A Goofball Called Pluto

by Bruce Moomaw
Cameron Park (SPX) May 18, 2007
One of the first pieces I wrote for SpaceDaily, over seven years ago, was on the already increasingly goofy debate over whether not to continue calling Pluto a "planet". Well, the debate has now proceeded to an official vote by the IAU that gained worldwide headlines by changing Pluto's title -- and it remains as goofy as ever.

Indeed, the description of that fateful IAU meeting in the Sept. 1, 2006 Science makes the IAU meeting sound rather like an episode of "Romper Room" in which Miss Virginia lost control of her class:

"Earlier [in 2006], the IAU Executive Committee asked seven people (including award-winning scence writer Dava Sobel) to write [a formal definition of 'planet]...Chaired by Owen Gingerich, a professor of astronomical history at [Harvard-Smithsonian], the committee met in Paris on June 30 and July 1 and unanimously agreed that Planet Club membership would be open to any sun-circling body big and massive enough to become spherical under its own self-gravity.

That would include Pluto and [Michael Brown's larger 2005 Kuiper Belt discovery that would soon be named 'Eris"], but also Ceres, the largest member of the rocky asteroid belt", and even Pluto's moon Charon -- thus creating the Solar System's first official 'double planet'.

"IAU presented the resolution to its General Assembly on August 16, giving the roughly 2500 attendees more than a week to discuss it. But the committee expected clear sailing...Instead, the '12-planet proposal' went down in flames. Critics objected that planets should also be defined by their orbital dynamics, not just by their size and shape. All eight 'major' planets, they pointed out, were massive enough to sweep up, fling away, or gravitationally control all the debris in their parts of the early solar sysem, but Ceres and Pluto [and other candidate 'planets' among Kuiper Belt Objects] were not.

"Many astronomers lambasted the resolution during a tumultuous lunchtime meeting on Augsut 22. To Gingerich's argument that the proposal rested on physical criteria, asteroid researcher Andrea Milani of the University of Pisa in Italy literally screamed, 'Dynamics is not physics?' Other astonomers protested the committee's neglect of extrasolar planets, only to be angrily silenced by outgoing IAU President Ronald D. Ekers, who declared such issues to be 'out of order!'...'It should never have become this emotional, says astronomer George Miley of Leiden University in the Netherlands.

"On the morning of August 24 -- the day of the vote -- IAU issued a revised resolution adding gravitational dominance to the requirements for planethood and omitting any reference to Charon... Ceres, Pluto, [Eris] and other spherical sun-circling bodies were labeled 'dwarf planets'."

The resolution pased by a wide margin, but not without leaving considerable, er, ill feeling. Alan Stern -- the new head of NASA's Space Sciences Division, who is also arguably the world's leading Pluto expert and the chief force behind the near-miraculous reversal of NASA's planned cancellation of its Pluto probe -- says, "This is a sloppy, bad example of how science should be done." The Science article goes on to say: "In protest, [Stern] and others have already withdrawn articles from an upcoming edition of a professional solar system encyclopedia after the editors requested them to change Pluto's status in the articles. A petition against the accepted planet definition is already circulating among planetary scientists."

At this point in mid-2007 -- almost a year after Pluto officially became a non-planet -- feelings don't seem to have simmered down much. Should they have? And what right has a non-scientist to stick his oar into the debate?

Well, I think the absurdity of the current situation can be summed up very neatly by considering what will happen if and when, as is very possible, we discover an object in the Kuiper Belt or the more distant Oort Cloud -- or more than one object -- that is actually bigger than Mercury. By the IAU's current definition, these will be officially called "dwarf planets" while the smaller Mercury remains a regular "planet", thereby providing endless fodder for the likes of Jay Leno. (Indeed, given the vast distance of Oort Cloud objects, it's a safe bet that we will literally never be able to say with confidence that such objects do not exist in our Solar System -- or that there are none that are not bigger than Mars as well.)

