"Pluto is dead" - Mike Brown, It's official Options

[alan]

Traditionally, back in the 1800's, Ceres, Pallas, Juno, and Vesta were listed in almanacs and journal by order of semimajor axis between Mars and Jupiter. For a while this made Jupiter the 9th planet and Neptune the 12th. After many more were discovered in the mid 1800's a numbering scheme was created and the new asteroids were put at the end in order of discover which was more convenient.

Marsden proposed giving Pluto minor planet number 10000 for a similar reason: he thought it would be more convenient if it was included in the minor planet database because Pluto kept being 'discovered' by automated asteroid search programs.

Traditionally, when a planet is found to be part of a large group, or belt, of objects it becomes part of that category instead of a planet. See the "unjust demotion' of Ceres for example.

[JRehling]

Traditionally, when a planet is found to be part of a large group, or belt, of objects it becomes part of that category instead of a planet. See the "unjust demotion' of Ceres for example.

That's not a truism and an example. That's a "rule" induced from a single example. If we had found a 10-25 earthmass body about every 10 AU out past Neptune, I doubt if the fact that they constituted a large group would have led anyone to revoke the "planet"hood of all of them, along with Uranus and Neptune, too, leading us to conclude that Mercury through Saturn were planets, but Uranus, Neptune et al were not.

When we try to induce why Ceres was demoted, we're theorizing in cognitive science. Which is actually the proper field for all of this, not astronomy.

To go down that road, the many-ness of the MBAs (main belt asteroids) was a likely factor, but size was as well, with Ceres being about 19% the diameter of Mercury. (A history of the accuracy of those measurements would be interesting; I don't have that information, but I don't think it fluctuated as wildly as the Pluto size estimates.) If it had turned out that the MBAs had many objects about the size of or larger than Mercury, then I guess we would have had a "is Mercury a planet?" debate 150 years ago. The large gap between Mercury and Ceres prevented that.

If it turned out that Pluto were about the size of Earth and we had kept finding KBOs that totally closed the size gap between Earth and boulder, howevers, I don't think we ever would have had the "is Earth a planet?" debate. Well, except for much earlier when it was realized that Earth was a globe surrounded by space instead of a planar surface overlying Hell.

The essential matter is that it seemed reasonable c. 1850 to exclude some small bodies from planethood, and the then-apparent gap made it look like nature had handed us some natural categories. The current embarrassment comes from the fact that the "natural category" assumption is not being questioned. Appealing to quantities that had never been a part of anyone's definition in order to reverse-engineer a desired outcome isn't good astronomy, good cognitive science or anything.

[john_s]

Well here's my contribution to the debate...

Planetary Society blog

John.

[djellison]

We gloggers all have to have a bit of input into the debate Jim had a go at it, so did Rosaly, now your turn....I'm hoping that by October, the IAU will have reduced the status of Pluto to that of a large onion so I can have a stab at the subject as well

Doug

[ugordan]

Well here's my contribution to the debate...

I like it. It looks as one of rare positive views on the whole definition thing. Most others just seem to bash the planet definition not because it's plain bad (though I admit, it is sloppy), but because Pluto isn't a planet anymore and they're just trying to find reasons to attack it. I get the feeling they'd be more than willing to live with the definition if Pluto remained a planet by the definition's standards.

Just my 2c...

[odave]

Very well written, John. I agree that the "neighborhood clearing" statement is clumsy. If they'd taken the time to word that phrase better, IMHO, fewer teeth would have been gnashed.

Thanks for your perspective.

[JRehling]

John, your statement of support is probably the best I've seen... which means it's the best place from which to try to take the discussion further.

Let's say that one principle is that a meaningful category can be induced when a large gap is seen in the sizes of the objects within a local population. In the inner solar system, we have a gap between the size of the Moon (or Mercury, if you want to exclude non-solar-primary orbits) and the largest inner-SS asteroid (which is about 5 km in the longest axis?). In the outer solar system, we have a large gap between Uranus/Neptune and Ganymede, or (sun-primary-only) Xena. Past Neptune, we have no such gap (smooth continuum from Xena down to gravel, probably).

