Pluto is a planet


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In an op-ed today, the principal investigator for the New Horizons’ mission as well as his co-author for the history of that mission explained in detail why the definition for planet as imposed by the International Astronomical Union (IAU) is flawed and unworkable.

In 2006, the International Astronomical Union (IAU) announced an attempted redefinition of the word “planet” that excluded many objects, including Pluto. We think that decision was flawed, and that a logical and useful definition of planet will include many more worlds.

We find ourselves using the word planet to describe the largest “moons” in the solar system. Moon refers to the fact that they orbit around other worlds which themselves orbit our star, but when we discuss a world like Saturn’s Titan, which is larger than the planet Mercury, and has mountains, dunes and canyons, rivers, lakes and clouds, you will find us — in the literature and at our conferences — calling it a planet. This usage is not a mistake or a throwback. It is increasingly common in our profession and it is accurate.

Most essentially, planetary worlds (including planetary moons) are those large enough to have pulled themselves into a ball by the strength of their own gravity. Below a certain size, the strength of ice and rock is enough to resist rounding by gravity, and so the smallest worlds are lumpy. This is how, even before New Horizons arrives, we know that Ultima Thule is not a planet. Among the few facts we’ve been able to ascertain about this body is that it is tiny (just 17 miles across) and distinctly nonspherical. This gives us a natural, physical criterion to separate planets from all the small bodies orbiting in space — boulders, icy comets or rocky and metallic asteroids, all of which are small and lumpy because their gravity is too weak for self-rounding.

They go on to explain the flawed history of the IAU definition, and how it has simply not been accepted by astronomers and planetary scientists alike. The definition makes no sense, and excludes the thousands of exoplanets discovered orbiting other stars. They also point to a proposed new definition that is simple and admits to reality.

A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.

Whether or not the stuffed shirts at IAU ever officially endorse this definition, it is the one that human beings are using now, and it will be the one they use into the never-ending future.

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8 comments

  • Orion314

    The best way to foul the science & art of Astronomy, (the mother of all science) is to mix politics with it.
    Pluto’s demotion to whatever the DemoPlutocratic “scientists’ deemed it to be, was nothing more than free exposure for the so-called scientific left. Pluto payed the price for it.
    An easy way to put my argument to test is:
    “If Pluto had been discovered by a black Astronomer instead of Clyde Tombaugh, Imagine the outrage of demoting the significance of such a discovery.” They would riot in Berzerkly.
    This observation is with all due deference to the self titled “black Carl Sagan” , Neil d Tyson , a man who bragged he single handedly demoted Pluto’s status. Apparently , for no other reason than the left’s blind approval, even though his entire story is nothing more than egyptian cotton weaved from “horse apples’ , with respect to BTB :)
    PS
    Years ago , when the subject of Pluto’s reclassification came to rise , my first thought was : Why not grandfather in the original 9 planets with the title of Planet , with a capital P ? ,As a bow to antiquity , every planet discovered subsequently, will have a small p , for planet. Hardly brilliant , but certainly practical.

  • Localfluff

    I was present at that IAU conference, I know how it happened. At the end of the session someone asked:
    – Dr. Dr. chairman, what is a planet, actually? I’ve never seen a definition.
    – Oh by Lord Euler! Don’t we know what a planet is? And we are supposed to be astronomers. We must get a formal definition now, or else this might escalate to some kind of media scandal that could hurt the IAU’s reputation.

    So they defined a planet as the properties of other planets, and things. What is a planet or not depends, not what it actually is, but on where it is and where other stuff is. Also, planets only exist at the Sun, it is indeed a very unique star! Exoplanets are not planets. Also, it has to be round. Vesta was formed round, but later disfigured by a huge asteroid impact, a deplanetizing event.

  • Localfluff

    Also, there cannot be more than about ten planets. That was very important for the definition. So a planet isn’t a planet if there are too many other planets. The definition always depends on something else than the properties of the planet itself.

    OT: Ted Cruz speaking at the Humans to Mars event:
    “The first trillionaire will be made in space!”
    https://livestream.com/viewnow/HumanstoMars2018

  • Edward

    The way that I heard the story, at the time (I was not present at the IAU conference), is that some astronomers noted that the Kuiper Belt was about to be explored and that there could be quite a few planet-sized bodies in there, perhaps a hundred of them the size of the Earth. They panicked, thinking that no grade school student would bother to memorize, say, 110 planet names,* so how could astronomers keep schools teaching planets and their names? Their solution was to redefine “planet” so that only a few astronomical objects fit the definition. Unfortunately, in their panic, they rushed the job and messed up the definition.

    Panic, of course, results when you don’t know what else to do.
    https://www.youtube.com/watch?v=BKaQLYPf5hM#t=111
    Fortunately, The Hitchhiker’s Guide to the Galaxy recommends “Don’t Panic.” Unfortunately, astronomers do not seem to be Douglas Adams fans.

    Now they have made a mess of things, and someone has to clean it up.

    * From the USRA planet definition PDF link: “Certainly 110 planets is more than students should be expected to memorize, and indeed they ought not.

  • Jack O'Leary

    It should be “principal investigator” NOT “principle”. /grammar

  • Phill O

    I am stuck on the Plutanoid planets term.

  • Jack O’Leary: Thank you. Post is fixed.

  • wodun

    you will find us — in the literature and at our conferences — calling it a planet. This usage is not a mistake or a throwback. It is increasingly common in our profession and it is accurate.

    This is stupid. Just because moons are more interesting than people used to think they are doesn’t make them planets. Being a moon isn’t an insult.

    The scientists studying moons shouldn’t feel like they are lesser and shouldn’t give in to the temptation to overcompensate for their doubt over self worth. Studying moons doesn’t make you less of a scientist, there is no need of the puffery here. Maybe they need comfort goats?

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