A "What is a Planet?" definition debate thread

[Ara Pacis]

Since it seems that every thread about any planet, in this star system or not becomes a comparison debate about different definitions, some of us thought a formal debate thread may help. The idea is that anyone can post their theory for a definition and debate can ensue. To make this thread a better archive for such comparisons, I'll try to list them all in the OP with links to the individual posts. I'll try to keep the definition theories divided by object based and system based criteria. After a set time, we might start poll thread to vote for the leading theory types.

Rules

• I think this should be a living thread, so...
  
  You can edit your theory to add or subtract parts of your main theory in your Policy post based on new data or analysis as well as clarity. Please include addenda noting the changes.
  
  If you want to decide that you agree with someone else, then please keep your main theory in your Policy post, but add an adendum that you have withdrawn it and I will note it in the OP.
  
  Please don't edit debate posts except for clarity issues, per forum rules.
  
  Your definition policy or theory can use any reason, including personal, arbitrary, historical or other reasons, but we would prefer that it be more than simple whim. Some of us would prefer more empiricism based definitions, but even the IAU seems unsure.
  
  Preferrably, the definition policy should be universal and simple, but you can debate this. (Thanks Halcyon Dayz)
  
  Clearly identify how many Solar system objects you claim as planets.
  
  Please Keep Policy posts (with the main theory for planetary nomenclature) separate from Debate posts (arguing against and rebutting) for clarity.
  
  Let me know if you want more structure, and I'll post it here.

Policy Posts
Post - username - definition/type - # of planets in Solar system

• Object based
  2 - Ara Pacis - rotochemogravisphere non-fusor with subcategories - 10+
  3 - dgavin - non-fusor, LL/D>=2000km, with subcategories - 10+
  9 - Parallaxicality - any non-fusor, RC/A/G=yes/active, UL/C=metallicity - 9+ ?
  11 - Macro Mouse - gravispherical non-fusor, with subcategories - 9+
  16 - zorbo - any non-fusor, LL/D/V=Pluto - 9+
  47 - Hugo Drax - non star, LL/D>3000, RC/!=comet - 8+

• System based
  10 - baric - spherical non-fusor, LL/M=Mercury or Pluto, with orbit requirement - 8 ?
  98 - hhEb09'1 - rotosphermastoidindentafron (spheroidal non-fusor) - LL/R=200km, with orbital requirement - 10+ ?

Glossary

• LL = Lower Limit /characteristic
  UL = Upper Limit /characteristic
  RC = Required Characteristic /characteristic
  Characteristics =/Atmosphere /Composition /Diameter /Geology /Mass /Population /Radius /Volume /! (not)

[Ara Pacis]

Planet = a dense rotochemogravispheroidal mass (solid or fluid) that is not currently large enough to sustain fusion and has never sustained fusion. This means that a non-fusing brown dwarf is a planet, but a neutron star, is still a star. However, if a brown-dwarf gathers enough material to start fusion it would become a star and no longer be a planet. Both stars and planets can radiate EM energy. Note: this definition exludes stellar precursers and stellar remnants by definition (gas and dust nebulae, neutron stars, blackholes and other forms of singularity). After this are subcategories and subsubategories.

Regular Planet
If there is more than one orbiting a star then their orbital planes are all within 10° of each other. Orbits are generally circular with relatively low eccentricity. These planets orbit a star (including former stars) and not other planets. These planets are not part of a population (having one or many moons is not a population to which the axial planet would be part). This definition allows for double-planets and at least two requirements need to be met: one can not be less than 10% of the combined mass, and the barycenter can not be inside the radius of either object. This also allows for twin-planets.

Twin Planet: Any object that would be considered a regular planet on it's own but which happens to be co-orbital with another object that also meets the definition of a regular planet. I'm not even sure if this is possible or if it would be stable if the situation attained.

This may be further subcategorized into, Terrestrial, Terrestrial Giant, Gas, Gas Giant, Ice, Ice Giant (?), and others.

Irregular Planets and Objects
Generally, this is basically anything not a Regular Planet, including anything significantly off the common plane of the regular planets, or anything that has a relatively high eccentricity (including parabolic and hyperbolic orbits), or anything that is part of a population.

