Planet-sized problems

Jay, Oro Valley, AZ — Thu, 08 Jun 2006 03:34:41 GMT

All right, if a shepherd moon orbits within a ring system, we still call it a moon, don't we? So why should a planet that orbits within an asteroid or cloud system not be a planet? That doesn't even strike me as a valid argument. The obvious natural cut-off point is where an object directly orbiting a luminous body has enough mass to pull itself into a sphere. Any other deliniation would just be chosen according to personal preferences. If they want to call the little ones "minor" planets or something of the like, fine, but I think we should keep in mind that the rules are set by nature, not our arbitrary numerical systems.

Planet-sized problems

A. Garcia — Thu, 08 Jun 2006 04:05:44 GMT

Be glad Zena & Pluto weren't discovered by a Russian. The Soviet Republics would make sure this problem were some kind of ethnocentrism against the Soviet Republics and the governing body would call Pluto and Zena a planet with some unpronounceable Russian name just like the back side of the moon. Personally I think Walt Disney's ancestors should have a say in this case along with Lucy Lawless.

Planet-sized problems

cory mcneil — Thu, 08 Jun 2006 04:13:38 GMT

The shuttle is--we hope--headed for the ISS and I've wondered for a long time . . . what is that ISS thing doing up there in the first place? It does a little of this and a little of that and costs a bloody fortune, money that NASA could better spend in just about any way I can think of.

I guess it answers the question: can elephants fly? Yes, we have a great big white elephant going round and round and round. Why it's there, nobody knows.

Planet-sized problems

Andy Vollmer , Phoenix , AZ — Thu, 08 Jun 2006 05:27:58 GMT

I think we have allowed the argument to get too big . An easy definition is : A planet orbits a star and is round due to gravity . It rotates in its orbit and this causes a bulge at its equator . It is the largest body if it is in a multiple body association (ie Earth and Moon) . Seems simple to me , sorry for being glib .

Planet-sized problems

Chris House, St. Paul, MN — Thu, 08 Jun 2006 11:01:14 GMT

I believe that the term planet would be too harshly watered down if we allow Pluto and all of the larger Kuiper Belt objects to be called planets.

I believe that the term planet should only be used to refer to objects large enough to become spherical under their own mass, yet that permanently exist within the 'warm' zone of the star they orbit; to the degree that if there is an atmosphere present, it will be persistent throughout the entire orbit about the primary (star) - and even if there isn't an atmosphere it's within the range that would allow it (from a temperature perspective) to have a persistent atmosphere.

Also the object must not be part of a debris field such as an asteroid belt - which may in fact be the remnants of an older planet that was destroyed by the tidal forces generated by neighboring bodies.

An additional rule would be that the object must have already established harmonic resonance with its 'neighbors', so new arrivals to a system should be considered rogues until they have achieved at least a quasi-stable orbit around the primary, even if they fit all other definitions for planet.

Anything large enough to begin deuterium fusion could be called a brown dwarf or a 'gray dwarf' (as in barely luminous but much brighter than a brownie) depending on the state of the reaction within it. This should be the case even if the object otherwise would fit all other criteria to be named 'planet', even if it was orbiting a full blown star.

Anything large enough to have a full blow fusion process underway should of course be called a star.

Anything large enough to become spherical under its own mass that does not exist within the range where a stable atmosphere is possible (such as Pluto and possibly some of the other 'new ones') should be considered a 'cryoplanet' or whatever new term is decided to be best descriptive; indicating that it's just far too cold to really be considered a 'proper planet'.

Any object that would otherwise be called a planet, yet is itself in primary orbit of another planet or dwarf rather than the primary, should be known as a moon - whether there is an atmosphere present or not.

Other smaller objects that are on the path to becoming spherical via aggregation of smaller bodies could be known as a proto-planet, a proto-cryoplanet or simply a planetoid - depending on the estimated orbital location and final mass & temperature once the object had swept up all aggregate material within its orbital path.

