Could Pluto Regain Its Planethood?

Feb 23, 2017

By Mike Wall

Advocates of Pluto’s planethood are about to fire another salvo in the decade-long debate about the famous object’s status.

Scientists on NASA’s New Horizons mission, which performed the first-ever flyby of Pluto in July 2015, will officially propose a new definition of “planet” next month, at the 48th Lunar and Planetary Science Conference in The Woodlands, Texas.

The new definition would replace, or supersede, the one devised by the International Astronomical Union (IAU) in 2006. A planet, the IAU determined, is a body that orbits the sun without being the moon of another object; is large enough that its own gravity has rounded it into a sphere (but not so large that it undergoes fusion reactions, like a star); and has “cleared its neighborhood” of most other bodies.

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9 comments on “Could Pluto Regain Its Planethood?

  • @OP – link – “Oh god the stupid Pluto stories are back. Yes, someone has proposed making Pluto a planet again. No, nothing is changed or new,” Brown tweeted Tuesday (Feb. 21). “Also, I should note, that proposal would make the moon a planet. Which is about 500 years out of date. But, ok #MakeTheMoonGreatAgain,” he added in another tweet.

    I must admit that was my reaction to this!

    The present definition is rough and ready, distinguishing major planets in approximately circularised orbits from asteroids, comets Kuiper Belt objects and Oort Cloud objects.

    As an alternative, the researchers will propose a “geophysical definition” — one based solely on an object’s intrinsic characteristics (and not on how it interacts with its environment). Here it is:

    “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.”

    This is also rough and ready but reclassifies the various moons as “planets” along with the larger asteroids and Kuiper belt objects (but not the smaller ones!), reducing the clarity of the definitions.

    Indeed, the number of officially recognized planets would balloon from eight to about 110, the researchers wrote.

    With no clear defining separation from the thousands of slightly smaller objects in the same zones, of the same types, and in the same types of orbit.



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  • Yes, either the definition they made which removed Pluto from planet status was premature or this change back is, do they plan on naming the myriad of other objects of similar size planets also. Really doesn’t bother me one way or the other but they should make up their mind. Pluto of course will not be changed one bit by what we choose to call it. However these semantic games may be harming the credibility of scientists is they keep chopping and changing. To me it seems it would be a far better idea to add a suffix or prefix to the particular type of planet it is which is what I thought they had done by calling it a dwarf planet.

    Reading some of Dawkins books you get a sense of the frustration in the biological world with different methods of biological classification. Fortunately these seem to have gone unnoticed by the wider community. My fear is this debate is just going to add fire to creationists and AGW deniers who will point to this as an example of scientific uncertainty. A modification of current terms to incorporate new knowledge would be better from a PR perspective I think.



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  • Reckless Monkey #2
    Feb 26, 2017 at 4:16 am
    . . . To me it seems it would be a far better idea to add a suffix or prefix to the particular type of planet it is which is what I thought they had done by calling it a dwarf planet.

    Yes, Reckless Monkey, the term ‘dwarf planet’ was a good compromise. If there are other objects in the solar system similar to Pluto, they should each be considered case by case for dwarf planet status; but the terminology should remain as it is. It serves its purpose well, and changing it again so soon will only cause further confusion and may begin, as you suggest, to look to the ill-informed and to various enemies of science like uncertainty among scientists about facts!



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  • Reckless Monkey #2
    Feb 26, 2017 at 4:16 am

    Pluto of course will not be changed one bit by what we choose to call it.

    As you point out these are rough and ready semantic labels – in this case on “planets” essentially by position of orbit and significant size.

    While spheroid bodies of the same size may be planets or moons, the moons (being closer) have much stronger interactions with their larger planet (and each other in multiple systems), than a similar body in a solar orbit.

    However these semantic games may be harming the credibility of scientists is they keep chopping and changing.

    The perceptions of “chopping and changing”, are properties of the discontinuous mind, rather than properties of the variation in material objects.

    To me it seems it would be a far better idea to add a suffix or prefix to the particular type of planet it is which is what I thought they had done by calling it a dwarf planet.

