Fight for Pluto !, A Campaign to Reverse the Unjust Demotion Options

[MahFL]

Well I still think of Pluto as a planet, small, cold and far far away.

[Alan Stern]

Thanks Alan. That was great! (Except for the Snapple ad on the first page obscuring the text -- I had to copy and paste that part of the article to read it.)

You two don't actually sound all that far apart in this article. Especially at the point where you
want to make a term for "planetary bodies." Is there any special reason not to push the term
"planetoid" for that? I know Mike Brown has proposed it at least once.

http://www.gps.caltech.edu/~mbrown/whatsaplanet/revolt.html

I realize it's an old term for an asteroid, but surely that usage has long expired. Non-fusing bodies large enough to be in hydostatic equilibrium really deserve a sexy name -- and at the moment they've got none at all. Heck, it would even be easier to explain to kids, "Well, you know scientists usually talk about planetoids, not planets, because planet is also about location, and location doesn't really matter that much."

Greg-

I agree, Mike is coming around.

As to your question about the term "planetoid," I'm not keen on it because it sounds like a
small thing.

But when I say something is a "planetary body" (PB) I include everything anywhere from a dwarf
just big enough to be in hydro equilibrium to a giant like Jupiter. Some PB's orbit their stars, some
orbit other planets (e.g., Titan, Triton, Io, etc.), and some are simply orbiting freely in the ISM
owing to ejection from planetary systems.

This naturally suggests a 3x3 matrix of PB types; I give some example assignments in the table
below:

Satellite Planet Unbound Planet
Dwarf Triton Pluto TBD
Terrestrial Titan Venus TBD
Gaint None Jupiter TBD

Just for ease of round numbers, I put the cutoff between Dwarfs and Terrestrials as a mass
of 0.1 Mearth (just below Mars) and I put the cutoff for Giants at 10 Mearth (just shy of
Uranus and Neptune).

One could go further with more categories, but this is the simple system I advocate.

-Alan

[SFJCody]

Just for ease of round numbers, I put the cutoff between Dwarfs and Terrestrials as a mass
of 0.1 Mearth (just below Mars) and I put the cutoff for Giants at 10 Mearth (just shy of
Uranus and Neptune).

One could go further with more categories, but this is the simple system I advocate.

-Alan

I still like the system I came up with last year:

http://homepage.ntlworld.com/sfjcody2/planetarycatesmall.png

[Greg Hullender]

What is the boundary on the ability of a planet to hold a hydrogen atmosphere? If I remember right, there's an exponential function of time in the equation, so the boundary should be fairly sharp. That seems like the most logical boundary between Terrestrial planets and Giant planets. Or is that already 10 MEarth?

As for "planetoid" meaning small, I know that was the historical meaning, but I think most people know that in general -oid just means "similar to." Lay people don't think "humanoids" are little humans, and I'm pretty sure scientists don't think spheroids are little spheres. :-) I think "planetoid" could work -- meaning that everything from Ceres to Jupiter would be a planetoid -- if the right people got behind it. That might just be you and Mike. :-)

[JRehling]

What is the boundary on the ability of a planet to hold a hydrogen atmosphere? If I remember right, there's an exponential function of time in the equation, so the boundary should be fairly sharp. That seems like the most logical boundary between Terrestrial planets and Giant planets. Or is that already 10 MEarth?

It depends upon temperature. Go out to where it's 2K, and a pebble of solid hydrogen would hold itself. Orbit just above the corona and Neptune couldn't hold it together. The latter type of world has been named "chthonian", in case we find any.

I raise an eyebrow at the idea that temperature would become such an important determining characteristic of "planet". But then I raise an eyebrow at any characteristic that is ex post facto to usage and tries to "reverse engineer" what people mean. When you find yourself needing such factors, it's time to concede that there isn't a natural category that deserves a precise definition. I don't know why people have such a hard time even addressing that point. I've heard people refer to "Nobel Prize fever" before. I think the main phenomenon we have here is "definition fever". Something big is (possibly) going to be decided and people would rather be a part of defining something than deciding not to define something. So the awkward definitions come rapid fire.

[Guest_Kevin Heider_*]

What would happen if we took Jupiter (as it is today) and put it in place of Mars? Would Jupiters strong gravity and magnetosphere help protect Jupiter from the now stronger solar winds and radiation? Or would Jupiter be reduced to a 10 Earth Mass object in 1 billion years as it is stripped of most of it's hydrogen and helium?