Other medium sized objects existing within regions such as asteroid belts or Kuiper Belt type regions where the gravitational interference by another large body in the system (such as Jupiter) prevents them from moving forward in their agregation process should either be categorized as asteroids or planetoids depending on their surface gravity, and not their total mass. Some specific cutoff level should be determined, based on the escape velocity - basically if an astronaut in a space suit can leap off the surface and escape the object it should be considered asteroid, but if they cannot it should be a planetoid regardless of shape. Although an exception to this rule would be that even if within the equivalent of a Kuiper or asteroid belt, if the object has completely cleared its orbital path clear, it could qualify as a cryoplanet or even planet proper depending on minimum temperature.

Basically we can't begin granting each and every little hunk of roundish space junk orbiting a star or dwarf 'planethood' - however we do need to come up with a simple discrete set of a half dozen terms that will allow an amateur or professional alike to immedately understand the nature of the body being described.

Eventually, we need something similar to what is used in Star Trek, where Earth would be a 'Class M Planet' and each of the other bodies would have a code corresponding to the temperature, mass and atmospheric composition of the object.

Planet-sized problems

Me You, harrisburg, penslavinia — Thu, 08 Jun 2006 14:31:39 GMT

couldn't they just all be satelites?

Planet-sized problems

Carson Baker, Seattle, Washington — Thu, 08 Jun 2006 14:39:20 GMT

I think this whole debate is overly complicated. (Though of course I am just an ignorant 13 year old.) I think there should be two qualifications for a planet. First, it's role/orbit in the solar system. For instance Jupiter is a planet because it orbits the sun directly. Jupiter's 61 moon are singular objects that though do orbit the sun, they do so indirectly. But, beause they are not "bunched together." They are considered moons. However the small space rock that orbits Jupiter in clumps are it's rings not moons.
So out in the edges of the solar system, neptune is a planet not because of it's size, but that it orbits alone, there aren't millions of Neptunes in the same orbit. PLuto is (sorry for all you pluto fans.) a comet, along with all the other comets iin the Kuiper Belt. Because it does still release nitrogen and other gases, it has a tail. And even more, it travels with millions of other "Plutos."
Okay point two. If there is any dispute with the first point, the it should re-direct to this. How it was formed. I have to go to school now. Bye.

Planet-sized problems

Chris Munroe, Lynnfield, MA — Thu, 08 Jun 2006 15:15:16 GMT

It is best to apply the principle of Occam's Razor to the definition of what a planet is. So, I think that the simplest definition would be the best. The criteria for the definition would be that any object that has sufficient mass so as to have the necessary amount of gravity so as to assume a spherical shape ought to be called a planet. The key would be to determine by math and physics what the minimum amount of mass and matter to have sufficient gravity for the assumption of a spherical shape is. Any object that has that minimum mass/gravitational strength would be bound to be a sphere.
Having said that, I would like further amplify that any such object that is orbiting a star or a brown dwarf should be called a planet. If such an object, regardless of size is orbiting an even larger planet, then it should be called a moon.
Such objects floating alone out in deep space, amongst the stars ought to called rogue planets. A Really small planet, smaller than Pluto, such as Ceres, should be called a minor planet, planetlet, planetino, or some other diminutive term that conveys the notion of its relatively small size.

Planet-sized problems

Jeremiah, CO — Thu, 08 Jun 2006 15:39:50 GMT

I have always enjoyed the links that you post on this blog. Are you discontinuing this practice?

