Who else wants Pluto to be a planet again?
2006 mythoslegends.com. Illustration by Pat Rawlings. Used with permission.
“The International Astronomical Union knows its new planet definition is indefensible,” said S. Alan Stern.
Alan is a professional astronomer, long–time space advocate and old friend of mine. He also happens to be Principal Investigator for NASA’s New Horizons space mission to Pluto, which launched a year ago and which is scheduled to encounter Pluto and its moon Charon in the year 2015.
Alan called Earth & Sky yesterday with an update on what some American and European astronomers are currently thinking about last summer’s decision of the International Astronomical Union (IAU) to rewrite its definition of a planet, thereby stripping Pluto of full–fledged planet status.
According to Alan, the IAU has now formed a committee to clean up its new definition. That’s when he said the word “indefensible.” In fact, many astronomers believe the new planet definition – voted into official existence last summer by only a few hundred astronomers – needs work.
Alan and I talked about some impending events, centered on Pluto and its lost planet status.
One is a special session at this year’s European Geosciences Union conference in Vienna in mid–April. The special session will be called “What is a Planet?” Session organizer Karsten Seiferlin explained to Alan in a note: “Last year’s decision to take away the status of being a ‘planet’ from Pluto … left both the public audience and some planetary scientists surprised and in some cases embarrassed.”
And if the Europeans are embarrassed, American astronomers are likely to have even more conflicting emotions about Pluto. Alan confided that some American astronomers are unhappy that the IAU “kicked out,” as he said, the only planet discovered by an American astronomer, Clyde Tombaugh.
Meanwhile, in Clyde’s hometown of Streater, Illinois, the Streator Tourism Council has announced plans to bring a Planet Pluto Expo to the city on May 18 and 19, 2007.
What is happening here? Wasn’t Pluto demoted from planet status forever?
In science, very little is forever. According to Alan and other astronomers with whom we’ve spoken recently, some astronomers are simply choosing to ignore the IAU’s decision. Alan and his colleagues in astronomy are hoping to organize a meeting of their own, so that professional astronomers can come together to discuss Pluto’s status.
To understand the arguments better, download this great (pdf) factsheet about Pluto from astronomer Ken Croswell.
Astronomers like Alan and Ken are trying to keep the public aware that Pluto’s status as a planet is still an open debate.
And speaking of the public reaction, Alan mentioned that he’s heard from parents whose children think that Pluto has just … disappeared. He seemed sad about that.
Pluto hasn’t disappeared, of course. It’s still out there on the edge of the solar system, with a moon nearly as large as Pluto itself, with its two smaller moons, and with mysterious surface features only now being glimpsed by our telescopes.
A lot of us are sad that the planet that was so popular among children is no longer a planet.
Meanwhile, Alan has more reason than most of us to feel unwilling to accept the IAU’s decision. He worked toward a space mission to planet Pluto for much of his adult life. When the New Horizons mission launched on January 19, 2006, it was headed toward a full–fledged planet in our solar system. Now, according to the new definition, the spacecraft is headed toward a “dwarf planet.”
Maybe there’s nothing to these labels, after all. All just sound and fury, signifying nothing? These bodies in space have pursued their majestic orbits for billions of years. When you think about it in that light, our little notion of whether Pluto should be called a planet or not doesn’t seem very important.
But astronomy is a peculiar kind of science. Although it has a few practical applications, its main use is to give us a larger context for our existence on this Earth. Astronomy is for people. It’s not for a few hundred professional astronomers, voting in a room somewhere. What is their work in astronomy for, after all, if not to expand our horizons?
That’s why, to me, the IAU’s decision last summer to give us a solar system of eight planets instead of nine seems … wrong. It seems contrary to what astronomy is supposed to do.
What do you think? Who else wants Pluto to be a planet again?
Please get Pluto back in the line of nine. What happened to the planet Xena and her moon Gabrielle. Did those a$#@^% screw that up also. Good luck, you have my support.
AV
Bring back Pluto!
My whole family wants Pluto to be a planet. In fact, my son’s sixth grade class took a vote. If it were up to these young stargazers, Pluto would never have been excluded in the first place.
“Xena”—which is slightly larger than Pluto—has been renamed Eris, the goddess of strife and discord. The moon “Gabrielle” has been renamed Dysnomia, the demon spirit of lawlessness and the daughter of Eris.
So at least some astronomers have a sense of humor.
Note that because Eris is larger than Pluto, then if Pluto is a planet, so is Eris. Thus the real choice is between eight planets and ten planets. Nine is no longer, um, tenable.
Alan,
I hate to inform you of this, but Xena has been renamed Eris, and Gabrielle has been changed to Dysnomia. In Greek mythology, apparently, Eris reigns as the goddess of strife, and her daughter Dysnomia’s name means lawlessness.
As things stand now, Pluto, Eris and the asteroid Ceres (I presume it’s still an asteroid) are categorized as dwarf planets. For whatever reason, dwarf planets are NOT planets, but dwarf stars are still stars.
