Planet debate shifts focus

Posted: Wednesday, June 10, 2009 4:29 PM by Alan Boyle

SwRI

A map of Pluto's surface,
based on brightness.

The main players in the planethood debate gathered together this week to look back at Pluto's woes - and look ahead to fresh discoveries on the solar system's farthest frontiers.

Among the speakers at Tuesday's forum, held at the American Astronomical Society's summer meeting in Pasadena, Calif., were Neil deGrasse Tyson, the astrophysicist and planetarium director whose Pluto-less planetary display contributed to the controversy, and Alan Stern, the planetary scientist who heads NASA's New Horizon science mission to Pluto and its neighbors on the solar system's edge.

In all, seven speakers revisited the International Astronomical Union's decision three years ago to issue a definition of planethood that excluded Pluto because it hadn't "cleared its orbit" of other objects close to its size.

Pluto and other round objects beyond Neptune were instead classified as "dwarf planets," and later as "plutoids" - sparking protests from Pluto's planetary defenders.

It's safe to say that no one's mind was changed up on the speakers' platform: Tyson traced Pluto's troubled past - including a progressive downsizing of size estimates that led astronomers to joke during the 1970s that the planet would disappear completely by 1984. As it happened, more objects like Pluto were found starting in 1992, climaxing with the 2005 discovery of another ice world that is bigger than Pluto. (The uncertain status of the newfound object, now known as a dwarf planet named Eris, is what sparked the IAU's decision a year later).

Moving on?
In Tyson's view, it's time to move on. He noted that the solar system contained a wide diversity of objects that could be sliced and diced into categories ranging from shape and composition to geology and suitability for life.

"This is what we should be thinking about now, not arguing over the fricking definition of a planet," Tyson wisecracked.

Stern agreed that the solar system's diversity should be the focus of the debate. But he said the way we think of planets should reflect that diversity. The way he sees the issue, objects like Pluto make up the most numerous class of planets in the solar system - and objects like Earth should be considered the true oddballs. "I think there's a bit of a Copernican revolution," Stern observed.

Other speakers added their own particular perspectives:

Charles Beichman, executive director of NASA's Exoplanet Science Institute, noted the difficulties in using any single criterion for defining planets. The IAU backed off from setting up a standard for extrasolar planets three years ago, and Beichman joked that the standard once cited by a Supreme Court justice for obscenity might have to be used: "I know it when I see it."
The University of Arizona's Renu Malhotra noted that Pluto ended up sparking a revolution in the way scientists thought about planets. Today, Malhotra and other astronomers say Pluto and its kin were pushed outward in a grand migration of the solar system's outer planets. "The planets did not form where we find them today," she said.
Mark Sykes, director of the Planetary Science Institute, argued that a broad definition for planets - that is, a definition that accepts anything that is massive enough to crush itself into a round shape due to self-gravity - should win out because it's easier to understand and leads to more logical groupings of celestial objects. He sketched out a Venn diagram categorizing objects that had geological processes, atmospheres and the potential for life. "It makes sense to group all these things together as planets," he said.
Jean-Luc Margot, an astronomer at the University of California at Los Angeles, took a fresh approach to the whole definition question: Stick to the IAU's view on planethood, and set aside the word "world" to apply to round objects like Pluto as well as satellites like Saturn's Titan and Earth's moon. The world definition also would apply to free-floating objects that don't orbit any star. "Some worlds are planets, others are not," he said.
Vanderbilt University's David Weintraub traced the twists and turns in the concept of planethood over three centuries, noting that scientists often got it wrong. (For example, by thinking that planets had to follow a distance pattern known as the Titius-Bode rule.) Weintraub - author of the book "Is Pluto a Planet?" - said he'd like to see a sensible definition of the word "but I'm not sure we're anywhere close to being able to do that."

Tuesday's forum was remarkably free of rancor. In fact, the loudest exchange came between Tyson and Bill Nye (the Science Guy), who was sitting in the second row of the hall and went on a mock rant over the term "trans-Neptunian object," which scientists use to refer to solar system objects even if they don't cross Neptune's orbit.

"'This is not complicated," Nye said "'Trans' means across, 'ultra' means beyond!"