Planet-sized problems

Steve Moniz, Leavenworth, KS — Thu, 08 Jun 2006 15:49:14 GMT

How do you define roundness? Earth isn't perfectly spherical. I can't come up with a definition that isn't human-centric.
It might be easier, and more universal, to measure mass. Consider a planemo system that has "cleared out" its orbit to the extent that it outmasses its co-orbitals. It should be called a planet, no matter how round it is. Of course, this requires a definition of "orbital path". Perhaps a toroid with a radius that reaches the nearest gravitational balance point with the nearest massive body.
(Note 1) By planemo system, I mean a body and its moons or possibly a double planet. I would define a large moon as a planet if it and its more massive partner orbit each other around a point that is between the two. The "fulcrum" for the Earth-Moon pair is some 400 miles below the surface of the Earth, so Luna is a moon by this definition. Pluto and Charon would be a double planet, if they outmassed the rest of the Kuiper Belt (not).
(Note 2) Earth, rather Earth/Luna, hasn't completely cleared its orbital path. Google 2002 AA29 or 3753 Cruithne.

Planet-sized problems

Wade Whitlock, Aberdeen, MD — Thu, 08 Jun 2006 16:25:47 GMT

OK, one more time. If it orbits a star and has a size sufficient to collapse it to a spheroidal shape - it's a planet. Now, let us set up a descriptive system based on material and size:
S type - silicon crust - Mercury, Venus, Earth, Mars
J type - gaseous - Jupiter, Saturn, Uranus, Neptune
I type - icy - Pluto, Xenia, Sedonia, etc.
Modify the type the way star types a modified with alphanumerics for atmosphere, gravity, etc.
For instance, Earth could be a planet type SB1. Silicon crust, breathable atmosphere, gravity =1. Too gaia-centric? SNO32.

The possibility for puns is almost too great to resist! Would the theory be a: Brane-teaser, Brane-storm, ...? Check out David Drake's Lt. Leary series of books where starships use masts and sails in the Matrix to travel by Casimir energy! Great military adventure in the style of the Hornblower books. Make a fantastic movie!

Planet-sized problems

Chris Eldridge, Harrisburg, PA — Thu, 08 Jun 2006 17:00:46 GMT

I think our whole idea of a solar system is a bit limited. Earth is 1 AU from the sun and Pluto 40. However, the Sun's gravitational influence extends outward 50,000 to 100,000 AU. This makes it very likely that we will find not only other Pluto’s but other earth-, Neptune-, and even Jovian-sized worlds as well. These additional worlds may not have formed here in the traditional sense, but could have been captured as they floated by. It is not until we have telescopes like the massive ESO OWEL that we will be able to make an accurate inventory. I do l like the idea of categorizing planets but how about a list of primary and secondary planets. This is simpler and leaves out all the fancy “belt names” that I still don’t know how to spell myself. Maybe any spherical body (>700km) that “stands out” from the others in a given orbit can be primary planets but I think even that is too narrow and eccentric orbits seem to be much more common.

Planet-sized problems

Kip Hansen, Garden Grove, Ca — Thu, 08 Jun 2006 17:21:27 GMT

A planet should be defined as a body in orbit around a star with enough mass to form a spheriod body and its orbits perigee/apogee is no less than .75 (not sure if this excludes pluto, I could only find the mean distance). Or if you want to upset everyone... there are no planets.. we are all moons of our sun with some of the moons having moonlets.

Planet-sized problems

ROGER HARTFORD CONN — Thu, 08 Jun 2006 17:57:52 GMT

ALL PLANETS SMALLER THAN PLUTO BUT LARGER THAN CERES SHOULD BECOME KNOWN AS PLANETOIDS (MY DEFINITION).

Planet-sized problems

CLIFF CHAMPNESS, WEST SAYVILLE, NEW YORK — Thu, 08 Jun 2006 17:57:55 GMT

Leave it alone, already!
I'd like to see more astronomical efforts aimed at exploring those 200+ extrasolar planet sized objects currently observed and which (if any) may be located within their respective star's "Goldilocks Zone" - as is our Earth, for supporting life as we know it.

Planet-sized problems

Nadein Ramos, Austin, Texas — Thu, 08 Jun 2006 18:02:24 GMT

A planet should be any object with enough mass and matter to have a spherical shape (unlike an asteroid) and its own gravity, but not enough mass to start thermonuclear fusion, and orbits a star (not another planet) in its own, unique orbit (not shared with other bodies like the Kuiper Belt objects), regardless of the orbital path (circular vs. elliptical). A planet and a moon could be the same size. As long as they meet the above definition, the only difference would be what they orbit, not their size or mass.