I find it ironic that the vague definition of “planet” has brought some “strife” and “lawlessness” to planetary nomenclature.
After doing a search, it looks like Ceres is no longer an asteroid, now that it has been reclassified to dwarf planet.
This is a great illustration and captures the essence of the article.
In my very humble opinion, I would like to see Pluto And Eris have be elevated to “planet” status.
So if Pluto and Eris were re-classified as planets, would the other dwarf planet – Ceres – also become a planet?
Are those the only three dwarf planets … or are there more?
I think the IAU did the right thing when they “demoted” Pluto, and I think they should be applauded for their courage. Revoking Pluto’s full-fledged planetary status shows that science is self-correcting. If Pluto had not been discovered until this century, it most certainly would not have been classified as a planet just as Sedna, Varuna, Quaoar, or Orcus have not been. Planetary demotion is not unprecedented either. When Ceres was first discovered, it was hailed as a planet, but as more asteroids were discovered, it became clear that it is starkly different from the other planets, and it was demoted. Ceres does not dominate the vicinity of its orbit, and that is exactly the same criteria that ousts Pluto from the club. Most arguments that Pluto should remain a planet are based on emotion rather than science, but if people are persuaded by emotional arguments, I’ll conclude with one: If Pluto is “un-demoted” the only logical thing to do is “promote” another 30 or 40 celestial objects to planetary status as well. Perhaps then fifth graders everywhere will recant, as they would have to memorize 50-some-odd planets instead of only 8.
So if Pluto and Eris were re-classified as planets, would the other dwarf planet – Ceres – also become a planet?
No. At least, not in MY opinion. People who claim that Ceres was once considered a planet fail to note that the word “asteroid” was coined just a year after the discovery of Ceres—to describe Ceres and its ilk.
If you don’t believe me, look up “asteroid” in the dictionary. It entered the English language in 1802.
Ceres was discovered in 1801.
Right, and the reason the word “asteroid” was coined was so that Ceres could be classified as one instead of being called a planet. Today we have the term Kuiper Belt Object, and Pluto without question belongs to that category.
What scientific basis is there for calling Pluto a planet but not calling Ceres one?
“What scientific basis is there for calling Pluto a planet but not calling Ceres one?”
Well, none that’s strictly scientific. I, personally, don’t have a major problem with classifying Pluto and Eris as “dwarf planets”, even if the official definition has technical flaws and needs to be reworked.
But there’s also an argument to be made for creating a cultural/historical category called the “Traditional Planet”. It would define “planet” the way we always did, before last August: any object that orbits the Sun, and is at least as large as Pluto. That would give us ten at present count: four terrestrial planets, four gas giants, and two “ice dwarves”.
The IAU ordinarily wouldn’t be expected to create such a cultural category, but because school children might well end up memorizing the names of the “Traditional Planets”, the IAU might want to go ahead and put its official stamp of approval on the idea. Plus, if an eleventh “Traditional Planet” were to be discovered, the IAU would have guidelines on how to name it, in the traditional manner.
Great idea, Bob!
“What scientific basis is there for calling Pluto a planet but not calling Ceres one?”
Pluto is a lot bigger than Ceres, as the fact sheet notes.
The analogy between Ceres and Pluto is a poor one. Only one year elapsed from the discovery of Ceres to the discovery of the next asteroid (Pallas). In contrast, sixty-two years elapsed from the discovery of Pluto to the discovery of the next trans-Neptunian object—despite the poorly known fact that Clyde Tombaugh kept looking for new planets until 1943, thirteen years after he found Pluto.
Had the International Astronomers Union (IAU) accepted the IAU’s planet-defintion committee’s original proposal for the definition of a planet, we would now have 12 planets in our solar system: the 8 classical planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus & Neptune) and these 4: Ceres, Pluto, Charon, Eris. Pluto-Chiron would have been classified a double planet, because its barycenter (shared center of mass) lies in between these bodies. (The Earth-moon barycenter lies inside of Earth, so the moon is a satellite of Earth.)
To be a planet according to the recommendations of the planet-definition committee, the solar system body had to have sufficient mass for self-gravity to shape it into a round structure (hydrostatic equilibrium). Second, that round body could not be a satellite of a planet.
The IAU decided, however, to add another requirement: for this body to predominate enough to “clear the neighborhood around its orbit.” That additional requirement removed Ceres, Pluto, Charon, and Eris from attaining planetary status.
I’m confused about that last requirement … about “clearing the neighborhood” around Pluto’s orbit. What hasn’t Pluto cleared away, that eliminated it from planet status?
And doesn’t Jupiter has stuff in its orbit … the Trojan asteroids … why wasn’t Jupiter disqualified from planethood for that reason?