A problem I see with your explanation of the current definition is that these are two or three different populations. It's fine to come up with a term that distinguishes Mercury, Venus, Earth, and Mars from the Apollo, Aten, and Amor asteroids, and another that distinguishes Neptune from Pluto, but there are two unexplained conceptual leap:

That local-relative size is the sole distinguisher of a category that has global (for the inner solar system, for the outer solar system, and possibly elsewhere: "planet" in all cases).

Suppose we found, way out there, an zone with tight concentric orbits with sixteen 10-Jupiter-mass-sized objects, and one Venus-sized object. The gap between the "biggies" out there and the Venus-sized object is bigger than the gap between Neptune and Xena, and I think this brings out the problem with the "local qualification... global term" problem. Now the far-out Venus is not a planet, but the way-inside Mercury is, despite the former being absolutely larger.

This seems to me to be extremely hard to defend (aesthetically, which is what this exercise is all about). The original sin that begs for this problematic "local qualification... global term" quirk is, of course, the fact that the gas giants are so much larger than the terrestrial planets, but the terrestrial planets are not that much bigger than the big KBOs. To spell out the numbers, and to focus on size alone, the geometric mean of the radii of Neptune and Pluto falls between that of Mars and Venus: Mars is more "like" Pluto than it is like Neptune. Certainly Pluto and Xena are more like Mercury than Earth is like Neptune.

The "local qualification... global term" problem fixes the answer a certain way by comparing Xena and Pluto to Neptune, but comparing Mercury only to Icarus and Geographos.

As you note, the definition is already counting on there to be no big KBO out there that muddies things. But Sedna may already do that, by working the "local" loophole, with nothing else (maybe) around it. And a really big KBO would attack the "global" problem with the definition. But we can see the problem without even having those new discoveries turn up.

[Superstring]

This is my first post here. I have thought long and hard about this debate, and although I am probably late in chiming in on this board, I do feel some things need to be said. Perhaps it already has been said, perhaps not.

First off, the problem with using dynamics as the fundamental characteristic in defining planets is simple: it is inconsistent. Each stellar system likely has its own unique dynamics, in fact a lot of them so far are rather different from our own Solar system. It doesn't make sense to make a definition that caters to only one system and no others. I know that's what the IAU did and intended to do, but I fail to see why. Our knowledge of other stellar systems is rapidly increasing and it won't be too long before a plethora of Earth-mass objects are found around other stars. So to me, it would make sense to go ahead and come up with a universal definition that is applicable to any system or place; otherwise, we're not really coming up with a definition per se, but more of a standby filler as we wait to gather more information.

The inconsistency in stellar system dynamics brings up several noteworthy, hypothetical scenarios. Systems with less accretion material, especially those involving small brown dwarfs as the center object, could easily have a Ceres-sized object that has "cleared its orbit." On the other hand, objects in our own system beyond Neptune are larger, and the possibility remains that a Mars or even Earth-size object in the region is waiting to be found. Yet, even these objects would be kept as dwarf planets merely due to location. It is worthy to note that had our system not had Jupiter or Saturn to absorb and slingshot material, the inner region very well could have consisted of a swarm of large objects as opposed to four, neatly-arranged worlds. The lack of such giant planets beyond Neptune is the reason why smaller bodies were able to spread out into the wide, chaotic region we know as the Kuiper Belt.

The actual criteria of "clearing an orbital neighborhood" is not only vague and suspect, it is arbitrary. What exactly constitutes a cleared orbit? Citing that one can see it and know it in our Solar system is not an acceptable answer since, as I aforementioned, it is best to come up with a definition that applies anywhere. Obviously, other stellar systems may not be so clear-cut on orbital dominance. Any boundaries we set regarding the width of a belt, maximum size ratios, etc will all be completely unscientific. Another issue with this is that it is difficult, if not impossible, to tell whether an exosolar object has "cleared its orbital neighborhood" or not. We could find several Earth-size planets, only to find out that all of them are "in a belt" and thus be forced to downgrade them to dwarf planets. Where is the logic in this? I should also point out that Sedna is currently the only object in its orbit. According the IAU definition, that technically makes Sedna planet, even though other objects are expected to be discovered in its region in the future.

Clearly, the definition needs to be based upon something more consistent, more applicable, and more scientific than dynamics and location. Some have suggested origin, but this will be even more difficult to determine and categorize. There is really no way of knowing how or where many of the known substellar bodies originated, so it would be ludicrous to base a whole classification off of that trait.