[Bart]

What would happen if we took Jupiter (as it is today) and put it in place of Mars?

"A diagram for the evaporation status of extrasolar planets" A. Lecavelier des Etangs
http://fr.arxiv.org/PS_cache/astro-ph/pdf/0609/0609744.pdf

Bottom line: not a whole lot would happen

(to Jupiter that is )

[Guest_Kevin Heider_*]

My reply to Mike Brown is in an interview we did together for Air & Space, see: http://airspacemag.com/issues/2006/october...PlutoDebate.php

Stern: it creates a situation—untenable, in my view—that a given object can be a planet in some circumstances and not in others. For example, the Earth, by this definition, would count as a planet at its current distance from the Sun. But if you moved the Earth out into the distant reaches of the solar system and discovered it there, it would not be a planet [because it wouldn’t have cleared its orbit]. And the same is true if you put Jupiter in the Oort cloud.

If it were orbiting another planet, I would call it a satellite that’s a planetary body.


Brown: Earth further away, Jupiter further away—I would agree you wouldn’t classify those as planets by this definition. But that actually makes it very interesting. Why would you not classify a Jupiter in the Oort cloud as a planet? Because it behaves very differently. It has had a very different history than these other eight planets.

But if you put it in orbit around another planet, it suddenly becomes a satellite. If you really go with the physical criteria, you don’t care where it’s located.


Heider: Mars in orbit around Jupiter. Titan in orbit around Saturn. Huya a Planet. Equal rights for equal mass? Perhaps the definition either needs to be ALL inclusive or dynamically exclusive!

-- Kevin Heider

[Superstring]

Heider: Mars in orbit around Jupiter. Titan in orbit around Saturn. Huya a Planet. Equal rights for equal mass? Perhaps the definition either needs to be ALL inclusive or dynamically exclusive!

I agree with this. My original definition made any non-fusing gravitationally-rounded object a planet (including moons)...with sets of sub-categories based on orbit and physical traits. Although I still maintain this is the most scientific scheme...it lacks cultural acceptance. Do you really think people will start calling the moon a "secondary planet" as opposed to "a moon"? That won't fly. Since we have to make a separation between moon and planet...two categories for the same physical kind of object...why not further distinguish spheroids that are orbitally dominant and those that are part of a swarm? Ceres, Pluto, Eris, Quaoar, etc are all planetoids...that is, they are large enough to be a planet, but they are not dynamically significant bodies and thus do not quite meet the requirement for planethood.

I think this concept is easy to explain. There are 3 types of large objects in any given solar system: Planet=orbits star, controls its own orbit. Planetoid=orbits star in a region of other objects and planetoids. Moon=orbits planet or planetoid and (IMO) also has to be round. The remaining small objects can be divided into asteroids, comets, and moonlets. All of the above classes can still have sub-classes.

I think this is a scientifically good scheme. It enables us to teach the 8 planets, plus the Main Belt (which contains many asteroids and one planetoid) and also the Kuiper Belt (which is larger and contains asteroids, comets, and planetoids).

[JRehling]

To one and (almost) all:

This planet definition issue isn't a scientific matter, really, but it wouldn't hurt to discuss it on a high level.

In a debate, the two (or more) sides advance arguments. The arguments, unless truly ludicrous, merit some rebuttal, and some kinds of appeal are more persuasive than others. I see a little of that in this thread. A little. But what I haven't seen anyone TOUCH is to seriously address this:

Why define planet at all?

There is a "countermove" which goes something like:

"Because as scientists, we must provide order to an otherwise chaotic universe. It allows us to agree upon terms when we discuss them, which is a basis of all discussion."

OK, counter-counter-move is:

"This only makes sense when the categories themselves make sense. "Prime number" and "composite number" truly are all-encompassing and mutually exclusive for the natural numbers greater than 1. But "planet", we are learning, is not such a category! And scientists deal with many such categories already. "River" is a great example. Serious journal articles are written on serious research on the hydrodynamics and biomes of rivers without any rigid definition discriminating between rivers and streams. We once thought "planet" was amenable to a rigid formal definition but the facts have consistently led us to the opposite conclusion, that it is like "river", a you-know-it-when-you-see-it category."

And I've found it VERY frustrating that no one has a counter-counter-counter-move to this, no one attempts one, but just goes on assuming that they can tinker with the definition with one hack or another and of course there has to be such a definition.

