that both Pluto and Orcus are planets in that the ORBITAL PATHS have been cleared of debris, not be Pluto or Orcus but by NEPTUNE. Eris may be in an independent orbit and because of Eris’ mass, may be appropriate to consider as a planet as well. The case for planetary status for Makemake and Haumea are very murky at best and I would be happy to call them Dwarf Planets.

    Under these new rules, the Planets would be Mercury, Venus, Double Planet Earth-Moon, Mars, Jupiter, Saturn, Uranus, Neptune, Double Planet Pluto-Charon and potentially Orcus. That would give us 12 Planets. Dwarf Planets would be Ceres, Vesta (Dawn has revealed signs of differentiation and after becoming a dwarf planet, it was smacked by a large body rendering it not absolutely ovoid), Haumea, Makemake, Sedna, Quaoar, 2007 OR and Orcus (if it is not to be defined as a planet).

    If we take the strictest defintion of “clearing the orbit”, the Earth fails (co-orbital with at least one asteroid), Mars fails (asteroids at L4-L5), Jupiter fails (large trojan and greek asteroid populations at L4 and L5), Uranus fails (asteroid at L4), Neptune fails large population so of asteroids at L4 and L5 as well as Pluto-Charon although orbital dynamics assures us that none of these ever coincide with the bodies that share the orbits of these planets (Earth, Mars, Jupiter, Uranus, Nepune AND PLUTO-CHARON).

    I do NOT favor an arbitrary size definition that makes the population of planets essentially static! (i.e., mass less than Pluto)

    1. I COMPLETELY agree with your last statement. The definition must be based on measurable physical properties and defined by any arbitrary cutoff. I also agree with your argument including barycenter falling inside of the primary of two co-orbiting bodies.

      1. … and if we apply the BARYCENTER rule, then the SUN is not the CENTER OF THE SOLAR SYSTEM. The barycenter of SUN-JUPITER is outside the surface of the sun so we would need to classify the Solar System as a “Double Star” referring to Jupiter as a FAILED STAR?

        If we use the “falls” towards the SUN rule, then Jupiter is JUST ANOTHER PLANET!

        So if we use the BARYCENTER RULE … what do we call Sun-Jupiter????

  21. Hi, I follow this website regularly, but I registered just so I could make a comment on this article 🙂

    I really don’t see how your proposal is better than any other random set of rules that have been proposed so far. To explain myself in detail:
    1) “I like Pluto” is not a good rule – and your rule for “minor planet” is EXACTLY that. Why would Pluto mark the limit? What’s so special about it? Why not Mercury? Your feelings about Pluto are scientificaly valid reasons. Also, setting (just as an example) Mercury as a “candle” would mean we wouldn’t need to add ANY extra planets (as opposed to your “just one”) 😉 .
    2) by including the words “is a planet”, your definition of planet includes all the other planetary subgroups (minor, interstellar and binary) – so you’re actually proposing hundreds of new planets (basically everything round orbiting a star)
    3) what does “orbiting around” mean? That one object in the binary system is “primary”? Does it mean Pluto would be a planet, but Charon would be a binary minor planet? Doesn’t it make way more sense to call the whole Pluto-Charon system “a binary planet”? But wait, that doesn’t go well with th “I like Pluto” rule…
    4) “insufficient mass to fuse hydrogen” – what mass is that, exactly? Does the object have to be fusing hydrogen at this very moment? What if it did fuse it some time ago? Are white dwarfs planets? What about brown dwarfs?
    5) “differentiated interior” – how to we determine that? As far as I know (and please do correct me if I am), the answer to this is usually taken from theoretical models – which often turn out to be completely wrong… It’s a rule without a ruler (or with too many rulers).
    6) “hydrostatic equilibrium – nearly round shape” – which one of these 2 rules are you considering? Is a 5km wide, round asteroid a planet? How do we know an object is (trully) in a state of hydrostatic equilibrium? What if we launch a spherical satellite (in a state of hydrostatic equilibrium, and of course, a differentiated interior) into an orbit around the Sun?

    P.S. What’s all the fuss about Pluto anyway? Why is it so important to call it a planet? Is it because it was discovered by an American? What’s so bad about the “dwarf planet” name? It’s not derogatory – or if you think it is, let’s just call them “little planets” 😉

    1. What is wrong is saying dwarf planets are not planets and blurring the distinction between objects that have all the features of planets on one hand but just happen to be smaller and shapeless asteroids and comets that do not have the same degree of complexity and are essentially rubble piles or ice balls.

      The geophysical planet definition does not in any way privilege Pluto. It is based on a physical attribute that is determined by an object’s mass. Is the five km asteroid to which you refer in hydrostatic equilibrium? If it is, it’s a small planet; if not, it’s an asteroid. If we can’t tell, the next generation of telescopes will be of major help in this area.

      It goes without saying that any definition should distinguish between natural satellites and artificial ones. In science fiction, there are cases of “artificial planets” being constructed to make room for growing populations. We can wait until such a thing actually becomes possible to determine whether artificial worlds like this should count as planets.