The last requirement confuses me – and probably many others, too. Debbie, I don’t know how someone would answer your point about the Trojan asteroids (asteroids sharing Jupiter’s orbit). I’d love for a member of the IAU who voted “yea” to enlighten us as to what “clearing the neighborhood” means.
Well let’s see, for thousands of years the planets referred to the five wanderers in the sky and the sun and the moon. Now because of advances of science we choose to redefine something that has had meaning for much longer than the time since Galileo. Go back to the old definition and leave it at that. We could choose to let the term become obsolete because of the lack of knowledge of solar system objects by our forefathers. Solar system objects (including the earth) should be scientifically defined but not as planets. The word planet already has its own meaning. We could choose to call solar system objects – rocky objects, gas objects, asteroid objects, Kuiper Belt objects, moons, the sun and comets. It would also be prudent to give proper names to our moon and the sun to reduce some of the ambiguity.
The Greek word planet means “wanderer,” referring to a celestial body that moves through the “fixed” stars of the Zodiac. Yes, in that sense of the word, the sun and moon were considered planets too, along with the five other known wanderers: Mercury, Venus, Mars, Jupiter & Saturn.
“And doesn’t Jupiter has stuff in its orbit … the Trojan asteroids … why wasn’t Jupiter disqualified from planethood for that reason?”
Well, I suppose someone supporting the IAU resolution—which I most definitely do NOT—would argue that the Jupiter/Trojan mass ratio is a lot greater than the Pluto/trans-Neptunian mass ratio.
The International Astronomers Union put out a question/answer sheet on the definition of a planet at http://www.iau.org/Q_A2.415.0.html. I can find no real clarification on the meaning of “clearing the neighborhood around its orbit.” But it does say “Ceres is an example of an object that does not orbit in a clear path. There are many asteroids that can cross the orbital path of Ceres.” By the way, does anybody remember when Comet Shoemaker-Levy smashed into Jupiter in 1994?
Not to mention that Pluto itself crosses the orbit of Neptune!
And how about the Apollo asteroids, whose orbits cross the orbit of Earth? According to Eric Weisstein, “it is believed that there are at least 2000 Earth-crossers with diameters of 1 km or larger, 100,000 larger than the Rose Bowl, and 70-80 million larger than a typical house.”
The article in full is at http://scienceworld.wolfram.com/astronomy/ApolloAsteroid.html
Magenta (comment #18) seems to understand the meaning of “clearing the neighborhood” – even if not in favor of the IAU defintion of a planet.
In his SCIENTIFIC AMERICAN article “What Is a Planet?” Steve Soter says the astronomer Michael Brown suggested that a planet be defined as “any body in the solar system that is more massive than the total mass of all the bodies in a similar orbit.” It was this and similar arguments, according to Steve Soter, that persuaded the IAU to define a planet in terms of “clearing” its orbital neighborhood. Apparently, no ratio has been set, so the author recommends that the IAU amend the definition to specifiy that “a body in our solar system is a planet if it accounts for more than 99% of the mass of its orbital zone.”
If you are a Pluto fan and are against the demotion of Pluto, make your voice heard by signing the petition at http://www.pleasesavepluto.org
Of course the 99% mass criterion is completely arbitrary. This underscores the truth that so many scientists seem to be running from: ANY definition of “planet” is arbitrary.
Thus, since the definition is arbitrary, it makes sense to choose a definition that says a planet is a Sun-orbiting object that’s the size of Pluto or bigger.
That preserves the current nomenclature and causes only one new world—Eris—to be added to the roster of planets.
I for one have no problem with Pluto not being a planet. As an icy body on the outskirts of the solar system it’s much more like a Kuiper object than one of its larger cousins near the center.
And then consider that ‘planet’ means wandering star. Thinking back on how that term came into existence, it would help if it were visible to the naked eye or at least a backyard telescope. Furthermore, it is admittedly hard to get excited about Pluto when all the descriptions we hear make it sound like an asteroid! (or group of asteroids)
I think what we all need to come to terms with here is that science, and reality for that matter, is rarely black and white. All things seem part of a continuum: that is, lots of things only a little different for each other, all thrown together in a parfait.
How big is our Solar System anyway? Where does it end? Where does the Sun’s magnetic field end? Where does any field end? How about it’s gravitational field? Closer to home, where does the Earth end? The Space Shuttle orbits in what looks like outer space (outer?) but there’s still an atmosphere there. Is that Earth?
It seems we constantly struggle to name, define and classify things. We like rules and borders. Sometimes they seem arbitrary or the reasons for the definition have little to do with the thing we’re defining. Example: children become adults at 18 years. Is that because we all get smarter and mature on our 18th birthday, (we don’t).
Our need to categorize has gotten us into trouble time and time again. Classes, races, sizes, colors (when does tan become brown?) As for me, I’ve never been clear on the difference between regular shrimp and jumbo shrimp. Then last summer, Wendy’s made their small soda 20 ounces! I’m lost!