Populations: Any objects that are relatively co-orbital and co-planar where no single object has a majority of the combined masses (51% to 90% open for debate). This includes trojans (mulitple objects in a lagrangian orbits). A population is a group, but not all groups are populations. If an object in a population is large enough to be a rotochemogravisphere, then it is specifically termed an Population Planet (e.g. Ceres). Populations may be further defined as regular, irregular, etc.

Rogue Planet: Any gravispherical object that would be considered a planet except that its orbit is parabolic or hyperbolic or in any elliptical orbit that is considered unstable or that crosses the orbits and risks collision with a regular planet. A rogue object would be the same but subgravispherical.

Planetoid: any object smaller than a gravispheroid but having a surface gravity of more than 1mm/s/s (arbitrary and debatable).

Planetesimal: any object smaller than a planetoid down to 1km diameter (arbitrary and debatable).

Meteoroid: any object smaller than 1km in diameter (not including molecules or atoms). May be further subdivided into Large, medium, small, or micro.

Moon: any object that orbits a non-star. May be subdivided into Planetary moon (if it doesn't reach the status of double planet, e.g. Luna), and other categories as nomenclature allows. This would include objects in secondary lagrangian orbits (not solar lagrangian).

Companion: Any object that shares a gravitationally related orbit with a regular planet (or irregular planet or object) but does not reach the definition of a regular planet or moon and is not a co-population object (e.g. Cruithne). This includes solar lagrangian objects and trojan populations (AKA Companion Population).

Related Terms
Group: A group has similar orbital or physical characteristics, but not considered co-planar or co-orbital, e.g. Oort cloud, Scattered Disc Objects. This is different than populations, which share an orbit and plane. A group may contain both regular and irregular planets

Comet (group term): An icy planetesimal or meteor that exhibits rogue-like behavior.

Supercomet (group term): An icy planetoid that exhibits rogue-like behavior

[dgavin]

Definition i'll be using soon on my Exoplanet site:

Per IAU working group on Extrasolar Planets: Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.

And for now to define the Lower Limit until IAU pins it down oifficialy: Objects 2000km (Pluto Sized) or larger upto 13 Jupiter masses will be classified as planets, those less then 2000km will be classified as Planetoids.

*Edit 02/07/2006 - Added another class and reorganized list*

Within the classification of Planet, will also use some Sub-classes which best fit what type of Planet it is. Hopefully the IAU will come up with a similar classification sceme that can be used here soon. There are 4 main classes used to classify planets by size, and then 3 additional classes for special casses not based on size.


Class Definitions for Size

• Dwarf (D) Dwarf Planets(worlds) are defined more by thier mass then size, any small planet 2000km but less then 1/2 the mass of Earth. Basically planets without enough mass to hold onto thier atmospheres. Mars even though half the diameter of earth would fall into this class, as it only has 1/10th the mass of the earth.
• Stock (S) Stock Planets(worlds) are planets that are Earth like (Stock meaning normal) with masses from 1/2 the Earth upto 5 Earth masses.
• Huge (H) Huge Planets are worlds that are from 5 times the mass of Earth upto 8 earth masses (Neptuned sized) that are not Gas Giants (or mostly there mass is from rock and not gases)
• Giant (G) Giant Plants are Neptune sized (8 masses earth) upto 13 masses of Jupiter. Basically this class is for gas gaints like Jupiter, Saturn, Neptune and Uranus.

Class Definitions for Special Cases

• Belt (B) Belt Planets(worlds) are planets like Pluto and UB313 which are planets originating in a belt or cloud of similar objects. Pluto, UB313, and Sedna all appear to originate from the Krupier Belt, leading to this logical classification.
• Double/Multiple(M) Double (or Multiple) Planet class is used to describe a two or more planet system, where at least two objects orbit each other around thier Bary Center, and the smaller is atleast half the size of the larger. The Pluto-Charon system would be a good example of this Class.
• Rogue(R) Rogue planet class is used to describe planatary objects that have been kicked out of thier solar system, or those still orbiting thier stars but the star has been kicked or is on the way out out of its galaxy.