I think the definition itself (not any one specific definition, but the idea of having one) has become like a toy that every child in the room wants in his or her own hand, and the idea of there being no formal definition threatens each person's personal ambition to have influence on the definition, so it is being waved off without a shred of rational consideration.

If someone has a counter-counter-counter-move along these lines, I would love to have some serious discussion with them. If simply ignoring this line, or repeating step #2 endlessly (we've moved past it... it's time for step #4) is all anyone has, I think they've stopped being a serious discussant of this topic, regardless of credentials.

[David]

If someone has a counter-counter-counter-move

I intend to stop using the word "planet" altogether, except in historical (pre-2006) contexts, and start using the word "world" instead, for which I will have my own, highly idiosyncratic, inconsistent, and fluctuating definition. I haven't yet decided what it will be, but it will probably combine rotundity (not sphericity), (potential) possession of an atmosphere, (potential) possession of moons, a low boredom rating, and possession of a cool name in ratios yet to be determined.

[Guest_PhilCo126_*]

Well, I guess the fight continues:

http://www.plutoisaplanet.org/

The Great Pluto Debate!

Location: Clay Center Lecture Hall
Date: Sunday, February 4th, 2:00 - 3:00 pm

Special Lecture Series Event: In partnership with the Society for the Preservation of Pluto as a Planet (SP3), on the birthdate of Pluto discoverer Clyde Tombaugh, February 4th, the Clay Center Observatory will hold a special "Pluto Multimedia Show and Friendly Debate."

Free admission. Limited seating!

[Bob Shaw]

As for names, I've always liked 'Planetoid' - and 'Worldlet' is quite poetic, implying as it does that such a body might grow up one day to become a fully fledged wossisname!


Bob Shaw

[Greg Hullender]

I really like "planetoid" too, but Alan Stern thinks it's too diminutive.

I guess we'll have a better idea of what taxonomy to use once we can get real details on extra-solar planets, but there do seem to be some obvious break points that merit some recognition. e.g.

1) Star (big enough to fuse hydrogen)
2) Jovian Planetoid (big enough to have a hydrogen atmosphere)
3) Terrestrial Planetoid (big enough to be round)
4) Asteroid (not big enough to be round)

You could divide 3 and 4 between "made out of rock" and "made out of ice".

Then you could have separate categories for graviational/associational properties. e.g.

1) Independent Planetoid (dominates it's region of space).
2) Twin Planetoid (has a companion at least 1/25th its mass but not more more than 25x).
3) Bound Planetoid (forced into some sort of orbital resonance with a larger planetoid).
4) Satellite Planetoid (directly orbits a larger planetoid).

One could play around with the definition of "dominates"; there might also be value in distinguishing planetoidsin circular orbits above the equator of the star (as having formed from the accretion disk) from planetoids in tilted, elliptical orbits (as having formed separately from the original nebula). Some term to differentiate airless planetoids from those with atmospheres might also make sense.

The general idea, though, is that a richer taxonomy might be well worthwhile, and it's probably easier to get there if we use a term like "planetoid" rather than "planet."

--Greg

[JRehling]

I guess we'll have a better idea of what taxonomy to use once we can get real details on extra-solar planets [...]
The general idea, though, is that a richer taxonomy might be well worthwhile, and it's probably easier to get there if we use a term like "planetoid" rather than "planet."

--Greg

I think the species we already know about defy the notion of a biological-style taxonomy, even a shallower one.

Size, composition, orbital-clearing, nature of the atmosphere, and most other attributes are going to be freely cross-correlated to a considerable extent. There will be massive worlds that haven't cleared their orbits, there are going to be massive (hot) worlds with no atmosphere, tiny worlds that have ended up spherical, etc. There'll be a worlds representing the full range of hydrogen as an atmospheric component.

I think the exercise will prove to be rather like looking at an employee data base and searching for a way to relate salary, gender, and department. In the final analysis, you just end up with a bunch of metrics and you note how individuals measure according to each metric.

That might be what it takes to get us to discard the bogus categories in our solar system wherein nine data points happen to spell out some perceived categories but don't do so ontologically and certainly won't as more data comes in.

The stuckness that has to break is the notion that such categories have a service to play even when the universe provides no such categories. Larger KBOs may break the logjam there in the short run, but extrasolar discoveries will end up doing so in the long run.