I could go on, but we should just let it go. I understand the children might be confused with the change in Pluto’s status. Calling it a very big or very close Kuiper object would be accurate and open a discussion into just what a solar system is. If that doesn’t work, I might suggest lining up all the planets in a row with their sizes and shapes relative to each other. Then singing a little song: “One of these things is not like the other, one of these things just doesn’t belong…”
Just my two cents,
Steve Goscinski
Pluto is faint only because it is far away. ALL the planets of the solar system would be invisible to the unaided eye if they were as far as Pluto’s mean distance from the Sun.
Furthermore, if Pluto had the same orbit around the Sun as Mars, it would—like Mars—be one of the brightest objects in the sky. In fact, at its closest, Pluto would outshine every star in the night.
No one disputes that Pluto belongs to the Edgeworth-Kuiper belt. But Pluto can still be a planet.
The necessary criteria for a planet are reasonable. A body orbiting a star that:
1. is not a star
2. is not a moon (satellite) of a planet
Additionally the criterion of “clearing the neighborhood of its orbit” appears to be tightly coupled with the concept of orbital resonance.
Why not word the last criterion as “is not in orbital resonance with another planet?”
This would eliminate Pluto/Charon (2:3 resonance with Neptune). Can someone look into the orbital resonances of the big boys other than Pluto/Charon in the Edgeworth Kuiper belt and beyond (Eris, Quaoar, Sedna, Lxion, Varuna, etc.)?
Though I’m jumping outside our solar system, two planets orbiting the star Gliese 876 apparently show a 2:1 orbital resonance. These gravitationally-linked planets are said to have at least 0.5 and 1.8 the mass of Jupiter.
Story at http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=214
Yes, and the first extrasolar planets ever found—around that pulsar—are in a resonance. So a resonance should NOT disqualify a planet from being a planet.
Also, there’s a funny dance between Venus and Earth, such that Venus appears at nearly exactly the same place in our sky every eight years. I hope no one would suggest demoting Venus and Earth…
There is a method to the apparent arbitrariness of using resonance as a criterion.
Before I continue I’ll quickly comment on:
1. the bodies orbiting (“orbiters”) around Gliese 876 – By the resonance criterion, one of them would not be a planet (likely the less massive one).
2. Despite the dance between Venus and Earth, their motion is sufficiently random for not considering them to be in resonance.
The two dominant themes for classifying planets appear to be:
1) size and 2) planetary system formation
The hydrostatic equilibrium criterion includes (subsumes?) the size criterion. A body has to be massive enough for such forces to come into play in determining their shape.
The resonance criterion includes the formation concept (it also adapts well to captured objects). For instance, w.r.t. our solar system, Neptune has had a big impact on the orbits of other smaller bodies causing some of them to go into resonance with it (like pluto, et. al), and likely capturing one (Triton).
I will attempt to arrive at the definition of a planet.
Let an “orbiter” be a body that:
1. is not a star (ok, this doesn’t cover the rare system wherein the most massive body is not a star, but…)
2. orbits one or more stars in the system
3. large enough to achieve hydrostatic equilibrium
Conceptually, a planet is an orbiter whose orbit is dominated (solely) by its gravitational interaction with one or more stars in the system. Operationally, this would mean that a planet is not in orbital resonance with other planets.
Conceptually, a “nomad” is an orbiter whose orbit is strongly influenced by a planet in addition to one or more stars in the system. Operationally, this means that a nomad is in orbital resonance with a planet.
A moon (satellite) is a special case of a nomad wherein the influence of a planet is strong enough to cause it to orbit around the planet in addition to orbiting one or more stars in the system.
The choice of the word Nomad above is not random. We observe that nomadic people tend to associated with a specific region in the world (Bush people of the kalahari, Bedouins of the Middle East, Gypsies in Europe, etc.). In effect, we can think of them as “wanderers” with additional “constraint(s).” This co-exists well with the word planet being a “wanderer.”
Advantages of this classification:
1. Classifies all the large bodies in a system.
2. Avoids any arbitrary size related cut-offs.
3. Provides a smooth scale to classify bodies. It allows for moons to be bigger than nomads or planets, better matching observed phenomena.
4. Includes aspects of popular classification strategies based on size, formation, and capture.
Disadvantages:
1. Conceptually, the scheme depends on an idea of dominance. Operationally, this depends on the precision/randomness of determining orbital resonance. For instance, even though Venus and Earth appear to be in a regular dance, the variation is considered to be “random enough” to not be a resonance.
2. It classifies all major bodies, neccessitating a change to the concept of moon (satellite), i.e. what do we call an irregular small moon now?
3. It introduces another class of objects “Nomads.”
For the record, i am not particularly enamoured with resonance, but it appears to be the best operational metric for measuring the orbital influence of other bodies. Anyone have a better idea?
I wish to refer to Steve Soter’s “What is a Planet?” article in SCIENTIC AMERICAN, because he champions the “clearing the neighborhood of its orbit” criterian for defining a planet. To me, he does not seem to couple the “clearing the neighborhood in its orbital zone” idea with orbital resonance.