Planet-sized problems

Patrick Bishop, Caldwell, NJ — Thu, 08 Jun 2006 20:48:32 GMT

My Definition of Planet

Planet: a celestial object having undergone shaping by internal gravity, which presents vertical internal differentiation, and which has never had an atmosphere capable of initiating nuclear fusion.

This definition excludes asymmetrical objects like most asteroids (basically amorphous chunks of rock), and also excludes brown dwarfs etc. However, asteroids like Ceres (which has been shaped by gravity and is probably internally differentiated) would be listed as planets, as would all of the spherical moons in our solar system, including our own moon and those such as Europa, Titan, etc.

One may argue that these objects are not really sufficiently similar to be addressed with the same nomenclature, but one must admit that they do all have these three things in common; each has undergone geologic shaping and internal differentiation due to its own gravity, and each lacks an atmosphere capable of initiating a fusion reaction.

For further typological purposes, we could refine our definition somewhat by offering "categories" of planet-hood. A planet only orbiting a central sun is a primary planet, one in orbit around another planet is a secondary. Some objects satisfying my criteria may not be in orbit around anything (see below) in which case we can categorize them as tertiary planets. We could further categorize them by how much gravity they exhibit, density, and/or atmospheric characteristics (whether it's got one, how dense it is, what it's made of, temperature, or perhaps more simply whether liquid water could exist there).

We could use Earth as a baseline for some of these other measures (gravity, atmosphere, etc).

My definition provides meaningful criteria for differentiating certain kinds of celestial objects. I would also like to point out that in the eventuality that our sun dies and is reduced to a white dwarf, it still fails to satisfy the criteria because it formerly had an atmosphere capable of initiating nuclear fusion. However, whatever planets survive the dissolution of our sun will still be planets as they wander freely through space because my definition does not require that an object actually be in orbit around ANYTHING for it to be classified as a planet.

Planet-sized problems

Scotty, Laramie, WY — Thu, 08 Jun 2006 21:46:53 GMT

"Rules" are not defined by nature, but by human beings. If the IAU defines Pluto to be a planet, then we'll call it a planet. If it does not, then we'll call it a "minor planet," or whatever other name they can dream up for larger Kuiper Belt Objects. A categorization has nothing to do with the size, shape, or formation process of a body, until we agree that it does. It doesn't even exist until we agree that it does. One thing is certain. It makes no difference to nature what we call it. If Pluto is defined as a planet, then we'll continue discovering "planets" for a long time. It's impossible to estimate the number of larger Kuiper Belt objects out there. Perhaps there are thousands or even millions. It is a prospect that would be exciting, discovering a new planet every several months indefinitely, but we must draw the line somewhere.

However, I agree with the Jay from Arizona's opinion that the spherical-shape issue is an important one. Personally, I think that the "minor planet" designation might be a good compromise. It sets them apart from both the undisputed planets existing outside of belts, and planetesimals, which DO NOT have enough mass to pull themselves into spheres.

Planet-sized problems

Scotty, Laramie, WY — Thu, 08 Jun 2006 21:51:09 GMT

We do not call a shepherd moon a "moon," but a SHEPHERD moon, which undeniably sets it apart from a regular moon, which does not orbit in close proximity to a ring system.

Planet-sized problems

James Gunnells, Olympia, WA — Fri, 09 Jun 2006 00:06:13 GMT

Why shouldn't a moon be a planet? It is circular, and because it orbits another planet means that it also orbits the sun. And why can't a planet in the Kuiper belt just BE a planet in the Kuiper belt?
Lots of us need to find a job. I'd love a full-time position discussing this stuff.