How did the planetary science community paint itself into this absurd corner? I think the answer lies in its futile search for some clear, unambiguous dividing line between "planets" and "non-planets". But ever since the discovery of the Kuiper Belt in the 1990s -- one of the most spectacular discoveries in 20th -century Solar System science; it's like living in your home for two centuries and then suddenly discovering an entire new living room that you didn't know about -- no such dividing line can ever be maintained.

Before then there was a nice big gap between the average diameters of Pluto (2390 km) and Ceres (950 km). But now a large number of Kuiper Belt Objects ("KBOs") have been discovered with all kinds of intermediate diameters, and Eris is now known to be a bit bigger than Pluto. It's a pretty safe bet that other KBOs will be found that are also bigger than Pluto -- maybe a lot bigger than Pluto. As I say, there is a real possibility that we will find a KBO bigger than Mercury; and there is no way of knowing whether such objects exist in the vastly more distant Oort Cloud trillions of kilometers from the Sun.

So any definition of "planet" and "nonplanet" based on size, or on mass, must be entirely arbitrary. Can we find a clearer one? Stern -- who played a major role in encouraging the IAU subcommittee's original definition of "planet" -- tried to define such a clear dividing line using the "gravitational self-rounding" principle. The trouble with this is that there are an impressive number of objects in the Solar System which have only partially rounded themselves gravitationally, depending on both their size and the rigidity of the material (rock or ice; solid or fragmented) that makes them up. Ceres varies in width from 909 to 975 km, depending on which axis you measure it. Pallas -- the second biggest asteroid in volume -- has a diameter varying from 500 to 570 km. Vesta -- the third biggest -- has diameters ranging from 458 all the way up to 578 km. Iapetus -- the third biggest moon of Saturn -- varies in width from 1425 to 1498 km. Proteus -- the second biggest moon of Neptune -- is shaped rather like a marshmallow, 404 to 440 km wide.

And as you get down to somewhat smaller objects, their degree of semi-rounding can vary greatly, especially if they are asteroids or Kuiper Belt Objects ("KBOs") that have suffered collisions and had large pieces of themselves knocked off. We have just started to make fuzzy measurements of the average diameters of KBOs; but just a short time after the IAU meeting, one of the biggest discovered KBOs -- 2003 EL61 (which has no official name yet) -- was found to be a bizarre elongated-egg object which comes close to being as wide as Pluto along its longest axis of 1960 km, but is only about half as wide on its shortest axis! Now, if you say that a "planet" is an object which rounded itself gravitationally, and you regard Ceres' degree of rounding as adequate for it to fit this definition, do you also call 2003 EL61 a planet or not? Stern's definition is hopelessly fuzzy unless you set a specific degree of "rounding" -- that is, the percentage difference between its largest and smallest diameters must be below a certain figure -- and any such degree must be arbitrarily chosen, just as with the "size" definition.

The next attempt at coming up with a firm dividing line -- the "dynamics" definition embraced, for the moment, by the IAU -- looks at first as though it would be firmer. A "planet", by this definition, must be an object that has also gravitationally "cleared its orbit" -- by which is meant that there are no longer any objects significantly approaching it in in size that might collide with it some day if the Solar System lasts long enough; all such objects since the formation of the Solar System have either already crashed into it (or into other planets), or flown by it (and then perhaps by other Solar System objects) close enough to be permanently flung into an orbit that will never allow any possibility of a collision.

