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View Full Version : Should the IAU abandon the term "planet"?



Mountaineric1969
2010-Mar-31, 10:45 AM
There is STILL a lot of anger over Pluto's "demotion" to "dwarf planet" by the IAU last year. Perhaps the IAU should instead completely abandon the term "planet" altogether. It is after all rather archaic. The term comes from the Latin for "wanderer".

Today we know of at least 4 or 5 completely different types of objects that orbit stars all made of different stuff. You have the rock balls like Earth, the gas balls like Jupiter, and ice balls like Pluto (here "ice" refers to any solid that would become liquid or gas in normal Earth conditions). We also have hunks of rock, metal and/or ice that are to small to be round. Perhaps terms like "Rock Ball" "Gas Ball" "Ice Ball" "Hunk of Rock" and "Hunk of Ice" would be better (I also dislike the term "asteroid" which is Latin for "star like". Clearly "asteroids" are not at all like stars). This would avoid the term "planet" and give us nice clear categories to place new discoveries into (including items found orbiting other stars). It would also avoid the supposed stigma of demotion.

Perhaps we can even avoid "English centrism" by using Latin. Latin might even sound better than "Gas Ball" which to my ear sounds rather crude.

I know this leaves out moons but everybody already agrees that if you orbit something in orbit around a star that you are a moon (or you're a moon if you orbit a moon).

astromark
2010-Mar-31, 11:02 AM
No, they got it right. Pluto is smaller than some of the moons orbiting Planets of this solar system. Dwarf Planet is perfectly clear and reasonable.
The excepted term Planet. describes to me a orbiting body of sufficant mass with a clear orbit about its star. Stars do not have moons. Planets do.

iniaes
2010-Mar-31, 11:08 AM
Planet does not mean "wanderer" as far as the IAU are concerned. They use the word to define any celestial body that orbits a star, has enough mass to assume a spherical or oblate spheroidal shape, and has cleared its orbit of planetoids, debris and other minor celestial bodies. A dwarf planet (pluto et al) is all these things but has not cleared it's orbit of debris.

It is common to use unofficial nomenclature to differentiate between the planets, ie, Terrestrial planet, gas giant, ice giant and so on. And the asteriods have their own subtypes, chondtrites, carbonaceous, silicateous, or stony to name a few.

moreover, a body that orbits a planet is not a moon, it is a sattelite. The moon got it's name because it was originally used to time the passing of a month (moonth), and the term is often incorrectly used to describe the sattellites of other planets.

So, should the IAU dispose of the label "planet"? I think not, as given their definition, it is more than adequate.

AndreasJ
2010-Mar-31, 11:42 AM
The term comes from the Latin for "wanderer".Greek, actually. But the etymology is entirely irrelevant to modern use (my favorite example of an etymological meaning that's completely lost in the modern usage is "nice", from Lat. nescius "ignorant").

As for abandoning the word, nobody would stop using the word if the IAU went back to not providing an "official" definition; it'd just be used with even greater lack of consistency than it's now, which wouldn't help anyone.

Swift
2010-Mar-31, 01:58 PM
Mountaineric1969,

Your post seems more a "question" for general discussion, rather than a specific astronomy question that needs a specific answer. I've moved the thread to Astronomy, where we've had plenty of past discussions about Pluto.

John Jaksich
2010-Mar-31, 03:01 PM
I find your question interesting because ultimately if (?) we were to come to a point where we had to systematically catalog -the Universe in the same manner that chemists use the periodic table (e.g. properties and reactivity based on periodicity), then substituting a rigorous fashion of naming might be better. But, in my humble opinion, we have been using a naming-type "characterization" for practically everything since humanity could form polysyllabic words. It just seems to make things easier on people--in general.

EDG
2010-Mar-31, 11:06 PM
What annoys me is that people keep coming up with needless sub-classifications for things that aren't very well defined ("planemo" is one that really irks me. It's a planet that doesn't orbit a star, it doesn't need a special name - "rogue planet" will do). IIRC the current IAU definition of "planet" doesn't even technically apply outside the solar system, and the true planet/dwarf planet divide isn't well considered. Basically what it should boil down to is that if it's a large member of a "belt" of similarly sized objects then it's a "dwarf planet". IIRC Alan Stern came up with a more rigorous way to define it based on how well the object had cleared the space around it than what the IAU adopted.

Romanus
2010-Apr-01, 02:18 AM
Science and classification go hand-in-hand; a planet definition of some type is needed, IMO. Furthermore, science generally (as it should) errs on the side of philosophical pragmatism. Unfortunately, this leads to the problem of no definition possibly pleasing everyone; cf the interminable conflict in biology (especially paleontology, in which remains are scarce to begin with) between "lumpers" and "splitters".

Short answer: the planetary definition is a bitter pill to some (I'll admit, Pluto will always be a planet to me), but it's necessary.

laurele
2010-Apr-02, 02:13 AM
The IAU definition is a terrible one, which is why it is not accepted by a good portion of the astronomy community. Even Neil de Grasse Tyson describes it as "flawed" and has publicly stated he never took the position that our solar system has only eight planets. People are angry because the process used to adopt this definition violated the bylaws of the IAU and was rushed through by a minority of members at the last minute. Before last summer's General Assembly, the IAU was asked by astronomers including Stern to reopen the debate, and they adamantly refused, leading to a large contingent boycotting the General Assembly entirely. Not wanting to reopen the debate is itself suspect. What does the IAU leadership fear if they believe their definition is solid?