I quote: “So far observers have found about 20 systems with more than one planet. In most, planets have orbits that do not intersect, and in the three exceptions, the overlapping orbits appear to be resonant, allowing the planets to survive without colliding. All the known nonstellar companions of sunlike stars are massive enough to deflect nearby small bodies. They would probably qualify as planets by the criterion of dynamical dominance.”
I’m not so sure that it’s Pluto’s orbital resonance with Neptune that disqualifies this dwarf planet from full-fledged planetary status. I suspect it’s because Pluto’s mass relative to the combined mass of all the other bodies in its orbital zone is not deemed sufficient enough to give Pluto “dynamical dominance” over its orbital neighbors.
Mind you, I’m not necessarily agreeing with the “clearing the orbit” requirement for defining a planet. At this stage, I’m just trying to understand what it means.
Just to clarify, my use of orbital resonance is an outcome of trying to find a concrete metric to determine dominance, not the other way around.
My basic thesis is simply a second category of bodies influenced by the “non-star others” in their systems.
I simple terms: – Planet — Orbit dominated mainly by interaction with star(s). – Moon — Orbit influenced so strongly by a non-star body that is also orbits around it as well as the star(s). – Nomad — Orbit dominated by interaction one or more boies other than the star(s).
Here’s another depiction: a) 1 interaction dimension for a planet, b) 1.x dimensions of interaction for a nomad, and c) 2 dimensions for a moon (1 for the star, 1 for the planet/nomad).
I’m sorry, this is all WAY too complicated for me.
A planet of the Sun can be defined in ten words: a Sun-orbiting object that’s the diameter of Pluto or bigger.
According to that definition, the Sun has ten known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, and Eris.
Please, let’s keep things simple!
The simplicity of the above definition certainly has its appeal!
Regarding orbital resonances, it seems to me that a 1:1 resonance indicates the bodies share the same orbital zone. On the other hand, I would venture to guess that the 3:2 orbital resonance of Pluto and Neptune indicates these system objects travel in different orbits.
Thank you, Bruce!
Some of us have to teach this stuff to kids!
You’re certainly welcome, Mangenta!
I’d like to think we’re all children of the Earth, leaning from one another. One of my favorite sayings is this old Hindu proverb:
Nobody knows everything; nobody knows nothing.
As far as teaching goes, the relatively simple classification of planets, nomads, and satellites will work.
The notion of atoms, electrons, nucleus, orbitals, etc. are more complicated than the above, yet we manage (over time) to teach them to kids.
Applying Magenta’s size classification, we can simply pick Mercury as the standard instead of Pluto, and we’d end up with the eight planets :).
Magenta if you want a simple way to teach kids, here is how i’d put it:
1. Planet – a big round object that orbits the Sun
2. Nomad – a big round object that orbits the Sun but pushed around by another big object
3. Moon – a big round object that orbits a Planet or a Nomad.
Kids will easily pick up on the “pushed” around concept :).
Thanks, but I HUGELY prefer my simple, common-sense definition (which is hardly original with me): a planet is an object that orbits the Sun and whose diameter is Pluto’s or bigger. Roundness and “clearing the zone” have NEVER been criteria for planethood.
I think those who want a more complicated definition should get a job with the IRS, so they can complicate an already overly complicated tax code, so that we can spend yet more time filling out our taxes, so that we won’t have any time to argue about planethood! :)
The roundness criteria is an approximation for size (if a body isn’t big enough it can’t become round).
The common-sense argument doesn’t hold up in picking Pluto. If we have to pick a hard size cutoff, I’d choose Mercury over Pluto.
Michael E. Brown, Professor of Planetary Astronomy (who, along with Chad Trujillo and David Rabinowitz, discovered Eris, the largest dwarf planet), gives his thoughts on the great planetary debate at http://www.gps.caltech.edu/~mbrown/eightplanets/
Bruce, thanks for the URL to Mike Brown’s interview. It is interesting that he also likes to include moons (satellites) in the debate.
Thought Shaman, glad to hear that you enjoyed the interview!
Actually, it makes more sense to draw the line at Pluto rather than at Mercury, for the simple reason that that’s where the line has always been drawn.
Magenta wrote: “Actually, it makes more sense to draw the line at Pluto rather than at Mercury, for the simple reason that that’s where the line has always been drawn.”
Please Magenta, this is a strange argument.
We’ve known about Pluto only since 1930. That doesn’t qualify it as “always.” Ceres was classified as a planet on its discovery in the mid 1850s. In short order based on the discovery of other asteroids, Ceres was demoted.
Classification (and reclassification) is a continuous process that takes into account new information. In science, there is very little that is “always.”
No, Ceres was discovered in 1801. In 1802, a mere one year later, the word “asteroid” was coined to describe it and its ilk.