The question on many people's minds was whether there would be an effort to modify or overturn the IAU's definition at the organization's next meeting, set for August in Rio de Janeiro. Based on the views aired on Tuesday, neither side sounds anxious to have IAU officials revisit the issue in Rio.

"I think they should quit while they're behind," Stern said.

The last word
If the IAU's deliberations are being left behind, what's ahead? Caltech astronomer Mike Brown, whose discovery of Eris kicked the whole debate into high gear, said today during a different AAS session that he tries to avoid talking about whether or not Pluto should be a planet. (After the 2006 decision, he declared himself satisfied with the outcome.)

Now Brown is focusing on Pluto's kin at the solar system's edge - fellow dwarf planets such as the fast-spinning, football-shaped world named Haumea, and a mysterious far-flung object called Sedna.

Sedna inhabits what appears to be something of a no-man's land between the solar system's Kuiper Belt and the cometary Oort Cloud. However, statistical analyses suggest that there should be about 40 objects out there that are at least as big as Sedna. Some of those objects may even be big enough to reignite the "What is a Planet" debate yet again.

After his talk, Brown told me he's looking forward to the Hubble Space Telescope's return to service this summer, because he has several observational opportunities coming up that could shed more light on the solar system's farthest, dimmest frontier. He gave Hubble a high rating on the telescope scale. "It's the coolest one out there," he said.

I've written a whole book about this planethood issue, titled "The Case for Pluto," It should be out in November. I mention this only because I feel as if I'm short-changing you when it comes to the whole discussion that took place this week. To make partial amends, I've put together a bare-bones Web page that combines all of my Twitter updates from the planet forums (and the updates I would have made if I hadn't violated Twitter's tweet-per-hour limit).

You can watch some of Tuesday night's action via Ustream's video archive.

Join the Cosmic Log corps by signing up as my Facebook friend or hooking up on Twitter. If you really want to be friendly, give some thought to "The Case for Pluto."

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Comments

"In Tyson's view, it's time to move on." Curiously, this what Republicans say when people bring up torture, corruption and other malfeasance. I think we need a Pluto Reconciliation Commission.

Stephen S. Power, Maplewood, NJ (Sent Wednesday, June 10, 2009 7:38 PM)

"Moving on" from a bad decision by letting that decision stand is a terrible idea and makes no sense. The fact that this issue continues to be debated illustrates that the IAU decision satisfied few people and created tremendous confusion. Margot's approach (I listened to the recording online) does not really offer much that is new. By sticking with the IAU definition and then adding the term worlds plus the caveat "some worlds are planets, and some are not," he puts us right back at the problematic point we started from--the fact that the IAU definition classifies objects by where they are while ignoring what they are. If we're going to stick with the term planet, why not just come up with more adjectives to describe the many different types of planets we are discovering? Satellites like Titan and Ganymede can be referred to as "secondary planets" since their primary orbit is around another planet, and their secondary orbit is around the sun.

Brown's view is problematic altogether, first because he was for dwarf planets being planets before he was against it, but even more because so much of his discussion of dwarf planets centers on himself. He repeatedly makes statements such as "I changed the solar system" and constantly refers to himself as having "put the nail in Pluto's coffin." The title of his blog, "Mike Brown's Planets" along with the description of "full and dwarf" suggests by default that he views dwarf planets as planets, just smaller ones.

Avoiding this topic will not make it go away. A mistake was made, and it must be corrected, whether by the IAU, by another body, or by public consensus and usage. There are many people, including professional astronomers, amateur astronomers, and lay people who will never accept the IAU definition and will continue to fight it. I'm proud to be one of them. We are nowhere near "the last word" on this topic, and as Weintraub said, we shouldn't be, as we know far less about the many types of planets out there than many so-called experts think.

Laurel Kornfeld, Highland Park, NJ (Sent Wednesday, June 10, 2009 10:01 PM)

Everyone wants to set new ‘standards’ and be the new guy on the block. That’s the way it is. But until we ‘old people’ die off, Pluto is a planet and that’s the way that is. After we’re gone, do what you want. Just don’t mess it up.

Dave BC WA (Sent Wednesday, June 10, 2009 10:48 PM)

I listened to some of the talks online, as well as the Great Planet debate last year. Really I haven't heard any real super-compelling definition that was better than the IAU's. Although I think the wording of IAU's definition should be tweaked, I don't think it should be revisited unless the pro-Pluto planetary scientists can make a really compelling argument for a new definition.