A discritive word may be used in front of the class, that helps to describe the type of planet it is. Such as Cold Dwarf Planet Mars, Ice Belt Planet Pluto, Habitable S-Planet Earth, etc...

[01101001]

Originally Posted by Ara Pacis

Since it seems that every thread about any planet, in this star system or not becomes a comparison debate about different definitions, some of us thought a formal debate thread may help.

Great idea! I'm so happy we now have a place to chase the planet-definition debaters off to.

I just wish you had titled it: The "What is a Planet?" definition debate thread

The topic's been done to death, but here's the place where it can be done some more. I wish you better luck than the IAU working group had in reaching a consensus.

[Ara Pacis]

Originally Posted by 01101001

Great idea! I'm so happy we now have a place to chase the planet-definition debaters off to.

I just wish you had titled it: The "What is a Planet?" definition debate thread

The topic's been done to death, but here's the place where it can be done some more. I wish you better luck than the IAU working group had in reaching a consensus.

Changed.

I know it's been done to death, but I figure this could be an archive for the various arguments for one definition over another.

[Saluki]

Do we really need yet another thread like this? Why not just merge it with all the others?

[Halcyon Dayz]

Just a comment.
Whatever definition we come up with,
I think it should follow these two basic rules.

1. It must be simple.
It should enable you to determine
planethood by a mere glance.

2. It must be universal.
You should be able to apply it to any system
we could come across. Even those we haven't
thought of yet.

[The Supreme Canuck]

Originally Posted by Saluki

Do we really need yet another thread like this? Why not just merge it with all the others?

The point of the thread is to consolodate all discussion. It's so dispersed right now that it's useless. So it is to be collected here.

[parallaxicality]

Here, for the record, is my preferred definition:

A brown dwarf is an object of or approaching stellar metallicity that cannot fuse

A planet is an object of non-stellar metallicity with either evidence of ongoing internal geological activity, an atmosphere above one microbar, or both.

A planetoid is an object of non-stellar metallicity that neither demonstrates evidence of onetime or current internal geological activity nor has an atmosphere above one microbar.

[baric]

Labels like planet, moon, comet, asteroid, etc are used as a means of classifying objects for convenience and clarity. Ultimately, all objects are unique, meaning that there will be objects that do not neatly fit into any classification or perhaps seem to fit in more.

The inability to neatly account for 100% of all observed objects is not a weakness of the classification as much as the weakness of attempting to classify things in general.

So if we want to call something a planet, when need to ensure that it is sufficiently similar to objects that we already call a planet.

The #1 rule that everyone agrees on is that a planet is not so large that it can sustain fusion in its core. We have another name for objects that large: star.
1. Too small to ignite

The #2 rule that most, if not everyone, agrees on is that the candidate planet be large enough to be highly spherical. This does not mean that being spherical makes the object a planet, but that being non-spherical disqualifies it.
2. Large enough to become spherical

The leaves us with an upper and lower limit for mass, and from those bodies we need to figure out which of those we want to call 'planets'. To do this, we need to look at how we have historically determined what are planets.

The #3 rule is that the body should orbit a star. A clear distinction has been historically made between planets (orbiting stars) and moons (orbiting planets). And despite its small size, the large asteroid Ceres was considered to be a 'planet' despite being far smaller than many known moons.
3. Must orbit a star

With the discovery of the asteroid belt, it became clear that Ceres (and 3 other asteroid 'planets') was simply the largest member of a group of thousands of bodies. Since this situation did not apply to any of the other 8 planets and Ceres was tiny compared to them, all of the asteroid-planets were demoted. One of the following rules can be deduced from this action:
4a. Must be of sufficient mass (>= Mercury?)
-or-
4b. Must be the dominant body in its orbit (ex: Ceres is 40% of its orbital mass, Earth is 98%, all others are 99.9%)

It's important to note that either one of these rules (4a or 4b) would have been enough to demote Ceres, but both applied. Were they both applicable, or was it just one? It would take over a century for another candidate planet to be discovered and provide the necessary refinement.