In contrast, Pluto has been classified as a planet for the last 77 years. It should stay so.
I agree that the IAU’s decision definitely was wrong, even more so since it was done in a “backroom deal” fashion with only 424 out of 10,000 members voting, most of whom were not planetary scientists. Their planet definition was created with the deliberate intent of excluding Pluto. There is no reason we cannot have a third class of planets (in addition to terrestrial and gas giant planets) that are also Kuiper Belt Objects. The most sensible choice is including both Pluto and Eris as the solar system’s ninth and tenth planets.
Thanks Magenta for correcting the date of Ceres’s discovery. The fact still remains that it was originally though of as a planet. It matters not that the correction happened within a year or two (or in Pluto’s case 77).
“In contrast, Pluto has been classified as a planet for the last 77 years. It should stay so.”
Science directed by historical sentiment or by fiat is not a good idea (Modern science does not like arbitrary boundaries either).
Either we decide that planet is just a literary/cultural term, in which case people can call any non star body as a planet (for the most part they’ll probably stick to round objects) in which case we’ll have at least 16 (just counting off the top of my head).
Or, we decide that we need to formulate a scientific definition, in which case, it is extremely hard to make a case for Pluto et. al. giving us eight planets at present.
I agree with Laurel Kornfeld: Pluto and Eris are the Sun’s ninth and tenth planets.
It’s not history but science that argues that Ceres is not a planet but Pluto is. Only one year elapsed from the discovery of Ceres to the discovery of the next asteroid. That’s history, but it reflects a scientific truth: lots of other objects like Ceres orbit the Sun between the orbits of Mars and Jupiter. Thus, it’s no surprise that little time elapsed from the discovery of the first asteroid to the discovery of the second.
In contrast, sixty-two years elapsed from the discovery of Pluto, in 1930, to the discovery of the next Edgeworth-Kuiper belt member, in 1992—even though Tombaugh kept looking for new planets until 1943. That’s history, and it also reflects a scientific truth: Pluto is a really BIG member of the Edgeworth-Kuiper belt. It took astronomers seventy-five years after its discovery to find the only known member of the Edgeworth-Kuiper belt that is larger than Pluto, Eris.
Drawing the line at Pluto’s diameter is arbitrary, but it is no more arbitrary than defining “round” or defining “clearing the zone.”
Furthermore, science is full of arbitrary definitions. As proof, look up the scientific definition of a meter.
we think pluto shood steel be a planet !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
we think pluto shood steel be a planet !!!!!
The IAU definition has rapidly taken hold all over the world and will stay, only the hastily drafted wording might be improved eventually. But the essence of the decision is highly unlikely to change: There are eight bodies in our solar system that dominate their ‘neighborhood’ and two belts full of minor objects, none of which being in a dominant role.
Pluto was just “lucky” that its apparent brightness made this one easier to find than its neighbors (though I’ve since found out that the next-brightest one was only a factor of 2.5 dimmer in the 1930’s – Tombaugh might have missed 2005 FY9 just barely). The IAU definition has brought order into the solar system as never before, and those still bickering should take a deep breath and get a life …
Daniel Fischer
science writer
Germany
Utter nonsense. Nearly as many astronomers signed the petition protesting the IAU vote as took part in the vote!
No, the IAU has not brought order into the solar system. The truth is exactly the opposite—it has created chaos with the ridiculous statement that a dwarf planet is not a planet at all (linguistically impossible) as well as a double standard since Neptune, Jupiter and even Earth can also be viewed as not clearing their orbits (Neptune of Pluto; Jupiter and Earth of asteroid fields). In fact, “clearing its orbit” was never an issue regarding planet definition until the IAU’s conference. I strongly believe this decision will not stand. It’s easy to tell people to “get a life” when they challenge your position. Things would be much more convenient for you if those of us who dissent with the IAU decision just “move on.” Well, that’s not likely to happen. Dr. Alan Stern, the author of the petition protesting the IAU decision, plans to hold a conference of 1,000 astronomers later this year to address this issue, and it almost certainly will resurface at the IAU General Assembly in 2009, where you can expect a lot of members who never previously showed up for these conferences. This debate is far from over.
Laurel Kornfeld: Exactly! Well said!
I totally agree with both Magenta and Laurel. The IAU decision was stupid because taken, without proper consultation and preparation, on a subject that has wide implications – zillions of schoolchildren and their teachers, not to mention parents, in 192 countries registered in the UN system.
The very simple solution would have been to classify Eris at the 10th planet… but why be simple when you can show your “erudition” by being complicated?
The people who are quibbling about “round” and “clearing the zone” ought to read Michael Brown’s views on the topic before drawing conclusions about the current definition’s criteria being arbitrary.
Thanks to Bruce McClure, here is the relevant information:
“Michael E. Brown, Professor of Planetary Astronomy (who, along with Chad Trujillo and David Rabinowitz, discovered Eris, the largest dwarf planet), gives his thoughts on the great planetary debate at http://www.gps.caltech.edu/~mbrown/eightplanets/”
Why is it that people continuously bring up “teaching to kids” as a justification for changing a definition in this context?