KC (Sent Wednesday, June 10, 2009 10:51 PM)

As a Qualified Observer (observatory volunteer) at my astronomy club, I'm teaching as many kids as possible that Pluto and all dwarf planets are planets. That's not to mention my five-and-a-half-year-old and two-and-a-half-year-old nephews, whom I'm making sure to teach the solar system the correct way. The older one knows all about the debate and understand that if an object orbits the sun and is round, it's a planet. Don't count on a generation dying off to lead to acceptance of the IAU definition; new generations are being taught Pluto's planethood as well. And just so the dynamicists are aware, longevity runs all through my family, so (knock on wood) expect to see my nephews still around in 2100!

Laurel Kornfeld, Highland Park, NJ (Sent Thursday, June 11, 2009 1:11 AM)

I still do not understand the point of this argument. What difference does it make what we call Pluto? The relevant point is this - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune - those 8 objects form a category. Pluto, Sedna, Eris, etc., form another group of objects. Whether we include both groups in the term "planet" or only the 8 big ones, either way, we're going to have to recognize the 8 big ones with some term. "Planet" is as good a term as any, but we could call it "Major planet" just as easily, and include Pluto, Eris, etc., as "Dwarf planets". Either way, Pluto is now known to merely be the second-largest known object of its type. I see no problem with the term "dwarf planet", the only issue being whether we should consider "dwarf planet" a subcategory of "planet", or whether we should consider it a separate category.

Either way, there's not 9 planets. There's either 8, or dozens, probably growing to hundreds if not thousands as we discover more. I like the definition that keeps the number small, but it would make no difference to me if we simply had to erect a new subcategory of "major planets" to include the 8 big ones. Whatever we call it, the 8 big ones deserve a name of their own separate from the category that includes Pluto and other similar small objects.

Paper Hand, Merriam, KS (Sent Thursday, June 11, 2009 4:27 AM)

This reminds me of my definition of a lawyer: one skilled in the manipulation of words. None-the-less, I was taught that Pluto is a planet (define that word as you will) so to me it is a planet. I pretty much, however, agree with the entry made by Laurel Kornfeld earlier.

Bill Stanley (Sent Thursday, June 11, 2009 7:11 AM)

The best answer would have been to simply define a Planet (with a capital P) as a non scientific common term which describes the 9 bodies which were historically considered to be planets. Then define terms that describe the various bodies in the solar system. Some of the terms might or might not use the term planet,such as "terrestrial planet" for the rocky inner earth like Planets, "jovian planets" fo the gaseous outer Planets and "dwarf planets" for icy bodies that resemble the Planet Pluto. Such a system is logical and consistent because it does not try to take 9 dissimilar objects and group them into a single definition that coincidentally fits an obsolete view of the solar system.

Michael M, Kent oh (Sent Thursday, June 11, 2009 7:43 AM)

For those that believe in God being the sole Being with the real design of all this phenomenal work honestly if we humble ourslves we shall agree that our knowledge is limited to some so many areas and therefore we its wise for man not to rule out on anything so far just becos its not may be lining up with a certain standard or behaviour like what the IAU's are coming up with.

SAM western kenya (Sent Thursday, June 11, 2009 9:24 AM)

Am I the only one that thinks this arguement is the result of astronomers with nothing important to do? Not that it's the astronomer's fault, my point is quite the opposite. When there's not enough funding to keep as productive as we have the manpower to be, we end up wasting the genius of these professionals on a free debate over an unimportant issue.

Alex Dickerson, Cedar Falls, Iowa (Sent Thursday, June 11, 2009 9:39 AM)

why must there be only one word to describe round objects orbiting our star - or any star for that matter ???? if the reality doesn't all fit into a simple word then there should be modifiers connected to that word or other words the fully express what we are describing. words are only symbols used to describe something - complex things are rarely adequately or precisely described by one simply word.
maybe more emphisis should be placed on discribing what we see rather then craming what we see into a particular word

stranger (Sent Thursday, June 11, 2009 9:44 AM)

Yes, the definition of "planet" must be tweaked. Now, a planet must have cleared its orbit of other bodies. This means only Mercury and Venus, locally, are planets. The others all have moons, and Jupiter has the Trojan asteroids in its orbit. It would be much simpler if the IAU defined "planet" as a body whose gravity has caused it to become a spheroid, and let it go at that. Who cares if a new mnemonic must be invented for schoolchildren (or adults) to memorize the names of the planets in our solar system? Consistency should rule.