After Ceres, the next big planetary test was Pluto. Pluto was originally considered to be larger than Mercury and it was very alone in its orbit outside of Neptune, so it clearly looked like a new planet. However, Pluto has always been considered an 'oddball' because of its orbital eccentricities. For example, it has a large 17% inclination (over twice as extreme as the worst planet, Mercury) and it was unique in that its orbit was lopsided enough that it actually moved closer to the Sun than Neptune for part of its orbit!

As improved measurements made it clear that Pluto's mass was just a tiny fraction (<5%) of Mercury's, it was clear that Rule 4a no longer applied to Pluto. Yet these discoveries created no outroar to demote Pluto from its planetary status. Rule 4b was clearly enough to preserve that status, and Rule 4a was obviously not relevant.

With the discovery of the first Kuiper Belt Objects in the 1990s, Rule 4b was put to the test for Pluto. It was soon clear that Pluto was a large member of a population of thousands of related objects in the Kuiper Belt. This was very analogous to Ceres and, if Pluto was not demoted, we faced the prospect of hundreds of planets. That alone is not a problem, but if 992 of your 1000 planets share many features but are all fundamentally different than the remaining 8, you have to question the utility of lumping them into the same category.

With the discovery of just a fraction of the Kuiper Belt, Pluto seems to be hanging onto its planetary status by the slimmest of margins. It seems clear that this status will become difficult to justify as more Pluto-sized KBOs are found. Time and research are not on the side of Pluto, which makes it fate inevitable.

My conclusion from the extent of the controversy is that Rule 4b applies. With respect to our Solar System, the most consistent rules of planethood seem to be:

1. Too small to ignite
2. Large enough to become spherical
3. Must orbit a star
4b. Must be the dominant body in its orbit


With the discovery of other planetary systems, there is a big push that our definition be universal and applicable to other star sytems. I think this is a honorable, but misguided, goal. This entire issue is being driven by the existence of tiny, icy bodies in the Kuiper Belt, not other star systems. Any planet we find around other stars will be easily larger than Mercury, so there is no ambiguity for extrasolar planets.

And that brings up a very important point. The data that we can collect for KBOs in our system is very limited. Here is the type of data we can collect, with the associated difficulty:

1. Orbital data - easy, always known
2. Size & mass - generally comes with moderate observation
3. sphericity - difficult to discern precisely, but can be estimated from mass
4. composition - difficult to discern, may not be found for some larger KBOs for decades

My point is this. Any method for determining planethood is only as good as the data it uses. Basing the 'what is a planet' criteria primarily on orbital data and mass seem to be the best way to go. Requiring difficult data, such as composition, is only going to create headaches and non-classifications.

[Macro Mouse]

It's a topic overly-discussed, but it's a good idea to have everyone's ideas in one convinient thread. So if you have an opinion on planetary definition, and have or have not posted it, post it in this thread.

I'm still sticking with my idea: a planet is a gravispheroid non-fusor.
Under this broad term, there are 3 major subclasses of planets distinguished by orbit:

• Classical - orbits a fusor
• Rogue - does not orbit a fusor
• Moon - orbits another planet

But in the end, they're all worlds...they're all planets. Further sub-categories based on structure, composition, special circumstances, etc can be used, but may vary a little from person to person. For instance, many would say our solar system's known classical planets can be divided into:

• Terrestrial Planets (Mercury, Venus, Earth, Luna, Mars, Ceres, Hygeia)
• Gas Giant Planets (Jupiter, Saturn, Uranus, Neptune)
• Kuiper Belt Planets (Pluto, Charon, Varnua, Ixion, Quaoar, Orcus, the many unnamed, etc)
• Inner Oort Cloud Planets (Sedna, etc)

I'd hesitate to include all moons in these categories; some easily fit (ie Luna as a terrestrial), but most are born in unique conditions in-situ to the gas giants. But again, these categories are opinionated anyways so I'm not going to stress over the matter.

Also, some objects like Vesta, Pallas, Iapetus, and Proteus are not quite spherical. Under my definition, these objects would not be true planets. So it is possible and acceptable to have planetoids larger than planets. Planetoids themselves I'd define as irregular, macroscopic objects...the universal term for asteroids, meteors, centaurs, KBOs, comets, etc. These sub-categories will still remain for clarity.