The concept of atoms, electrons, orbitals, spin numbers, etc. are more complicated than the current definition of a planet and yet we manage to teach them to our kids.
Are we forgetting Einstein’s “Make everything as simple as possible, but not simpler.” here?
Are far as picking criteria for classification, we ought to stick with classifiers that are characteristics of the domain of interest. Size (approximated by roundness), dominance (approximated by ‘clearing the zone’), etc. are concepts within the domain of astronomy.
Pluto as a planet was simply an accident of our record. There is nothing special about the size of Pluto within the domain of interest. It is as unremarkable as the size of Varuna or Quaoar, or 2005 FY9. As such, it does not make a good classifier.
On the other hand, picking the unremarkable value of the size of Pluto as a classifier is detrimental to educating kids in the scientific process. The only way to justify this is along the lines of “Because I/we said so…” This is Science by fiat and really harms the process of enquiry.
I respect the discoveries Mike Brown and his colleagues have made. However, the fact is, he’s gone back and forth on this issue. Before the discovery of Eris, he said Pluto was not a planet. Then, after the discovery of Eris, he said it was a planet. Now, after the unfortunate IAU vote, he says it is not a planet. So, with all due apologies to a certain unsuccessful presidential candidate, he was for Pluto before he was against it.
Setting Pluto’s diameter as the defining property of a planet is indeed arbitrary. But so is setting a certain wind speed for the defining property of a hurricane. Furthermore, roundness and “clearing the zone” require two more arbitrary lines to be drawn. How round is round? If the equatorial axis is 5 percent greater than the polar diameter, is the object still round? What if it’s 10 percent? 20 percent? Where do you draw the line? Wherever you draw the line, it’s arbitrary.
Likewise, where do you draw the line concerning “clearing the zone”? If an object has cleared out 1 percent, 10 percent, 50 percent, 90 percent, 99 percent of its mass? Wherever you draw the line, it’s arbitrary.
There is a very good reason to draw the line at Pluto’s diameter: Pluto is a very large object in the Edgeworth-Kuiper belt. It is the tenth largest object that orbits the Sun. Varuna, Quaoar, 2005 FY9, Sedna, etc., are all smaller, with diameters typically two-thirds that of Pluto. Drawing the line at Pluto thus maintains a fairly high threshold for any aspiring planet to cross. Thus, this definition preserves the current connotation that a planet is an important object. (In all fairness, so does the IAU proposal.) However, conferring planethood onto all sorts of sub-Pluto objects—which has been derided as the “no iceball left behind” policy —dilutes the significance of what “planet” means. That’s why so many astronomers were irate over the proposal by the Gingerich committee. That proposal was so extreme that it provoked an extreme backlash: the demotion of Pluto and Eris. Had the Gingerich committee instead come up with a moderate, sensible proposal—drawing the line at Pluto—it likely would have been accepted.
Magenta makes many excellent points. There are many scientific classifications that are actually arbitrary. Furthermore, the requirement of an object clearing its orbit sets up a ridiculous situation in which objects are defined by where they are rather than by what they are. Objects further from the sun are far less likely to “clear their orbits” unless they are gas giants (even those may not meet this qualification—Neptune does not clear its orbit of Pluto). So an Earth-sized object under this definition would be considered a planet in Earth’s current orbit but not a planet if it orbited beyond Neptune. That definition is very arbitrary and chaotic. Also, it is noteworthy that David Rabinowitz, co-discoverer of Eris along with Mike Brown and Chad Trujillo, disagrees with Brown’s position on Pluto. Rabinowitz is one of several hundred planetary scientists who signed Stern’s petition against the IAU definition. So even the discoverers of Eris themselves are not in agreement on this.
Mike Brown’s emphasis on concepts is important, not whether he waffled on what a planet is. Besides, he acknowledges his bias in the article.
The concepts are size and dominance. The “roundness” is simply a symptom of size. For instance, 2003 EL1 is egg-shaped, but there is no doubt that hydrostatic forces are a dominant influence on its shape. For hydrostatic forces to come into play, an object must be big enough. In fact, another astronomer (Basri) proposed that this be the lower limit for a planet a few years ago, which would be about 435 miles (or is it Km?).
As Magenta points out, it is reasonable to desire some distinguishing characteristic between the heavyweights and the also-rans. This is where dominance comes into play. I will refer the reader to my earlier posts (#s 29, 31) for some of my thoughts on dominance.
Magenta’s threshold criteria applies equally well in picking Mercury, or 2005 FY9, or Varuna as the cut-off.
I agree that the IAUs “clearing the neighborhood” is not very well worded. All it underscores is that we need a better way to express the concept of dominance in operational terms.