Norman Fields, Bullhead City, AZ (Sent Thursday, June 11, 2009 9:49 AM)

You can make a case that Pluto is a planet. You cannot make a case that Pluto is one of nine planets. Eris is larger than Pluto, and dozens of similar objects are only slightly smaller.

Ideally, I'd divide them into 3 (or more) groups: 1) small, rocky inner planets. 2) Gas giants. 3) Kuiper Belt Objects (like Pluto and Eris). There may be room for more groups if more objects similar to Sedna are found. The question of which of these groups are "planets" is one of semantics. The question of whether Pluto is one of nine is not.

Greg, Cleveland, Ohio (Sent Thursday, June 11, 2009 10:20 AM)

Interesting article Alan! I liked that Neil deGrasse Tyson started the revolution what a planet is. Now that we've discovered so many Kuiper Belt objects the size of Pluto it was time to rename Pluto as a dwarf planet or plutoid. It's truly amazing at how our view of our own solar system has undergone such radical improvements over the past 50 years of spaceflight.

I'm glad that NASA has sent a probe to check out Pluto close up even though it will take many eyars still for us to get that data.

Eric, Salinas, CA (Sent Thursday, June 11, 2009 10:59 AM)

People are notoriously resistant to change. Pluto has only been a "planet" for what, 50 years? It has been a planet for most of my 35 years on this planet, however, I am in favor of a definition that does not give us dozens-upon-dozens of "planets" once New Horizons arrives at Pluto. Pluto truely is in a class of it's own, but that class is poised to become a big one, me thinks (hopes, in-fact). Assigning the term Planet to every "body" that can gravitationally make itself shperical takes away from the true "8" planets that are the primary components of our solar system.

Eric, Appleton, WI (Sent Thursday, June 11, 2009 11:00 AM)

I don't know what's more fascinating about this whole story to me; the merits of the debate itself among scientists qualified to have the discussion because they make a living caring about such things, or the inevitable blowhards in these boards who've inexplicably convinced themselves that they have some really strong opinion on this.

I only know that the solar system is vastly more interesting and diverse a place than I learned about when I was a kid.

Also I kind of like the business about planets and worlds as a solution. Only I think maybe it should be the other way around ;)

Blah.D.Blah (Sent Thursday, June 11, 2009 11:43 AM)

Well, the very word planet is the english equivalent for the latin "Planeta" or "wanderer". There are a lot of wanderers out there. So many that you couldn't possibly name them all or catalog them

Thomas Ashby (Sent Thursday, June 11, 2009 12:25 PM)

What do you tell all the millions of people that grew up learning in school that Pulto is a planet? Also, I wonder how long it took, before people of long ago excepted the explation that the world was round and not flat! Was this learned by a new generation before it was excepted?

Mark (Sent Thursday, June 11, 2009 12:46 PM)

Laurel Kornfeld,
I agree that the current definition is lacking. And I'd like to see it changed. That being said, the definition isn't supposed to satisfy any emotional fancy. It provides common ground so that communication can take place. There are some ice balls out there and if we want to talk about them we just need to know what to say so we can be understood by everyone. There *is* a definition, right or wrong, and with it we can accomplish communication. Maybe we have to do it with a frown on, but we can do it.
You, and that group you belong to, have a legitimate, relatively inconsequential gripe. It's rather like refusing to move into a house because the baseboards are all wrong. Even if you don't like them they do the job they're supposed to do.
It's nice that you have a cause to champion. I'm sure if you're successful many people will enjoy fuller, healthier, more prosperous and happy lives.

Tim Rommes, Washington, UT (Sent Thursday, June 11, 2009 12:56 PM)

I am afraid I don't understand why people are so emotional about Pluto being removed from the planet list and called a dwarf planet. It is still there, and it will affect the solar system in the same way.

Rakesh Sharma, San Mateo (Sent Thursday, June 11, 2009 2:43 PM)

The IAU doesn't need a "new" definition. Just dump the orbit clearance garbage. No orbit has ever been "cleared". New stuff migrates in all the time.

