Pluto: The Honorary Planet

 

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Pluto – New Horizons – 14Jul15 (Johns Hopkins APL, via Wikipedia)

Watts Up With That (WUWT) highlights an article by Arthur Hirsch from a Johns Hopkins website called Hub, “Scientists make the case to restore Pluto’s planet status,” reporting on a new paper by JHU scientist Kirby Runyon. Pluto, of course, was demoted from planetary status by the International Astronomical Union (IAU) in 2006, following the discovery in 2005 of a new ‘Trans-Neptunian Object’, which was called Eris: it was in the same distant region of the Solar System, called the Kuiper Belt, and was a more massive object than little Pluto (though not quite as large in diameter), already the smallest ‘planet’ in the Solar System. It was becoming clear that Pluto and Eris were just rather large specimens of an increasing number of icy objects discovered in the Kuiper Belt, so it was decided to reduce the ‘official’ tally of Solar System planets to eight. Henceforth, said the IAU, to be a ‘planet’, a body must be in orbit around the Sun, rounded by its own gravitational mass, and able to have cleared its orbit of foreign objects.

This caused a fair amount of consternation among traditionalists, and also reasonable objections, like “Why restrict ‘planets’ to our particular Sun?” and “What official planet has actually cleared out its orbit of transgressors? Certainly not the Earth!” From Wikipedia:

Public reception to the IAU decision was mixed. Although many accepted the reclassification, some sought to overturn the decision with online petitions urging the IAU to consider reinstatement. A resolution introduced by some members of the California State Assembly facetiously called the IAU decision a “scientific heresy”.[73] The New Mexico House of Representatives passed a resolution in honor of [Clyde] Tombaugh [the first to see Pluto on a photographic plate], a longtime resident of that state, that declared that Pluto will always be considered a planet while in New Mexican skies and that March 13, 2007, was Pluto Planet Day.[74][75] The Illinois Senate passed a similar resolution in 2009, on the basis that Clyde Tombaugh, the discoverer of Pluto, was born in Illinois. The resolution asserted that Pluto was “unfairly downgraded to a ‘dwarf’ planet” by the IAU. . .

In the course of an entertaining Comments thread to the WUWT post, ‘Moderately Cross of East Anglia’ writes,

As I recollect the demotion of Pluto was rather a dodgy stitch up late on the Friday pm session of the IAU when the proponents of the change knew most of the delegates would be on their way to catch flights home – it is questionable if their motion would have succeeded otherwise. And several people have commented rightly that on the grounds they stated for the exclusion of Pluto other planets too should therefore be disqualified. JBom correctly points out the interwoven orbits of the Earth and Moon. . .

with some, notably an equally cross ‘Chimp’ pointing out that

It’s estimated that 200 dwarf planets probably inhabit the Kuiper Belt, with up to 10,000 possibly lurking farther out, scattered around toward and in the Oort Cloud. . .

the implication being that if Pluto were reinstated, instead of remembering planets with a simple mnemonic, like “Mother Very Easily Made A Jam Sandwich Using No Peanuts” (where ‘a’ I assume stands for the Asteroid Belt), students would have to memorize the names of hundreds, or even thousands of wannabe planets, rather than lumping them together as ‘planetoids’, or ‘minor planets’, or some other derogation. In response, one Alastair Brickell chimes in, “Thanks to the IAU. . . Many Very Eminent Men Just Stuffed Up Numerous Planetariums!”

Now Runyon et al, in the new paper announced by Johns Hopkins, have proposed an ‘intrinsic’ definition of ‘planet’, eliminating any consideration of external factors (like its orbit or neighborhood):

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.

Note that this expansive definition will include objects usually called ‘moons’, as for example Titan or Earth’s Moon, or the largest asteroid, Ceres (once also discussed as a possible planet). So we end up with qualifiers, like ‘dwarf planet’, and ‘moon planet’, and the authors end up with “at least 110 known planets in our Solar System”:

Certainly 110 planets is more than students should be expected to memorize, and indeed they ought not. Instead, students should learn only a few (9? 12? 25?) planets of interest. For an analogy, there are 88 official constellations and ~94 naturally occurring elements, yet most people are content to learn only a few. So it should be with planets.

This is certainly reasonable on its face, but left many commenters dissatisfied. One Dean Kennedy writes,

OK, I support returning Pluto to planetary status but I don’t see is as an improvement to create a definition which ignores the common sense category of ‘moon’. We should recognize the planetary character of large moons, asteroids and rogue wanderers based on their intrinsic qualities but the neighborhood in which they are situated is also important, distinctive and should be acknowledged even if it could be transitory.

This is more than a terminological dispute. Clearly we’re now learning that the Solar System is not just a neat little paradigm of nine planets and an asteroid belt, but a collection of thousands of odd objects of varying sizes and characteristics, some so distant that they are not observable until they sneak in close enough (e.g. comets). And there is no obvious evolutionary tree on which to structure a taxonomy. But we could use one, as we are now starting to investigate other stars and their ‘stellar systems’, or whatever they’re called. As astute writer ‘AndrewZ’ says in the WUWT Comments,

. . . what we really need is a system of classification for non-stellar objects that can be applied consistently across all stellar systems and to objects that do not orbit any star. It would therefore need to define them in terms of their intrinsic properties and would give them a classification code rather than a name. I’m imagining something a bit like a Hertzsprung–Russell diagram for discrete non-stellar objects.

The all-inclusive definition from Runyon et al., based on intrinsic characteristics, is certainly a start. But absent such an agreed-upon classification (which will probably always be fuzzy), it is certainly more convenient for popular consumption, if not technical purposes, to keep the neighborhoods of planetary objects in mind. So I suggest we continue to refer to the ‘planets’ as those more-or-less big (spherical) objects that orbit the Sun (or a sun), and ‘moons’ as those (generally smaller spherical objects) that orbit around other objects (usually planets) and not the Sun (or a sun), and maybe ‘planetoids’ or ‘asteroids’ for objects that don’t orbit anything.

In any case, it seems to me that poor ol’ denigrated Pluto ought to be restored to ‘planetary’ status, at the very least Honorary Planet, since after all it was big news when it was discovered by pursuing the further perturbations Percival Lowell noticed in the orbit of Uranus, after the discovery of Neptune (he called the source ‘Planet X’), and that cannot be said of any of its putative Kuiper Belt congeners, like Sedna or Eris.

Indeed the name ‘Pluto’ (suggested by an 11-year-old girl in the UK) was considered particularly apt because the first two letters were the initials of Percival Lowell (the actual ‘discovery’ is credited to Clyde Tombaugh, but he was hired after Lowell’s death just to compare photographs). Percival Lowell should rightfully be allowed to retain the planet, qua planet, associated with him—especially since his other famous hypothesis, about the canals of Mars, has been (sadly) long since disconfirmed. /LEJ

PS Surely you can’t make a map like this of a mere planetoid!  Original caption:

This image contains the initial, informal names being used by the New Horizons team for the features and regions on the surface of Pluto. Names were selected based on the input the team received from the Our Pluto naming campaign. Names have not yet been approved by the International Astronomical Union (IAU).

Obviously some of them were H. P. Lovecraft fans, but I must object to ‘Cthulhu Regio’, the name assigned because it looked like a whale. Great Cthulhu Who Lies Dreaming does not look like a whale, and anyway, the region looks more like a dog.