With all of this said, it is my opinion that the best option lies in the physical attributes of an object. Here is the definition of the word planet I have come up with, written in IAU style:

A planet is a celestial body that has (a.) sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape, (b.) restricted mass so that it cannot support or sustain fusion reactions, and (c.) does not contain degenerate matter.

In laymen's terms, a planet is any object that is rounded by gravity and is not, or never was, a fusor. That's it. This is simple, scientific, consise, and groups a large, intriguing class of objects in our universe under one name. I fully realize this includes moons; that is my intention. Moons that are pulled round are the same class of object as rounded objects that orbit a star. The only difference is orbit, and I stand by my case that orbital traits should be left aside from the broad definition. Titan is just as much a planet as Mars, as are Enceladus, Umbriel, Quaoar, Pluto. They are all planets.

Since I have made planet a broad term, sub-categories are needed. There are two fundamental and necessary ways to divide planets: orbit and size.

Based on orbit:
1.) A primary planet is a planet that orbits about a point within a star or any fusing object.
2.) A secondary planet is a planet that orbits about a point within another planet.
3.) A rogue planet is a planet that drifts freely through space.

Based on size:
1.) A giant planet is a planet that has sufficient mass for its self-gravity to sustain an envelope of light gases that make up the majority of its total mass.
2.) A terrestrial planet is a planet that has sufficient mass for its self-gravity to sustain an atmosphere from gases present in its hill sphere*, but restricted mass so that it is not a giant planet.
3.) A dwarf planet is a planet that has restricted mass so that it is not a terrestrial planet.

*This limit is approximately 10^23kg, just slightly lower than the masses of Mercury and Callisto.

It is correct, but not necessary, to combine the two sub-categories when describing a planet. For instance, Earth is is a primary terrestrial planet. Enceladus and Europa are both secondary dwarf planets. The nice thing with this is that it's very flexible. One can easily group all the terrestrial planets or dwarf planets without worrying about orbit; conversely, one can group all the primary planets with orbit largely in mind. Another benefit of adding the major moons into the planet category is it would give them more public credibility. For some reason, discussion about a moon does not seem as fascinating to many people as a planet. This is demonstrated by how the Solar System is taught; students learn the terrestrial and giant planets that orbit the Sun, plus Pluto and maybe some asteroids, but rarely do the moons get such attention. This is a good example of why limiting this kind of definition based on orbit is so problematic.

If this definition is applied, our Solar System contains about 50 planets and likely more to be added beyond Neptune. In all honesty, I must ask: what's wrong with this? Astronomers aren't afraid to have billions of stars or billions of galaxies, but then the idea of a couple dozen planets is scary? This is flawed science. I imagine it stems from the notion that Earth is somehow in a special class of objects, but in reality, a planet is what it is. I also find it preposterous that Pluto and other similar round bodies don't qualify because they are too small. If we're going to get strict about size comparisons, how about we take note how small Mercury is to Jupiter. The size ratio between the smallest terrestrial planet and largest giant planet is actually about the same as the size ratio between the smallest dwarf planet and largest terrestrial planet (Mimas and Earth, respectively). The giant planets are so radically different that to include Earth and its neighboring large bodies in the same class, meanwhile excluding objects like Pluto, is insane.

Two cents.
-String

[john_s]

This seems to me to be extremely hard to defend (aesthetically, which is what this exercise is all about). The original sin that begs for this problematic "local qualification... global term" quirk is, of course, the fact that the gas giants are so much larger than the terrestrial planets, but the terrestrial planets are not that much bigger than the big KBOs. To spell out the numbers, and to focus on size alone, the geometric mean of the radii of Neptune and Pluto falls between that of Mars and Venus: Mars is more "like" Pluto than it is like Neptune. Certainly Pluto and Xena are more like Mercury than Earth is like Neptune.

Point taken. We really have not eight "planets" but four "terrestrial planets" and four "Jovian planets", and lumping these very different categories together is a bit artificial. But the other constraint we're working under is to have a definition of "planet" that's not radically different from the one that's currently part of the culture, and our culture has lumped together these two classes since antiquity. The IAU definition makes only a small adjustment to what currently counts as a planet, while making the definition cleaner. Hypothetical future discoveries could muddy the waters, but a roundness-based definition, for instance, produces muddy waters already, with a dozen known bodies that hover near the planethood boundary and whose status will always be controversial. This may make for good publicity, but doesn't make for a good classification scheme.