UP Pluto!

Celtic Curmudgeon, Aberdeen (Sent Thursday, June 11, 2009 4:00 PM)

Why not use Star Treks model of classification of planets. All the attendees at the IAU meeting probably have it memorized anyway along with their schematics to the Enterprise.

Curtis Milwaukee WI (Sent Thursday, June 11, 2009 4:07 PM)

The word "planet" has cultural significance that shouldn't be redefined by a scientific body. Sure the IAU might try to decide what astronomers deem to be planets, but the word was originally used to describe visible objects that were not fixed in the sky in relation to background stars. The word "planet" meaning "wanderer" has a rich history and deserves consideration on that level. Ultimately we should be asking, is the word "planet" a modern scientific term?

Humanity's emotional reaction to the IAU's ruling says it all... we're angered that a group of scientists casually re-wrote our history, poetry, myths, lore, and incalculable numbers of creative works.

Jim, Seattle (Sent Thursday, June 11, 2009 5:37 PM)

While I'm saddened that Pluto is no longer considered a planet (it always will be, in my book), I think that both Mark Sykes and Jean-Luc Margot both have interesting ideas on how to "define" planet-hood, which should be investigated.

However, I also agree that, despite the controversy, more attention should be paid to the diversity of the solar system around us. Having said that, it's still sort of "fun" to have these little debates that probably don't amount to much in the grand scheme of things.

I look forward to the IAU eventually coming up with a solid "definition" for planet-hood, but I equally look forward to hearing about all the marvels and wonders that are yet to be discovered in the solar system. I can't wait for Hubble to come back online.

Craig S., Emmaus, PA (Sent Thursday, June 11, 2009 8:03 PM)

Tim Rommes, the planet definition I support is not based on an "emotional fancy," and I strongly disagree with your claim that this definition does accomplishes what it is supposed to do. A good definition is based on, as Dr. Mark Sykes said, the need for a definition to be useful, and on scientific fact. Objects large enough to be rounded by their own self-gravity are fundamentally different from smaller ones shaped by chemical bonds. Why do people see a problem in having a category of planets that includes a large number of objects? We have billions of stars and billions of galaxies. It's only a matter of time before the number of exoplanets we discover becomes too large to count. Restricting a category artificially just for the sake of our own convenience is not an argument based on science. In fact, it could be construed as an emotional argument as well. As for change, not all change is good, just as not all change is bad. In the case of the IAU definition, the change was negative because it classified objects solely by where they are while ignoring what they are, introduced an extremely vague concept of "clearing an orbit," and makes the claim that dwarf planets are not planets at all. Why not use a broad planet definition like the one proposed by my astronomy instructor Al Witzgall: a planet is a non-self-luminous spheroidal body in orbit around a star." We can then establish multiple subcategories of planets--terrestrial planets, gas giants, ice giants, dwarf planets, super Earths, hot Jupiters, and likely many more yet to be discovered. No, our solar system does not contain 9 planets. It contains at least 13 and possibly more. Yes, many are very different from each other (Earth is more like Pluto than like Jupiter), and that is where the subcategories help us create a meaningful, understandable classification system.

Laurel Kornfeld, Highland Park, NJ (Sent Friday, June 12, 2009 12:53 AM)

Ok,, this is annoying. As much as the astronomers get to define what a planet is as far as its use in astronomy they do not get to change the definition of the word as it is generally used. The IAU, intentionally or not, defined Pluto as a planet. The use of the adjective "dwarf" modifies the noun "planet". As an example, if I tell you "that thing over there is a car" and you tell me you disagree and that you think it is not a car and then call it a "blue car" most people, myself included, will be annoyed with you. Why? Because a blue car is still a car. A blue car is a type or subset of the general catagory of car. Thus Pluto, by the IAU definition, is a type of planet.

Also, this idea about clearing the orbit is dubious in my relatively uninformed opinion. What would we do if we find two Jupiter mass planets crossing orbits around a star somewhere? Not a stable orbital situation you say? Sure isn't, but it could last for far longer than our lifetimes. So would we say the two planets are temporarily not planets?? I don't want to have to discuss the relative dynamics of an entire solar system in order to give someone an understanding of a particular celestial body. So I am complaining I should offer a solution. Keep it simple. Planets orbit stars and are big enough to be round due to gravity. (Exceptions can happen but must be voted on by a large group of cubscouts.) Use adjectives to further categorize. Major planets have more gravity than Earth, minor planets have less. Then maybe terrestrial vs. jovian or gas giants, inner vs. outer solar system planets, hot vs. cold and so on. This way if I tell you I found a major terrestial cold outer planet you have very clear notion of what I saw.
There,, rant over,, feel better now.