      Again, according to the geophysical definition, any object that ever conducted fusion in its core, whether of hydrogen or deuterium, is a type of star rather than a planet. Brown dwarfs are the lowest end of the stellar category even if they are not undergoing fusion anymore. White dwarfs are stellar remnants, and already have their place in the Herzsprung-Russell Diagram, so they are not planets either.

      Because Pluto and Charon orbit a barycenter outside of Pluto, they can be considered a binary or double planet system. Why not dually class Charon as both a large moon and as a small planet?

      The need for a better definition has nothing to do with Pluto having been discovered by an American.

      One idea to consider is developing something like the Herzsprung-Russell Diagram but for planets, to account for the many types ranging from small worlds like Ceres to gas giants just below the threshold for conducting fusion. Another possibility is to consider the “Star Trek” planetary classification scheme, which takes into account the existence of many types of planets. It would not be the first time we looked to science fiction for real world answers.

      1. Who’s saying dwarf planets are planets? The word “planet” is even in thier title! WHo is blurring the distinction between dwarf planets and asteroids? Ceres and Pluto (and other TNOs) are clearly distinguished from the “rest of the rubble” – I mean by the current IAU definition.

        I’m not sure which “geophysical planet definition” you are referring to, so I’m going to assume it is the one mentioned in the article. Sure, that definition doesn’t favour Pluto, but the “small planet” definition does – because it specifically limits (proper?) planets to Pluto-size or larger. This rule is grounded in emotions and nothing else. And if we exclude that one rule, we end up with dozens of planets.

        I have yet to see a scientifically sound rule that would not keep Pluto in the same category as the other TNOs (and Ceres).

        Calling Charon a small planet and a moon – does this mean our Moon would also be classified as a small planet? Would Titan be classified as a planet and a moon (since it is larger than Pluto)?

        I am not saying that the current definition is great (or good), but it’s the only one we’ve got for now. So we should stick with it until someone comes up with a better one (for example, the H-R diagram for planets – that would be awesome).
        You may accept it or not, but any rule that tries to push Pluto out of the TNO group is there only because it was discovered by an American. Scientifically, it makes no sense at all. There is no physical difference that would allow us to distinguish it from the other large TNOs.

      2. Who is saying dwarf planets are not planets? That would be the IAU, or at least the four percent who voted in 2006, rejecting resolution 5b, which would have established dwarf planets as a subclass of planets.

        Calling Ceres, Pluto, and similar objects “minor planets” and giving them minor planet numbers amounts to blurring the distinction between these dwarf planets on the one hand and asteroids and comets (what you call the rubble) on the other hand. The term “minor planet” is a synonym for asteroid or planetoid, objects the IAU now refers to as Small Solar System Bodies. Any definition that simply lumps objects like Pluto and Ceres into just KBOs or TNOs because of their location without acknowledging their intrinsic planetary properties lumps together two very different types of objects.

        Pluto and Ceres should NOT just be in the same category as other TNOs. They should be acknowledged as objects with much of the same features as the larger planets, just in smaller versions. Definitions that do not acknowledge this, especially solely out of “concern” with keeping the number of solar system planets small, are inherently flawed.

        The geophysical planet definition is slightly different from the one the writer proposes. It does not place a limit on planets being Pluto-sized or larger. It defines a planet as any non-self-luminous (not a star) spheroidal body orbiting a star, free floating in space, or orbiting another planet. If a celestial object is large enough and massive enough to be squeezed into a round or nearly round shape by its own gravity (hydrostatic equilibrium), it is a planet, even if it is smaller than Pluto.

        Once again, I question, what is the problem with having dozens of planets? Objection to something that likely is a reality in our solar system is motivated by sentiment and emotion, not by science. If our solar system has hundreds of planets, then that is what it has. Memorizing a list of names isn’t important; what is important is understanding the different subclasses of planets and their defining characteristics. We don’t ask kids to memorize the names of all the rivers or mountains on Earth, just to be able to explain what a river and a mountain are.

        According to the geophysical planet definition, spherical moons of planets can be considered secondary or satellite planets. But there is no problem with continuing to call them moons. Titan is a spherical moon that can also be considered a satellite planet of Saturn. In the case of the Earth-Moon system, the barycenter is inside the Earth, so I wouldn’t call it a double planet system–though it may become that someday as the Moon moves away from the Earth. In contrast, the barycenter between Pluto and Charon lies outside of Pluto itself, making the system a true binary planet system.

        I strongly object to the statement, “I am not saying that the current definition is great (or good), but it’s the only one we’ve got for now.” This statement privileges one definition as THE “current one” instead of acknowledging that there currently are several competing definitions in use. It’s better to acknowledge the ongoing debate and recognize that having such a debate is not a problem or a bad thing.