[dvandorn]

We really have not eight "planets" but four "terrestrial planets" and four "Jovian planets", and lumping these very different categories together is a bit artificial.

I'm not sure I agree with that characterization. I'd say we have four rocky planets, two gas giant planets, and two ice giant planets. Jupiter and Saturn are far more similar to each other than either is to Uranus or Neptune.

As an off-topic aside, we always hear that Jupiter and Saturn may well have rather small, rocky cores. Considering that most of the mass in the stellar disk probably migrated early to the gravitational center of the disk before the Sun ignited, is there a theory that the Sun has (or had) a rocky core in there somewhere? That would be a heck of a thing to see...

-the other Doug

[JRehling]

I'm not sure I agree with that characterization. I'd say we have four rocky planets, two gas giant planets, and two ice giant planets. Jupiter and Saturn are far more similar to each other than either is to Uranus or Neptune.

There's a technique called cluster analysis: There's no doubt that in our solar system, with size as the constraint, Jupiter and Saturn would be nearest neighbors, as would Uranus and Neptune, Earth and Venus. I'm sure, however, that the JS pair and the UN pair would be linked before the UN pair would be linked to the EV pair. Neptune is almost four times the diameter of Earth, but better than 40% the diameter of Saturn.

[Rob Pinnegar]

Titan is just as much a planet as Mars, as are Enceladus, Umbriel, Quaoar, Pluto. They are all planets.

In a way I'm surprised that this argument hasn't come up more often. Just about everyone seems to include "Sun-circling body" in the definition of "planet", but it doesn't *have* to be that way, really. Certainly it's not difficult to think of Titan as a "planet" -- in fact it seems more like a planet than a moon, and the same goes for Io and Europa at least. The "Small Solar System bodies" category then becomes effectively "moons of the Sun" that aren't big enough to be planets.

By the way, on the topic of Doug's point about whether the Sun ever had a rock-metal core: The silicon-iron inner layers of red supergiants (just prior to Type II detonation) could probably be argued in this fashion, though of course that's a very different situation from the proto-Sun because those elements were formed in place rather than settled under gravity. As for the Sun itself, I guess it depends on how strong the convection was -- it's an interesting point.

[David]

There's a technique called cluster analysis: There's no doubt that in our solar system, with size as the constraint, Jupiter and Saturn would be nearest neighbors, as would Uranus and Neptune, Earth and Venus. I'm sure, however, that the JS pair and the UN pair would be linked before the UN pair would be linked to the EV pair.

Now that's just what we need: a cladogram of the planets!

Which, I guess, would look something like this:

1.
1.1
1.1.1 Jupiter
1.1.2 Saturn
1.2
1.2.1 Uranus
1.2.2 Neptune

2.
2.1
2.1.1 Earth
2.1.2 Venus
2.2
2.2.1 Mars
2.2.2 Mercury
2.2
2.2.1
2.2.1.1 Eris
2.2.1.2 Pluto
2.2.2 -- All other minor planets and assorted solar debris

Every definition of planet proposed up to now has thus been paraphyletic -- and thus not really "scientific".

[alan]

Slideshow showing relative size of various objects ranging up to largest stars. Pluto is mentioned. Takes awhile to download.
http://sizeofoutworld.ytmnd.com/

[laurele]

- BREAKING NEWS: Pluto Demoted, No Longer a Planet

- Pluto loses status as a planet

"Pluto is dead???" What kind of ridiculous statement is that for a scientist to make? It sounds as if he believes he has the power to remove an object, no matter what it is called, from the solar system. Remember, "a rose by any other name..." It sounds like he is reflecting a personal bias against Pluto that borders on the irrational. Sorry, Dr. Brown, but you cannot "kill" a planet by deciding to change what you call it. Pluto is very much out there, the same as it always has been. The debate over the definition of the word "planet" and over Pluto's status is far from over. Meanwhile, I would expect a far more professional, rational attitude from someone of Dr. Brown's status.