Dave.

Dave, Albuquerque, NM (Sent Friday, June 12, 2009 4:44 AM)

This debate has been around for a few decades. I was looking through a 1981 edition of Patrick Moore's Atlas of the Universe p.182 and came across "And in any case, it now seems very doubtful whether Pluto should be regarded as a true planet. It may even be the brightest member of a whole swarm of small bodies.." So lets call it the Edgeworth-Kuiper-Moore belt.

However the debate is really where do you slice a power distribution into two segments. Sorting the solar system bodies in terms of size (I prefer mass personally) then where are the natural breaks in the progression. There is one between Europa and Titania but some of the ultra-Neptunian objects may fill that gap. Short of a natural break you are on a hiding to nowhere picking the cut-off point.

David, Adelaide, Australia (Sent Friday, June 12, 2009 6:11 AM)

It makes just as much sense to me to say instead of '8 planets and a dozen dwarfs' to say 4 terrestrial planets, 4 gas giant planets and a dozen dwarfs, scientifically, the destinction between the gas giants and the terrestrials is as significant as the difference with the dwarfs, so we get back in a round about way to having pluto a planet, just a dwarf one instead of a terrestrial or a gas giant one. I would want to add the category planetary moon to include places like Titan as opposed to, say, phobos.

John Delphia (Sent Friday, June 12, 2009 11:49 AM)

Laurel Kornfeld,
“Objects large enough to be rounded by their own self-gravity are fundamentally different from smaller ones shaped by chemical bonds.”
That’s why we’re not calling every rock out there a dwarf planet.

“Why do people see a problem in having a category of planets that includes a large number of objects? “
They don’t. The category in question is “dwarf planet” or “plutoid.”

“As for change, …”
Using your return key to break up your run on paragraph would be a good change. It would divide your subject matter into discernable, logical units and make it easier for all of us to follow what you’re talking about. Much like using the term “dwarf planet” or “plutoid.” But you do seem to be attached to the idea of lumping everything together.

Your, via your instructor, proposed definition includes the term “spheroid” which I am taking as a reference to hydrostatic equilibrium. There is no clear demarcation that defines roundness so this is vague. Could you explain the difference to me that makes this vague definition acceptable while “clearing an orbit” is not, other than it is “extremely” vague. Some objects will round out nearer the sun but fail to overcome those chemical bonds farther out in the cold. So part of the question of roundness has to do with where they are. Should we include in the definition of roundness that the objects self gravity would bring it into hydrostatic equilibrium outside the effects of external heat or tidal forces? Break it down to just mass and composition?

Out of curiosity, earlier you said you teach kids the solar system in the right way. How do you teach the earth and moon? Earlier still you griped that the current “IAU definition classifies objects by where they are while ignoring what they are.” Removing the caveat about where they are would make most moons planets, if you go with the vague idea of roundness. But our moon, the Moon, in particluar presents an added twist. It directly orbits the sun, meaning that it never crosses its own orbit. The fact that we refer to it as a moon is strictly positional. By your standards its not a question of 8 or 13 but dozens already.

Location has a say in every other aspect of defining a planet. And there are no parts of planetary definition that are clear cut and unambiguous. That’s why there’s a controversy at all. It would just suck if you referred to a certain group of celestial objects as “fishes” and I referred to them as “blockards” and neither of us realized that we were talking about the same thing. Our definitions only serve to give us common vocabulary so that we can talk about objects in general. And the current ones accomplish this in fine manner. They could be better, but they don’t need to be. Personally, I wish the term “planet” referred tocelestial objects that are in hydrostatic equilibrium and do not support fusion. Then we could subcategorize as rocky, gas, ice, rouge, primary, secondary, gravitationally superior, gravitaionally inferior, gravitationally co-superior, magnetic, non-magnetic, volcanically active, etc. Do I need a logical definition like that? No. If I want to discuss dwarf planets with you I can because when I say dwarf planets you know what I’m talking about. At least as much as you can know using vauge terms like spheroid.