Planet-sized problems

John Howell, Wynantskill, New York — Fri, 09 Jun 2006 02:10:49 GMT

I have a question regarding the speed of light and the age of the universe. Recent discoveries found galaxy clusters at about 10 billion years old. Thus 10 billion light years away. Well if it took 10 billion years for the light to reach us from the early years of the universe. Then how did we get so far out so fast (from the big bang) before the light from those clusters reached us.

Planet-sized problems

Tom Novella, New York, NY — Fri, 09 Jun 2006 02:51:00 GMT

I like the idea that a planet must orbit a star.
I also like the "sphere" constraint (thus requiring a threshold mass) on the definition of planet. Of course that would exclude every "planet" in the solar system, especially Saturn, whose rotational speed significantly elongates its equatorial radius.
The "object" must also be an object, not just a section of space or an area of luminosity.
I'd call a planet "a regular spheroid comprised of matter which produces gravitational effect, in circular or elliptical orbit about a star".

Planet-sized problems

Alan Boyle — Fri, 09 Jun 2006 03:33:45 GMT

Thanks for your comments ... I let a couple of 'em run on a bit longer than usual, but I hope you'll stick with it. Jeremiah from Colorado, I wanted to say that I'm continuing to provide the daily links, and maybe our new practice of linking to the feature items of the day is leading folks to miss out on the fact that there are other (shorter) items posted in the course of a day. So I'll change that practice later tonight and see if it makes a difference.

I also cleared a question on the Big Bang... actually, scientists think the Big Bang happened about 13.7 billion years ago. A lot of people have the wrong idea about the Big Bang ... that it actually was a three-dimensional explosion that blew us out from some central blast point. Instead, I think of the Big Bang as the inflation of four-dimensional space-time in an extradimensional space. This is probably worth another thread, but for the time being, here are some links on the subject you might find interesting:

New study super-sizes the universe
http://www.msnbc.msn.com/id/5051818/

Beyond the Big Bang
http://www.msnbc.msn.com/id/6392268

What is the universe expanding into?
http://msnbc.msn.com/id/3077398/

Planet-sized problems

Jim C , Philadelphia, PA — Fri, 09 Jun 2006 14:03:04 GMT

There are a number good ideas, many of them similar, but it still comes down to classifying what is a planet, and what is not. It's obvious we, as a world society, have become more sophisticated, articulate, and intelligent, in general. One item that caught my eye, that I think needs more thought was one of the discussions of life imitating art. We seem to be 'stuck' in trying to classify what is a planet because it's impossible to put together anything other than an extremely complex definition of what constitutes a planet. Instead why not put togther a planetary classification system that would provide a set of comprehensive definitions for all non-star space objects? It is very evident there is a significant amount of variability within our own solar system, and Kuiper Belt alone, not to mention what we may find as telescopes and space travel allow us to explore even further, that prevent us from simply defining 'what is a planet'.

I would propose we adopt a System of Planetary Classification that concisely and comprehensively defines all space objects. The few systems that I have looked at do not seem to have classifications for suns, and maybe the ones we currently have are adequate. They do provide definitions for the numerous type of space bodies within our solar system and Kuiper Belt, and even some that we might consider works of imagination. If you perform a search of 'planetary classification' you end up with numerous sites, some of which follow a Star Trek genre of classification. I would propose we start with the Star Trek classification as a basis and expand/contract the system to fit our needs.

Below are a couple of examples or the 'starting point':

http://www.sttff.net/planetaryclass.html

http://en.wikipedia.org/wiki/Star_Trek_planet_classifications#Class_S_and_T_Ultragiants

Planet-sized problems

Wade Whitlock, Aberdeen, MD, USA — Fri, 09 Jun 2006 20:09:59 GMT

Just a note to compliment you on the new format. This is one of the best sites for an intelligent person, yet! Well done!

Planet-sized problems

Nosakhere Ajani, Stafford, VA — Fri, 09 Jun 2006 21:07:12 GMT

I am seven years old and I am future astronomer. I believe that a planet should be at least 2000 km in diameter. Anything smaller should not be considered a planet. Anything larger, no matter where it is located, should be consider a planet.