So how will our solar system be taught to kids...and uninformed adults of course...with this definition? The best way is to say there are many planets, but only teach the main ones: Mercury, Venus, Earth, Moon, Mars (the rocky planets), Jupiter, Saturn, Uranus, Neptune (the gas planets), and then the "Icy" planets (just say that, don't go naming all of them). This isn't too hard or too big of a change; kids will still learn the main planets and their main groups, and also learn there's not one, but a whole fun group of icy planets after Neptune! Teachers can still cite Pluto as a cool example. People should also realize there are more interesting planets/moons around those planets...not to mention an asteroid belt. The fact that the belt contains 2 small planets could be a neat "wow" trivia in school. (with the image of a bunch of lumpy rocks, I would've been amazed at that as a kid!)

That's my take.

[Halcyon Dayz]

Originally Posted by baric

Any planet we find around other stars will be easily larger than Mercury, so there is no ambiguity for extrasolar planets.

Don´t underestimate what we might be able to in the future.

[baric]

OK, here is some data about the 8 accepted planets that may be helpful for anyone trying to come up with guidelines. Ceres, Pluto and 2003 UB313 are included for comparison...


Mass (in units of 10e21 kg)
1,899,000 Jupiter
568,500 Saturn
102,400 Neptune
86,830 Uranus
5,974 Earth
4,869 Venus
642 Mars
330 Mercury
?? 2003 UB313
13 Pluto
1 Ceres

Diameter (in km)
138,000 Jupiter
114,000 Saturn
50,000 Uranus
49,000 Neptune
12,740 Earth
12,100 Venus
6,780 Mars
4,880 Mercury
~3,000 2003 UB313
2,306 Pluto
950 Ceres

Oblateness% (0 = perfectly round)*
0.0% Mercury
0.0% Venus
0.3% Earth
0.7% Mars
0.7% Ceres
1.7% Neptune
2.3% Uranus
6.5% Jupiter
9.8% Saturn
?? Pluto
?? 2003 UB313

* - rocky planets will have less oblateness than gas planets. No data for Pluto or 2003 UB313

Orbital Eccentricity% (0 = perfectly round orbit)
0.7% Venus
1.0% Neptune
1.7% Earth
4.6% Uranus
4.8% Jupiter
5.4% Saturn
8.1% Ceres
9.3% Mars
20.6% Mercury
24.8% Pluto
43.7% 2003 UB313

Orbital declination (0% - exactly on ecliptic plane)
0.0% - Earth
0.8% - Uranus
1.3% - Jupiter
1.8% - Neptune
1.9% - Mars
2.5% - Saturn
3.4% - Venus
7.0% - Mercury
10.6% - Ceres
17.1% - Pluto
44.0% - 2003 UB313

% of mass in orbital region (100% - only body in that orbit)
100% - Mercury
100% - Venus
~100% - Mars
~100% - Jupiter
~100% - Saturn
~100% - Uranus
~100% - Neptune
98% - Earth
40% - Ceres
2% - Pluto*
?? - 2003 UB313

* based on estimates that total mass of KBO objects is approx 10% of Earth mass.


Anyway, that is some of the info I've looked at in reaching my conclusions. I hope that it helps other people as well.

(minor edits made)

[jkmccrann]

I for one really appreciate the effort you've gone to baric to compile that data in one place for easy comparison for all of us here.

I would actually vote for this thread to be a sticky thread here in the Astronomy section at least until the IAU Session in Prague when we might actually gain a firm definition to put this argument to rest. Of course, we may not get that firm definition but we're at least likely to see some movement on this front.

Originally Posted by Halcyon Dayz

Just a comment.
Whatever definition we come up with,
I think it should follow these two basic rules.

1. It must be simple.
It should enable you to determine
planethood by a mere glance.

2. It must be universal.
You should be able to apply it to any system
we could come across. Even those we haven't
thought of yet.

I second those guiding principles as well, although in terms of being universal its probably impossible at this stage to really be universal, but I would say the definition should strive to be universally applicable, and that if there are too many qualifiers to the definition, what that in reality ends up doing is confusing the issue and muddying the waters of the definition. A relatively simple definition should be something to factor in when searching for a definition to this question.