        Once again, no one is privileging Pluto because it was discovered by an American. People who advocate including Pluto with the planets do so because of its intrinsic properties, not because of its discoverer. The issue isn’t a matter of trying to push Pluto out of the TNO group; it is a matter of distinguishing that Pluto is not just another TNO, the same way Ceres is not just another member of the asteroid belt. Maybe some objects should be dually classed as TNOs and small planets. But there certainly are physical differences that allow us to distinguish large TNOs–not just Pluto–from the majority of tiny ones; these differences all hinge on the fact that these large objects are in hydrostatic equilibrium and have all the same features as their larger planetary counterparts; they just are smaller versions.

      3. Objects with several Jupiter masses are often called brown dwarfs by some and planets by others. Back in the dark ages of the 1990s, brown dwarfs needed to have ~13 Jupiter masses to be called such. What would society call an object with 2 Jupiter masses orbiting 60,000AU from the Sun? There will always be grey areas with any definition.

      4. Agree – no doubt, some classification system is needed. These high level definitions should come first and then a system such as you mention the HR as one. Sara Seager of MIT has developed classifications for atmospheres; also studied interiors. Interiors and atmospheres, size and mass, location and influence in or outside a stellar system, cannot define a definitive boundary between planet and everything else or just dwarf planet. I wrote a second article, the other took an opposing view. My overall conclusion was that you cannot define planet without some arbitrary cutoff if you want the term to be useful. While the physics of the question is important, there is no specific property or combination of properties that spell out planet and *not a planet* like fusing hydrogen provides a pretty good cutoff. The “lower” end defined by Pluto is a fair but arbitrary limit necessary for classifying planetary systems throughout the Cosmos. As Dawn at Vesta has shown us, an object can be differentiated and not round. Perhaps all one needs is round and not the clause “b) has a differentiated interior as a result of its formation.” That clause might discriminate against some odd bodies which are round, even much bigger than Pluto, but not differentiated. Astrophysicists could move on with this Pluto lower limit and classify planetary systems by stars, counts of “planets” and total mass, counts and spatial distribution of small body types (comets, asteroids, “cloud/belt” objects). It is possible that once statistical analysis is accomplished on many planetary systems, some body mass might stand out as a reasonable lower limit for planet. Pluto could then end up in the rubble heap of minor or dwarf planets but at least for some time, it can make a good lower limit to what we refer to as a planet. I would classify Pluto-Charon as a binary planet system, Type 2 [planet & minor planet]. Type 1 would be a two “planet” variety.

      5. I think all kinds of features which are not obvious from the most basic set of observations should be excluded from definition of the most basic of classes.
        Can we determine roundness just by orbit, mass and albedo ? Differentiation needs more examination and should probably be put with definition of subclasses or marginal cases such as Vesta.

  22. KISS Keep It Simple S.

    Killer whales (Orcas) are not whales but dolphins, very big (biggest species) of dolphins but biologically they are NOT whales.

    Many excellent articles on astronomy frequently refer to “other solar systems” but there is only one Solar System, the star system which has the star named Sol at it core.

    Electron “orbits” around atomic nuclei are not nice, simple circles but electron clouds, but I suspect a great majority of the (non-science) public would be confused and put off by an article that simply referenced electron clouds in place of orbits.

    Culturally, Pluto will remain a planet in the minds of most people until they die. Trying to force a change in common usage will do nothing to improve understanding but will confuse millions of people who are comfortable with high level overviews. School kids who are, hopefully, the future of science are more likely to be discouraged by the additional blanket of semantic technicalities.

    One of the most frequent complaints the public has of technical advisors is they cannot understand them because they speak in jargon. Economists. Many doctors. Consultants, and many more professions. 45 years as a consulting actuary have made me very sensitive to the necessity of finding familiar terms as a vehicle for communicating very complex concepts without using terms with a meaning that conflicts with the reality they represent.

    You say Orcas or Killer Whales and almost anyone over the age of 6 knows what you reference and introducing the fact they are a dolphin will not improve the conversation a whit. Let Pluto continue to be called a planet and no one is going to misunderstand. In those cases where some further knowledge is necessary to convey a technical point it is a given the audience is a subset of society in general with the scientific knowledge to understand Pluto’s technical specifications are, at best, marginal qualifications for the planet designation.

    I suggest the more productive ambition for those who worry about the semantics of planet or not a planet would be to hone communication skills that are competent for the larger body of the public to grasp their stories and participate in the learning process.

    1. In the name of simple :
      – if it’s big, round and shines it’s a star;
      – if it doesn’t shine, it’s a planet;
      – if it’s not round, it’s asteroid.
      This seems to me quite decent start for detailed classification.

      I think astronomers are trying to hide a fact that our solar system contains hundreds of planets. I wonder what interest could they have in that ?

  23. Obviously definition of planet needs refining. It should start with what is the most basic to the definition, that is :
    – round
    – not a star.

    Since public has generally accepted that the Earth is a planet, that is what defines a planet and it’s all astronomers ‘fault’, for which some of them have burned.

    For me, this has always been about Ceres since the first time i’ve known about it’s roundness.

  24. Two more issues :
    1. Dominant planet is a planet which dominates it’s orbit, duh ..
    2. Marginal case is Vesta, which is almost a planet, unless anyone knows about a more massive object, which is not round.

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