Planet-sized problems

jonathan jay, Kapa'a Kaua'i — Thu, 15 Jun 2006 09:23:45 GMT

Most discussions attempting to define 'categories of things' are by their nature based upon a false premise. The universe is much more fuzzy than firm. Typically the qualities of things occur along a spectrum, not in discrete types. Yet our hungry monkey-mind persists: what is the pattern? - which is the 'right' pigeon hole?! In this case, what is a planet.

Clearly there are many non-stellar bodies in our solar system (and others beyond), sometimes in groupings but often with overlapping qualities. Of the 'planets' we at present mostly agree on (Mercury through Neptune), there are a broad range of planets. For example, Jupiter and the Earth are very different, yet each are close to the heart of what we feel a planet is.

What are Planets? Perhaps a more pointed question is 'What is NOT a Planet' This debate shows neither are simple questions. Objects are felt to be planets (or not) based on a fuzzy calculus of their qualities and behavior, location, size, shape and mass. People place different weight on these aspects.

Imagine Saturn's MOON Titan were instead located in the middle of the asteroid belt, had absorbed (or ejected) most of the asteroids there, and (perhaps) captured a few of them into orbits around itself, there would be little debate on whether or not Titan was a planet. The Greeks would have been able to see the PLANET Titan with their naked eyes. They would have watched it 'wander' across the ancient night sky. Sure, it would be a bit more volatilized being closer into the sun, but Bode would have been happy. As things are, Titan in orbit around the PLANET Saturn is a MOON.

Here is my effort to define the broad category of Planet. Planets: (all of them) are sub stellar objects that through gravitational self-collapse, have achieved a spheroid form. All the other irregular bodies, aggregate gravity piles, flotsam, jetsam and shards, are not any sort of ‘planet’. Typically, Planets:

are highly internally differentiated
have semi-stable orbits around their star(s)
stay close to their local stellar ecliptic
gravitationally dominate their orbital sector
if massive may have many moons
have atmospheres &/or a magnetospheres

IF a newly discovered candidate for Planet-hood had ALL of these qualities, you can open up the history books and start re-writing. If it had only SOME of these typical qualities, there would be increasing debate on it's status. If it had NONE of the typicals, some diehards might still insist on Planetary-dom, but would never be able to convince anyone to agree. Here is an informative article: http://en.wikipedia.org/wiki/Definition_of_planet

Of the broad range of planemos presently semi-understood, Here is my 'vote' for…
4 Types of Major Planets: (w/Prototypical Bodies & Rough Mass Ranges):

Terrestrial: N/Fe & silicates, ~.05 EMass ~5 EMass
Mercury, Venus, Earth, Mars

Ice Dwarves: icy-tiny ~.1 PlutoMass ~ 10 PlutoMass
Pluto, Charon, Quaoar, Sedna, Xena, (formerly) Triton

Ice Giants: icy-big ~.1 NeptMass ~ 10 NeptMass
Neptune, Uranus

Gas Giants: gassy-big ~.1 JupMass ~ 10 JupMass
Jupiter, Saturn

Perhaps one day soon, there will be a Hertsprung/Russle diagram for Planets. Won’t that be nice.

jonathan jay lives inside the collapsed caldera of an extinct volcano on a tropical island. He is 40.

Planet-sized problems

J. Servis, Los Lunas, NM — Tue, 20 Jun 2006 22:18:09 GMT

Can anyone direct me to a site with photos of the minor planets, i.e., photos of areas where minor planets reside?

Planet-sized problems

Alan Boyle — Wed, 21 Jun 2006 02:41:48 GMT

J. Servis, you'll definitely find this page interesting: http://cfa-www.harvard.edu/iau/lists/InnerPlot.html ... the Minor Planet Center in general is a good place for info on the minor planets: http://cfa-www.harvard.edu/iau/mpc.html ... and "Nine Planets" has info and pics of some of the bigger minor planets: http://www.nineplanets.org/smallbodies.html

Planet-sized problems

Drew Slatton — Wed, 16 Aug 2006 14:31:14 GMT

While I like the recent definition from the IAU, the one thing missing is the idea of an atmosphere. All 9 of the objects currently classified as planets have one, and I believe if that requirement were added to the IAU's definition, I would have no problems with it.