[baric]

Originally Posted by jkmccrann

I for one really appreciate the effort you've gone to baric to compile that data in one place for easy comparison for all of us here.

Thanks! I have tried to look at a lot of data in order to come up with the most consistent definition for planet, especially considering the history of the term and how that translates to properties of the bodies in question.

Except for oblateness, you can see that Pluto & 2003 UB313 are jointly at the bottom of all of those lists, which is why I don't give them any consideration as planets, in the traditional sense. And I think that any rocky body of sufficient mass (this includes the lowly Ceres), is going to generally be more round than a gas planet, which is more easily flattened by the rotation on its axis.

Ceres has a very round planet-like orbit, but that should come with an asterisk because the entire asteroid belt is shepherded by Jupiter.

[zorbo]

My definition as an absolute layman is rather less elegant-sophisticated (compared to the others) but very simple. It is also the one which is easiest to understand even for our times' high-school graduates which don't know where (or what) Paraguay is.

A planet is an object with at least the size of Pluto; the upper limit is the (apparently consensual) barely non-fusing giant (which, in effect, is also a size limit).

To elaborate.

The shape - or composition/structure/tectonic activity/thermaldynamics, for that matter - is not of relevance; if we have an object the size of Jupiter that for whatever reason happens to consist entirely of massive chocolate and have the shape of Elvis then I'ld still call it a planet.

Orbits and/or relation (or lack thereof) to a star (star system) is not of relevance. If the planet happens to be wandering between solar systems, it would still be a (rogue) planet - even if we never found out about it.
However, I'ld be willing to compromise this criterium to limit planets to such objects which are found inside star systems - even though I would find this a slightly arbitrary limitation.

The non-fusing upper limit is consensus afaics; if it starts fusing, it's a star.

The two catches that I can see with my definition is the lower limit, videlicet, arriving at a consensual lower limit and defining "size".
Pluto has been called a planet ever since it was detected; people will want to keep it as a planet. Pluto, despite its shrinking over the last decades, happens to come in at a reasonable, handy size, which will cut off the large amounts of smaller objects (number will increase exponentially towards smaller sizes). Pluto is also a size which we can still safely call a planet without a bad gut feeling.

Which leaves us with the definition of "size".
Here we have several options; we could use mass, diameter or volume.
I'ld be willing to discuss this detail. I concede there are problems with each of these options.
Diameter can be tricky with irregular-shaped bodies; diameter and volume can also be a problem with gaseous objects.
Mass could put objects into the planet category which are definitely smaller than Pluto but hyper-dense. Not sure if we would want to have these in there.
Maybe my compromise would be at least the volume *and * mass of Pluto (cumulative criteria).
Non-fusing object with at least the volume and mass of Pluto. Yeah, that's it.

[baric]

External links discussing this topic (for reference)

Some are links to the discoveries of Sedna and Quaoar, to give you an idea of the reaction to those potential 10th planets. I'll add links to this post as I find them...

Wikipedia: The Pluto Debate
NY Times: Here a Planet, There a Planet - 2 Feb 2006 (reg. reqd)
The Space Review: Common sense planets - 21 Nov 2005
Space.com: Pluto's Planet Status could be Jeopardized by Sedna Discovery - 17Mar 2004 (interesting Brown quote)
CBBC: New Planet Sedna may have a moon - 16 Mar 2004
BBC: Astronomers discover 'new planet' - 15 Mar 2004
Space.com: Controversial Proposal Would Boost Solar System's Planet Tally to 12 - 27 Feb 2003
Wired News (cache:: Pluto: The Planet that used to be - 23 Jan 2001
CNN.com Biggest object since Pluto found in solar system - 7 Oct 2002 (interesting Brown quote)
Space.com:What is a Planet? Debate Forces New Definition - 2 Nov 2000
Nasa.gov:Much ado about Pluto - 17 Feb 1999
John Stansberry, Astronomer: Is Pluto a Planet? -1998

[Disinfo Agent]

There have been several discussions in the BAUT forums, as well.