    That is indeed why prefixes or secondary labels, are important to classify types. Pluto is not subject to the tidal heating of a moon like Europa, and we know that its icy composition is quite different to that of Mercury because of temperature differences.

    As probes supply new knowledge of orbiting materials, further sub-categories according to material composition, are likely to be built up – as in the case meteorites.

    http://geology.com/meteorites/meteorite-types-and-classification.shtml
    Although there are a large number of sub classes, meteorites are divided into three main groups: irons, stones and stony-irons.
    Almost all meteorites contain extraterrestrial nickel and iron, and those that contain no iron at all are so rare that when we are asked for help and advice on identifying possible space rocks, we usually discount anything that does not contain significant amounts of metal.
    Much of meteorite classification is based, in fact, on how much iron a specimen does contain.

    In future, there may be a system in the outer Solar-System for classification according to how much water ice or other frozen volatiles bodies contain!



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  • Garrick #3
    Feb 26, 2017 at 5:06 am

    Yes, Reckless Monkey, the term ‘dwarf planet’ was a good compromise. If there are other objects in the solar system similar to Pluto, they should each be considered case by case for dwarf planet status; but the terminology should remain as it is. It serves its purpose well,

    https://upload.wikimedia.org/wikipedia/commons/thumb/9/9c/TheTransneptunians_Size_Albedo_Color.svg/300px-TheTransneptunians_Size_Albedo_Color.svg.png

    Some await confirmation of details, – and there are probably more to be discovered.

    https://en.wikipedia.org/wiki/List_of_possible_dwarf_planets



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  • I see no problem with the classification of a planet being, inter alia, a body with sufficient gravity to assume a spherical shape, nor any need to reclassify Pluto again. There has to be some sort of size limit. I also see no need to change the definition of a moon as that of a smaller body orbiting a planet rather than orbiting a star directly.

    There is however one case that I think needs consideration which is that of a binary planet in the same way as there are binary stars. If two bodies of close to equal mass orbit each other and their common centre of gravity orbits a star then calling one of them a planet and the other a moon makes no sense. That leaves to be decided the mas ratio that would make it appropriate to call the smaller of the two a planet in its own right rather than a moon. The closest thing we have to that in our solar system actually involves Pluto again as its largest moon Charon is about 1/8th of Pluto’s mass. The next closest is Earth’s own moon which is 1/81st of Earth’s mass.

    The mass ratio problem does not affect binary stars because if both have achieved nuclear fusion then mass is irrelevant and they are both stars rather than the larger being a star and the smaller a planet.

    I’d suggest that if two co-orbiting bodies had a mass ratio of 1:4 or greater and both were sufficiently large to achieve spherical shape then the issue would need serious consideration. It seems very likely that somewhere out there such cases exist.



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  • Arkrid Sandwich #6
    Feb 26, 2017 at 6:23 am

    That leaves to be decided the mas ratio that would make it appropriate to call the smaller of the two a planet in its own right rather than a moon. The closest thing we have to that in our solar system actually involves Pluto again as its largest moon Charon is about 1/8th of Pluto’s mass. The next closest is Earth’s own moon which is 1/81st of Earth’s mass.

    This is why the definition is dependent on the position of the orbit, circularisation, and the distance from the star.

    Mercury is so close to the Sun that it cannot retain a moon.
    Out beyond Pluto the Sun’s gravity is so weak the even comet sized objects can have smaller bodies orbiting them.



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  • @OP As an alternative, the researchers will propose a “geophysical definition” — one based solely on an object’s intrinsic characteristics (and not on how it interacts with its environment). _
    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.”

    However, – given that many accretion discs are formed around second or third generation high metalicity stars, which contain substantial proportions of heavy elements which are the products of fusion and supernova explosions, and the surface and structure of many (if not all) moons and planets are determined by their historic interactions with their gravitational and radiation environments, this definition seems to have serious short-comings! Their “intrinsic characteristics, both chemically and physically would seem to be inherently tied to their historical interactions with their environments.
    (Orbital resonances, synchronicity, core and crustal structure, elemental make-up, acquisition or loss of volatiles, etc.)



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