Such an orbit doesn't necessarily have to be entirely inside or outside the planet's orbit; two objects orbiting the Sun can get into "resonant" orbits which cross each other, but have periods that are in a precise ratio that will forever keep the two objects from actually getting into the same point at the same time. Pluto and Neptune comprise such a pair -- Pluto's year is exactly 1.5 times that of Neptune, and their timing is such that Pluto will actually never be able to get as close to Neptune as it can to Uranus! There are many other newly discovered smaller "Plutinos" among the KBOs whose orbits are similarly related to Neptune's -- and in our search for planets of other stars, we have already discovered three pairs of giant planets whose orbits may possibly cross -- but two of them seem to have 2:1 ratios of their orbital periods which will prevent them from ever colliding, and the third pair apparently has a 5:1 period ratio that also will do so. However, Pluto's orbit is itself crossed by the orbits of other KBOs that may collide with it if the Solar System lasts long enough (and the KBOs will survive at such distances from the Sun even after it goes through its red giant phase).

The same thing is true of the asteroid Ceres, which may eventually get hit by another large asteroid if it survives long enough. And it is apparently true of every single smaller asteroid or KBO -- none of them has enough gravitational "clout" to have cleared out their neighborhood of other objects on the same size scale. In the case of the asteroids, Ceres might have cleared such a zone for itself by now, except that Jupiter's constant powerful gravitational tuggings keep the orbits of the asteroids "stirred up" and frequently crossing each other. In the case of the KBOs, it's simply that there has always so much room out there for objects to orbit around the Sun -- and it takes them so long to complete such orbits -- that none of them has ever even begun to come close to clearing the others out of its way.

The Museum of Natural History's Steven Soter, in his essay "What Is A Planet?", has developed this definition of "planet" in perhaps the most detail. He recognizes that all the planets, including Earth, still have small asteroids or comets (or, in the case of the giant planets, "Centaur" objects as much as several hundred km wide) that can eventually collide with them. So his argument is instead that we can call an object a "planet" if it has more than 100 times the total mass of all the Sun-orbiting objects that might be able to hit it someday -- and whose orbital periods are less than 10 times as long as the planet's own orbital period (its "year").

Now it is possible not only that there are really big objects in the Oort Cloud, but that a few of them -- like the much smaller comets whose highly eccentric orbits first alerted us to the existence of the Oort Cloud -- may someday be flung by the gravitational tuggings of passing stars into orbits taking them all the way back into the Solar System proper so that they could collide with one of the eight "proper" planets. For instance, we can't quite rule out the possibility that there is Oort Cloud object as massive as Mars that may someday in the very distant future be put into a super-eccentric orbit that will cause it to swoop back into the inner System and collide with Mars -- or even that there may an Oort object as massive as Earth that may someday hit our own planet (if Earth is still around after the Sun's red-giant phase has roasted it). In fact, we can't absolutely rule out the possibility that there are huge Oort objects already in such a super-eccentric orbit that could allow them to hit one of the four inner planets. But Soter rules these out as "planets" with the second of his numerical criteria: all of them must have orbital periods of tens of thousands, or millions, of years -- tremendously longer than 10 times the orbital periods of any of the planets.

By Soter's criterion, there are no asteroids or KBOs or Oort Cloud objects that even remotely begin to come as close to clearing their orbital neighborhoods as the eight "classic" planets have. The weakest of the eight planets (by far) is Mars, which has only enough asteroids or comets in its "orbital zone" -- that is, objects that are currently in orbits that could hit it, and that have less than 10 times the orbital period of Mars' 687-days -- to equal a mere 1/200 of its mass. For Earth, the ratio is less than two millionths! For Uranus and Neptune -- despite their closeness to the Kuiper Belt -- all the KBOs that might conceivably hit them, and which have orbital periods less than 10 times those two planets' very long years, only total a few ten-thousandths of their mass. On the other hand, the total mass of the asteroids or comets that might someday hit it -- and currently have orbital periods less than 10 times as long as Ceres' 4.6 year period -- is fully three times the mass of Ceres itself. Pluto and Eris have about 14 and 10 times their own mass in other objects in their own "orbital zones".