[Doodler]

Throw me in with Macro Mouse, he's close enough to the same page where I am that the differences aren't significant enough to worry about.

[baric]

Originally Posted by Macro Mouse

I'm still sticking with my idea: a planet is a gravispheroid non-fusor.

Originally Posted by Doodler

Throw me in with Macro Mouse, he's close enough to the same page where I am that the differences aren't significant enough to worry about.

The "large enough to become spherical due to gravity" is a bottom limit that is often proposed for planethood. I read an article recently that postulated this size as being about 700 km in diameter.

Given that, the following bodies in the Solar System would qualify as "gravispheroids":

the 8 traditional planets, plus the asteroid Ceres
these 15 moons: Luna, Ganymede, Callisto, Io, Europa, Titan, Rhea, Iapetus, Dione, Tethys, Titania, Oberon, Ariel, Umbriel, Triton
these 11 trans-Neptunian objects: 2003 UB313, Pluto, Charon, 2005 FH9, Sedna, 2002 TC302, Quaoar, Orcus, Varuna, 2002 UX25, 1996 TO66
possibly these 5 TNOs: 2003 EL61*, 2002 AW197, 2002 MS4, 2003 AZ84, 2004 XR190

In other words, we already know of 35-40 "gravispheroids" in our Solar System. Looking at the recent KBOs, it's not unrealistic to think that we could have over 100 in the next 5 years.

I'm not saying that "gravispheroid" is a bad test -- in fact, it's perfectly scientific and relatively easy to apply. The problem is that it sets the "planetary" bar much lower that it's been in the past, which might create a bigger public and scientific backlash than demoting Pluto!


*2003 EL61 is large enough to fit in this category, but it is clearly not a spheroid.

[Doodler]

Originally Posted by baric

The "large enough to become spherical due to gravity" is a bottom limit that is often proposed for planethood. I read an article recently that postulated this size as being about 700 km in diameter.

Given that, the following bodies in the Solar System would qualify as "gravispheroids":

the 8 traditional planets, plus the asteroid Ceres
these 15 moons: Luna, Ganymede, Callisto, Io, Europa, Titan, Rhea, Iapetus, Dione, Tethys, Titania, Oberon, Ariel, Umbriel, Triton
these 11 trans-Neptunian objects: 2003 UB313, Pluto, Charon, 2005 FH9, Sedna, 2002 TC302, Quaoar, Orcus, Varuna, 2002 UX25, 1996 TO66
possibly these 5 TNOs: 2003 EL61*, 2002 AW197, 2002 MS4, 2003 AZ84, 2004 XR190

In other words, we already know of 35-40 "gravispheroids" in our Solar System. Looking at the recent KBOs, it's not unrealistic to think that we could have over 100 in the next 5 years.

I'm not saying that "gravispheroid" is a bad test -- in fact, it's perfectly scientific and relatively easy to apply. The problem is that it sets the "planetary" bar much lower that it's been in the past, which might create a bigger public and scientific backlash than demoting Pluto!


*2003 EL61 is large enough to fit in this category, but it is clearly not a spheroid.

I concede that possibility, but I will offer this very often proven platitude as consolation.

There is no such thing as bad publicity.

Even tragedies lead to positive outcomes, when the lessons of those tragedies are properly learned. Controversy also generates publicity, so offering an update and accurate, if controversial, definition for "planet" that sends the counts up the wall, its not going to hurt the scientific community in the end.

Yeah, the public will look at astronomers like they've grown a second head, and I'm sure more than a few astrologers will have coronaries or nervous breakdowns, but the bottom line is, its good exposure.

[Kristophe]

The way I see it is, a car's still a car, even when it's in a parking lot. It doesn't magically become a go-kart. If there are objects out there that we'd call planets if they weren't contained within a belt, then we should be calling them the same if they are.

If we called the ice dwarfs plutonians, do you think the doom sayers would came back out in drovs?

[The Supreme Canuck]

Originally Posted by Kristophe

The way I see it is, a car's still a car, even when it's in a parking lot. It doesn't magically become a go-kart. If there are objects out there that we'd call planets if they weren't contained within a belt, then we should be calling them the same if they are.

