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)

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. 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)

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)

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)

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)

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)

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)

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 Find, Universe 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 System, Cornell 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.

Tim Reyes

Contributing writer Tim Reyes is a former NASA software engineer and analyst who has supported development of orbital and lander missions to the planet Mars since 1992. He has an M.S. in Space Plasma Physics from University of Alabama, Huntsville.

View Comments

  • 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)

  • 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.

    • 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.

  • 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.

    • 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.

  • 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.

  • 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?

    • 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.

  • 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!

    • "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?

      • 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.)

  • 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..

    • 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.

    • "...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.

  • 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.

    • 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.

      • 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.

        • 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.

    • 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.

  • 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.

  • 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!

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