Tim Rommes, Washington, UT (Sent Friday, June 12, 2009 4:48 PM)

Of the many problems with the IAU's definition of a planet, none is more obvious than its failure to include Trojan planets. Consider a solar system where the parent star is 100 solar masses, a brown dwarf of 50 Jupiter masses orbits at 30 AU, and there is one Jupiter planet (both in size and mass) at each of the LaGrange points L_4 and L_5. These two Jupiter sized planets have not cleared the neighborhood around their orbits since they would not have cleared out the brown dwarf at 30 AU nor would they have cleared out each other (at 30 AU). Any reasonable definition of a planet must include them but the IAU's definition does not include them. The problem with defining a planet as "nearly round" suggesting that pear shaped bodies in hydrostatic equilibrium do not qualify as planets, see http://www.josleys.com/show_gallery.php?galid=313. Saturn is over 12% oblate and this suggests its planet-hood is ambiguous because "round" is quite often understood as spherical. Although the IAU listed Saturn as a planet, Saturn's planet-hood should be unambiguously deducible from the definition of a planet and therefore not require its presence on a separate list. And the definition of planet should not be heliocentric. Currently, the IAU has one definition for planets in the solar system and another definition for planets everywhere else. In accordance with the Copernican principle, there should be one definition of a planet that applies everywhere. The linguistic problem with dwarf planets not being planets and the ambiguity of "clearing an orbit" demand that the IAU planet definition be altered. A more general problem is that there is now too much terminology for the outer solar system. For instance, there are now Kuiper belt objects, Trans Neptunian objects, Plutoids, Plutinos, Scattered disc objects, cubewanos, etc. There are other ways to fix up the planet definition problem and below is an example of one possible way of doing this. This approach defines a planet in context with other bodies in the universe:

A star is a celestial body that sustains, has sustained, or is capable of sustaining nuclear fusion.

A brown dwarf is a star that is only capable of fusing deuterium and/or lithium even if neither is present.

A celestial body that has a barycenter continuously inside the body of another celestial body is a satellite of that celestial body.

A planeton is a celestial body that is not a star and has sufficient mass for its self gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium.

A planet is a planeton that is not the satellite of another planeton.

A moon is a satellite of a planet.

These definitions could be supplemented by three different classifications for those people who feel that the number of `big' planets in the solar system should be a small enough list for school children to memorize:

A planet with a maximum width: > 30,000 km is a giant planet, < 3,000 km is a dwarf planet, otherwise is a midsize planet.

A moon with a maximum width: > 3,000 km is a giant moon, < 300 km is a dwarf moon, otherwise is a midsize moon.

A primary planet is a giant planet or a midsize planet and a primary moon is a giant moon or a midsize moon.

Definitions like these are backwards compatible with the history of astronomy, most text books, and even the IAU's flawed definition. Dwarf planets remain dwarf planets, for example. They also solve what many consider to be a problem of having "too many" primary planets in our solar system. Another advantage of an approach like this is that it simplifies the solar system: Going out from the sun there are four primary midsize planets, an asteroid belt, four primary giant planets, and a Kuiper belt. Simple. Imagine the problems that would arise if electrons were defined in a way analogous to the way the IAU has defined planets. The definitions might go something like this:

(1) A "electron" is a subatomic particle that: (a) is in orbit around an atomic nucleus, (b) has a negative electrical charge that is in a (nearly round) symmetric shape, and (c) has cleared the neighborhood around its shell.

(2) A "dwarf electron" is a subatomic particle that: (a) is in orbit around an atomic nucleus, (b) has a negative electrical charge that is in a (nearly round) symmetric shape, and (c) has not cleared the eighborhood around its shell.

In this way of thinking, a lithium atom has one electron in its outer shell and two dwarf electrons in an inner shell (and dwarf electrons are not electrons). Obviously definitions like these are deficient.

Randy Gerl, Redmond, WA (Sent Friday, June 12, 2009 4:54 PM)

Randy Gerl,
Nicely done. But I would say that anything held in a L-point is cleared. Actually captured. Just caged in space instead of accreted.