That's good point. So, Ceres is a planet then?

[baric]

Originally Posted by Kristophe

The way I see it is, a car's still a car, even when it's in a parking lot. It doesn't magically become a go-kart. If there are objects out there that we'd call planets if they weren't contained within a belt, then we should be calling them the same if they are.

But things aren't that black & white. What is the difference between a car, an el camino and a small truck? Or a car, a station wagon and a small van?

With regards to planets, there will be objects just on either side of "the line" no matter where you draw it.

When Ceres was discovered, it was assumed to be much larger than it actually is. As evidence of its small size and its membership in a larger "belt" became clear, a consensus developed to strip it of its "planet" status. Of course, several other asteroids were named planets before a consensus developed.

Now we see an identical situation with Pluto. It was originally considered to be much larger. Now we know it is quite small and part of a larger belt of similar objects. Are we really going to repeat the mistakes of the past, or learn from them?

My guess is that we will repeat them. But after about 5 or 6 KBOs are named planets in the next decade (and a few dozen close calls are averted), a consensus will develop among astronomers.

We can boot Pluto out now, or we can do it later. But it will happen. The writing is on the wall.

[The Supreme Canuck]

I'm going to ask an obvious question here. What's so bad about having a large number of planets?

[baric]

Originally Posted by The Supreme Canuck

I'm going to ask an obvious question here. What's so bad about having a large number of planets?

Thanks. That is a very good question.

On the surface, there is absolutely nothing wrong with having a large number of planets. But the point of a label like 'planet' is to categorize bodies that share similar characteristics so that everyone knows what you are talking about when you use the word. And, since this is not a newly-invented word, we need to understand the historical uses of the word so that our modified definition still fits.

We have 8 bodies in the Solar System that we all agree are 'planets'. And, until recently, there was agreement on Pluto. In addition, we have bodies that used to be called planets but are no longer (Ceres and the earliest-known asteroids).

Imagine we have two groups of bodies:

8 bodies that greatly share many orbital characteristics, but few mass and composition characteristics.
200+ bodies that greatly share many mass and composition characteristics, but no orbital characteristics with the first 8 bodies.

Now, why in the world would we use a single word to describe two distinct types of objects? It's like using a single word to describe cats and dogs as a single group.

[Doodler]

Originally Posted by baric

Thanks. That is a very good question.

On the surface, there is absolutely nothing wrong with having a large number of planets. But the point of a label like 'planet' is to categorize bodies that share similar characteristics so that everyone knows what you are talking about when you use the word. And, since this is not a newly-invented word, we need to understand the historical uses of the word so that our modified definition still fits.

We have 8 bodies in the Solar System that we all agree are 'planets'. And, until recently, there was agreement on Pluto. In addition, we have bodies that used to be called planets but are no longer (Ceres and the earliest-known asteroids).

Imagine we have two groups of bodies:

8 bodies that greatly share many orbital characteristics, but few mass and composition characteristics.
200+ bodies that greatly share many mass and composition characteristics, but no orbital characteristics with the first 8 bodies.

Now, why in the world would we use a single word to describe two distinct types of objects? It's like using a single word to describe cats and dogs as a single group.

We do use a single word to describe cats and dogs. Pets. To follow the analogy. I think there's a few people here who see "planets" as a term more analogous to "pets" than "cats" or "dogs".

[The Supreme Canuck]

I don't see why there should be a large distinction between bodies that have different orbital characteristics. I mean, we don't call a moon in a retrograde orbit something other than "moon."

[Doodler]

Originally Posted by The Supreme Canuck

I don't see why there should be a large distinction between bodies that have different orbital characteristics. I mean, we don't call a moon in a retrograde orbit something other than "moon."

That's part of why this is so jarring. Being forced to put an actual, meaningful definition to what a planet is can't be done in a vaccuum. Whatever the new definition is finally agreed to be, its going to redraw the boundaries on more than a few other classifications of sub-stellar objects as well.

[The Supreme Canuck]

And again, there's the question of finding a Mars-sized KBO. How can we not call that a planet just because of its orbit?