The problem is largely in the claim that dwarf planets are not planets at all. If the resolution were amended to establish dwarf planets as a subclass of planets--planets because they are large enough to be in hydrostatic equilibrium but of the dwarf subcategory because they do not gravitationally dominate their orbits--much of the opposition would dissipate.

Small planets like Pluto have more in common with the Earth than do gas giants like Jupiter. Pluto has a solid surface, is geologically differentiated, has weather, and even has a nitrogen atmosphere. It is the only planet in our solar system other than Earth to have had its moon formed via an impact with another large body. How can one justify placing Earth and Jupiter in the same category? Jupiter has no solid surface, has a composition of mainly hydrogen and helium like the Sun, and has its own little solar system of moons going. These moons formed along with the planet, unlike Earth's moon and Charon.

Also, the IAU definition precludes any exoplanets from being considered planets since it says planets must orbit the Sun, not "a star." And we have found giant exoplanets with orbits much more eccentric than Pluto's, including at least one system in which two giant planets orbit in a 3:2 resonance just like Neptune and Pluto.

The IAU bungled this badly and should take themselves out of the business of defining the term "planet" entirely. They never felt a need to define terms such as "star" and "galaxy." This was a bureaucratic and political decision stemming from a very unscientific reluctance to accept we may have a large number of planets in our solar system. If their definition is ignored by enough planetary scientists, it will fall by the wayside and be essentially meaningless.

agingjb
2010-Apr-02, 07:49 AM
Titan has more in common with Earth than rather a lot of planets and other bodies, but...

Taxonomy is certainly necessary, but I wonder if it is compatible with stable nomenclature. The generic names of some birds are now changing faster than their common English names.

baric
2010-Apr-03, 03:15 PM
There is STILL a lot of anger over Pluto's "demotion" to "dwarf planet" by the IAU last year. Perhaps the IAU should instead completely abandon the term "planet" altogether. It is after all rather archaic. The term comes from the Latin for "wanderer".

Four years ago, not "last year".

The level of "anger" is irrelevant. There is also still a lot of anger over Darwin's theory of evolution published 150 years ago.



Today we know of at least 4 or 5 completely different types of objects that orbit stars all made of different stuff. You have the rock balls like Earth, the gas balls like Jupiter, and ice balls like Pluto (here "ice" refers to any solid that would become liquid or gas in normal Earth conditions). We also have hunks of rock, metal and/or ice that are to small to be round. Perhaps terms like "Rock Ball" "Gas Ball" "Ice Ball" "Hunk of Rock" and "Hunk of Ice" would be better (I also dislike the term "asteroid" which is Latin for "star like". Clearly "asteroids" are not at all like stars). This would avoid the term "planet" and give us nice clear categories to place new discoveries into (including items found orbiting other stars). It would also avoid the supposed stigma of demotion.

It was not a "demotion"; it was a reclassification. This happens all of the time in science as we learn more. The only "stigma" assigned to it is by those people who seem to have an emotional attachment to the previous classification.

In no way does this reclassification change the compositional or orbital properties of Pluto.

Hornblower
2010-Apr-05, 01:26 AM
I see no need to abandon the term "planet". When I was getting acquainted with stars and planets as a child in the late 1950s, some writers referred to the big nine as major planets and to large asteroids such as Ceres as minor planets. I was not the least bit confused by such terminology.

I look upon the recent reclassification of Pluto as the functional equivalent of the reclassification of Ceres, Pallas, Juno and Vesta a century or so earlier.

Jens
2010-Apr-05, 06:07 AM
In no way does this reclassification change the compositional or orbital properties of Pluto.

Yes, but it may have hurt its feelings.

baric
2010-Apr-05, 03:59 PM
Yes, but it may have hurt its feelings.

If anyone should be upset, it should be Charon. She just found out that she's been hanging around for millions of years with a dwarf planet.


Charon: Plu, honey, is it true what they are saying?

Pluto: no, Chari baby, don't listen to those haters

Charon: but, but.. they say now that you're really a dwarf!

Pluto: no, no. I've got all of the diameter a sexy moon like you could ever want.

A chill sweeps over Charon as the couple moves progressively further away from the Sun.

Charon: I'm not in the mood. Nix! Hydra! Don't stray too far.

Pluto: hey, don't be so frigid. I've always been massive enough to keep you and the kids hanging around. Why so chilly?

Charon looks backs at the distant Neptune. For a moment, she thinks she sees Triton winking at her. "A body like that," she calculates, "would certainly not expose our barycenter for the entire system to see."


Will Pluto's demotion become permanent? Can Charon accept this change in their relationship, or will she pack her bags on hop over to Triton on the next Neptune fly-by? Who will get custody of Nix and Hydra? Find out in our next exciting episode of "As the Solar System Turns"