Tim Rommes, Washington, UT (Sent Friday, June 12, 2009 11:00 PM)

To Tim Rommes:

You said, “That’s why we’re not calling every rock out there a dwarf planet” in response to my statement, “Objects large enough to be rounded by their own self-gravity are fundamentally different from smaller ones shaped by chemical bonds.”

My problem, as stated earlier, is the claim that dwarf planets are not planets at all. This issue is causing many to view Pluto, Eris, etc. as nothing more than asteroids. Objects rounded by hydrostatic equilibrium are significantly different in composition and geological processes to shapeless asteroids. That meaning is obscured when we define dwarf planets as not being a subclass of planets. Instead, dwarf planets should be understood as small planets shaped by their own gravity but not gravitationally dominant in their orbits.

It seems some people have a problem with a definition that gives our solar system dozens of planets. That is the reference I was making. Why does the number of objects falling under the broad category “planet” have to be limited? If there are hundreds of such objects in our solar system, then that is what there are.

There are in fact many who object to a definition that "opens the floodgates" to hundreds of solar system objects being considered planets. This is not a scientific issue but solely a matter of convenience regarding memorization, which is not as important as many think in teaching children about the solar system. Understanding the different types of planets and what defines each subcategory is much more important.

Hydrostatic equilibrium does not mean an object is a perfect sphere. It can be an oblate spheroid. The idea is that the object is large enough to be shaped by its own gravity, and this is not very difficult to determine except for tiny bodies very far away. The threshold for objects becoming rounded by their own gravity is not very different in spite of whether the object is closer to or further from the sun. In any classification system, there are always a few objects on the fringes that are not clearly part of one or another category (such as brown dwarfs vs. stars). However, Pluto, Eris, Haumea, Makemake, and Ceres are not at that boundary. They are clearly in hydrostatic equilibrium.

In contrast, the "clearing its orbit" is far more vague. How "cleared" does an orbit have to be? Every planet has asteroids in its orbital field. Neptune does not clear its orbit of Pluto. Two objects in a binary planet system under this definition can never be considered planets. And the further away a body is from its parent star, the larger the orbit it will have to clear. If Earth were in Pluto's orbit, it would not clear that orbit either and therefore would not be considered a planet.

Many astronomers consider round moons, ie. those that have attained hydrostatic equilibrium, as secondary planets. I teach children that the composition of many of these objects is similar to that of planets; the difference is they orbit other planets instead of orbiting the sun directly. So at present, our solar system has 13 primary planets and many more secondary planets.

You say, "Personally, I wish the term 'planet' referred to celestial objects that are in hydrostatic equilibrium and do not support fusion. Then we could subcategorize as rocky, gas, ice, rouge, primary, secondary, gravitationally superior, gravitaionally inferior, gravitationally co-superior, magnetic, non-magnetic, volcanically active, etc."

Then we are in agreement, as this is exactly what I propose. Dwarf planets should be a subclass of planets that qualify as planets due to their being in hydrostatic equilibrium, but are of the dwarf subcategory because they are not gravitationally dominant in their orbits.

Laurel Kornfeld, Highland Park, NJ (Sent Saturday, June 13, 2009 2:39 PM)

“This issue is causing many to view Pluto, Eris, etc. as nothing more than asteroids.”
Many people are idiots, and you can’t help that. There is a clearly distinct category for all the crap that can’t pull into round.

“Dwarf planets should be a subclass of planets that qualify as planets due to their being in hydrostatic equilibrium, but are of the dwarf subcategory because they are not gravitationally dominant in their orbits.”
That’s essentially where we’re at. We just lack that specific heirarchy. Instead a a SUBclass it’s just a different class. Which makes a degree of sense. We have 1,2 and 3. You’re proposing that should be 1, 1a and 3. Of course, then we should have a 1b and let’s keep the same order. 1a is planets that dominate their orbits and 1b is planets that do not dominate their orbit. 3, renamed 2, is debris. We can renumber the definitions without changing the wording and satisfy your gripe. Do you really think the way the rules are numbered is scientifically important?

I agree with your points, I just don’t think it’s all that important. Annoying. Especially linguistically. Dwarf planets are not planets. It’s ridiculous to say. It’s a very important issue if you’re an English major. On the science side we have the distinction, vague as it is. But there aren’t many clear lines.

Tim Rommes, Washington, UT (Sent Thursday, June 18, 2009 8:43 AM)

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