Planet-sized problems

Richard Senate, Oak View, Calif. — Fri, 18 Aug 2006 17:58:05 GMT

I like the idea of a 12 planet solar system. This was the way it was envisioned by the ancients thousands of years ago. We should keep the Greek/Roman names going for new planets (Xena is welcome in Greek) and save names like "Elvis" and "Sagan" for another system. We could also use names of Egyptian gods as well, such as "Isis", "Thoth" and "Horus." Planets must be round in shape

Planet-sized problems

KL Baldwin, Titusville, Penna. — Sat, 19 Aug 2006 20:04:56 GMT

Much of the remaining nomenclature issue is pretty much based on whether we keep Pluto in the listing or not. Okay, then, so here's a thought:

+ Leave "planet" to public domain, whatever bodies are significant to the (most local?) populace.

+ Use another term for scientific purposes to define the gravitationally-rounded bodies, like planetoid or planemo.

+ Use one of the suggested classification systems to describe its type: rocky, icy, gassy, whatever.

+ If an object qualifies as a planetoid/planemo, but orbits another -- with barycenter inside the body of the latter -- call it a planemonic moon ("planemon"?) or secondary planetoid, something like that. (As differentiated from the primary-secondary-tertiary system already mentioned. Unless a moon orbits a moon, there's no tertiaries.)

Anyways, that's my take.

Planet-sized problems

Robert Young, Atlanta, GA — Fri, 25 Aug 2006 05:35:55 GMT

Steve, Please ditch the psuedo-elitist, revisionist jargon. The 'Moon' in English is...the Moon. To add a new name from another language (Luna) and pretend that that is the name is simply revisionism. Also, the idea of promoting Charon to a 'planet' because the barycenter is between the two objects (Pluto and Charon) is ludicrous and fails to account for the fact that the barycenter between the Sun and Jupiter is also between the two (does that make Jupiter a star? NO!).

Other neologisms like 'planemo,' especially when used in the overlapping sense below, add unnecessary confusion to the issue.

As for 'roundness, the Earth is naturally spherical due to gravity; any bulge is caused by centripetal/centrifugal forces due to spin.

I would define planets as any objects in orbit around a star that are not massive enough to be a star but massive enough to assume a nearly spherical shape due to gravity. Moons or satellites orbit planets, not stars. Planets could then be divided into 'regular' and 'irregular'...'regular' planets, regardless of size, orbit in nearly circular, nearly ecliptic, orbits. Irregular or minor planets (i.e. Ceres, Pluto, Xena) do not. Use of the term 'classical' is a misnomer (Ceres was discovered before Neptune!).

How do you define roundness? Earth isn't perfectly spherical. I can't come up with a definition that isn't human-centric.
It might be easier, and more universal, to measure mass. Consider a planemo system that has "cleared out" its orbit to the extent that it outmasses its co-orbitals. It should be called a planet, no matter how round it is. Of course, this requires a definition of "orbital path". Perhaps a toroid with a radius that reaches the nearest gravitational balance point with the nearest massive body.
(Note 1) By planemo system, I mean a body and its moons or possibly a double planet. I would define a large moon as a planet if it and its more massive partner orbit each other around a point that is between the two. The "fulcrum" for the Earth-Moon pair is some 400 miles below the surface of the Earth, so Luna is a moon by this definition. Pluto and Charon would be a double planet, if they outmassed the rest of the Kuiper Belt (not).
(Note 2) Earth, rather Earth/Luna, hasn't completely cleared its orbital path. Google 2002 AA29 or 3753 Cruithne.
Steve Moniz, Leavenworth, KS (Sent Thursday, June 08, 2006 11:49 AM)