The difference, as Soter says, is dramatic. But it still leaves us with the strong possibility I mentioned above: that we will discover Kuiper Belt or Oort Cloud objects bigger and more massive than Mercury -- or Mars, or conceivably even Earth -- which will nevertheless not be given the title of "planet". (If the IAU's definition of the moment stands, they will instead have to suffer under the humiliating title of "dwarf planet".) Cue Jay Leno.

Soter has tried to come up with an entirely new scientific definition of "planet". But his definition -- like Stern's new "gravitational rounding" definition that the IAU's subcommittee originally proposed to adopt -- depends on numerical limits (in this case, mass and orbital period) that are ultimately just as arbitrary as the size gap between Pluto and Ceres (and before that, between Mercury and Ceres) that served for so long as the planet/nonplanet dividing line before the rest of the Kuiper Belt besides Pluto was suddenly discovered. Most non-scientists think of "planets" in terms simply as Sun-orbiting objects above a certain diameter or mass, and this definition is really just as defensible, as Stern's and Soter's proposed new "scientific" definitions that also, in the end, rely on arbitrary limits.

So I suspect that in the end the IAU will finally have to beat another embarrassing retreat and fall back on the common-sense dividing line: call everything above a certain average diameter (and/or a certain average mass) a "planet", and everything below that size a "nonplanet". Average diameter is a lot easier to determine for distant KBOs than mass is, and so would be better for the purpose -- unless we're talking about the planets of other stars, where we often know the mass before we know the diameter. But the masses of all those extrasolar planets known so far are hundreds or thousands of times the mass of Mercury, and so we can safely say that a planet is either something that is known to be either above a certain diameter or above a certain mass.

We could set that dividing line at a diameter of 2000 km -- a nice neat figure only a little bit smaller than the diameters of Eris and Pluto, thus keeping a huge and confusing flood of other KBOs from crashing the Planet Club. (Or, if we want to get pickier, we could set it at 4000 km (keeping Pluto and Eris out, but keeping Mercury in.) And the lower limit for the mass of a planet -- should we discover a candidate's mass before we discover its diameter, as with the extrasolar planets -- would be the mass that a planet 2000 (or 4000) km across would have if it had the same density as Pluto (which is about half ice and half rock).

Everyone would have to recognize that this simple size definition was also strictly arbitrary -- anything even a tiny bit below the cutoff point would be called a "nonplanet", and everything even a tiny bit above it would be called a "planet". But there's no problem in everyone, including schoolkids, readily recognizing that fact -- and recognizing that the Solar System has actually become a more interesting place since the discovery of a whole new wing of it has made the definition of a planet forever fuzzy.

(One final note. In the case of Pluto and Charon, the IAU's subcommittee proposed to call them both planets because, unlike any of the other planets and their moons, they're close enough in mass that they circle each other around a center of mass -- a "barycenter" -- that is actually short distance outside the body of Pluto itself, as though they were two ends of a dumbbell. Some asteroids and KBOs are known that are a lot closer together in mass than Pluto and Charon are, and so their barycenters are a lot closer to the midpoint between such two "binary objects". But Charon was called simply a "moon" of Pluto for over a quarter-century after its discovery in 1978 -- and my size-cutoff definition of 2500 km would allow it to remain officially just a "moon", while at the same time leaving us free to tag a genuine "binary planet" in the Solar System if we ever discover two KBOs orbiting each other that are both more than 2000 km wide.)

Finally: note that we are already faced with the same problem where defining a namable "moon" of a planet as opposed to a "ring particle" circling it. The Cassini orbiter has already discovered objects in Saturn's rings just a few hundred meters wide, and we have every reason to think that similar objects exist in the rings of the other three giant planets. Do we call them moons, or not? This nomenclature problem is every bit as ambiguous as the planet/nonplanet nomenclature problem -- and in this case we can't even hope for some "scientific" definition like Stern's or Soter's to help us out. It's time we faced up to the fact that, where the Solar System is concerned, our categorization of objects by size is inevitably going to be our own arbitrary choice.

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Armagh, UK (SPX) May 18, 2007
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