I am perfectly fine with a definition that allows a non-planet to be bigger than a planet. Ganymede (of the Jovian system) and Titan (Saturn’s moon) are both bigger than the planet Mercury and I see no serious proposal to classify them planets. Heck, Titan even has a permanent significant atmosphere and Ganymede actually has its own magnetic field.
As far as arbitrary classifications go, in science whenever a better classification scheme comes along it tends to supplant one that has arbitrary formulations.
In the particular example of the Tropical Depression-Tropical Storm-Hurricane continuum, I haven’t heard of a proposal that is less arbitrary, which is funny because the Meteorologists themselves think that even a Tropical storm is significant enough to name.
In the context of a planet, the proposal jettisoning Pluto was less arbitrary than the previous status-quo.
Neptune so strongly influences the orbits of Pluto, Eris and their brethren that they look like its lost children. Similarly, planetary theorists credit Jupiter’s influence for preventing the formation of a planet where the asteroid belt currently lies.
I have no objection if someone can come up with a reasonable proposal that will allow us to designate Titan and Ganymede as planets, or include Pluto et al., in which case the classifier’s core concepts will have to change from the current basis of size and dominance.
However, insofar as dominance is still a core concept (which it obviously is, otherwise we would be talking of Titan and Ganymede too) it is logical to demote Pluto, Eris, et al.
The problem is, I object to both roundness and “clearing the zone” as defining characteristics of planets. Both lead to absurdities.
Historically, Uranus, Neptune, and Pluto were considered planets because they were a) large and b) orbited the Sun. These are the ONLY two criteria that should be used to define a planet of the Sun.
Here are three examples of the absurdities that result when employing the “clearing the zone” concept. The first of these has already been discussed by Laurel Kornfeld:
1. Imagine a planet with Mercury’s mass, density, and diameter—in other words, a perfect twin of Mercury, which everyone agrees is a planet—orbiting the Sun in the Edgeworth-Kuiper belt, or in some more distant belt of objects. Under the IAU proposal, the real Mercury would be considered a planet, while its twin would NOT. That is absurd.
2. Imagine, as some science fiction writers have, that there is another Earth orbiting the Sun on the other side of the Sun. This anti-Earth has exactly the same mass, diameter, and density as the real Earth, and it goes around the Sun once a year, just as the real Earth does. Under the IAU proposal, neither Earth nor this anti-Earth would be considered planets—because neither has “cleared its zone” of the other! Absurd!
3. Imagine that in 2010 a planet with the same mass, diameter, and density as Earth is discovered 200 AU—200 Earth-Sun distances—from the Sun. Is this new Earth a planet? Of course, you say. But wait. Until astronomers determine whether or not this object has “cleared its zone,” this object cannot be classified as a planet or a nonplanet. Instead, it will linger in limbo, probably for decades, because detecting smaller bodies at such an enormous distance from the Sun will require new technology.
In contrast, under the common-sense planet definition (planet of the Sun = Sun-orbiting body that’s Pluto-sized or bigger), in each of these three scenarios, the hypothetical object would quickly be determined to be a planet, once its diameter is measured and found to be larger than Pluto’s.
The roundness concept leads to similar absurdities.
Magenta the current classification uses size and dominance as its basis.
1. IAU’s current definition of a planet makes no mention of “roundness.” The property is “hydrostatic equilibrium.” As I pointed out in previous posts this provides a built-in size cutoff. Repeat – the property is not roundness, it is hydrostatic equilibrium.
In simple terms this means that a body will tend towards a spherical shape if it is large enough (think of it as a spheroid).
2. The Earth-AntiEarth in the stable configuration you describe is impossible to form via natural processes.
3. If an Earth-sized body is found at a later time, whether or not it is a planet depends on the absence of another big object “dominating” its orbit. This is the same logic as to why Titan and Ganymede are considered satellites (moons) and not planets. If Titan or Ganymede were in the presumed empty space by themselves past the Edgeworth-Kuiper belt they would qualify as planets (provided there isn’t another body pushing them around).
The wording of “clearing the neighborhood” is silly, but the concept of dominance is sound. Astronomers need to work on a better formulation for dominance.
4. If you would like to see Pluto back as a planet you would have to propose/support a framework which excludes dominance as a basis for the classification. In such a case, you would have to consider the possibilty that the number of planets could easily top 25 (8 classical planets, 7 moons that are bigger than Pluto, The big EK objects and asteroids(Eris, Pluto, Charon, Sedna, Quaoar, Varuna, Lxion, 2003 EL1, 2005 FY9, Ceres, etc.). Further, the framework would have to alleviate, as you put it, the “no iceball left” issue.
5. A proposal based on an arbitrary or historical size cutoff is likely not going to be acceptable, simply because that is exactly what people are trying to correct.
I don’t accept dominance or hydrostatic equilibrium as criteria for planethood. They have never been criteria for planethood. I see no similarity between Titan, which orbits Saturn, and Pluto, which orbits the Sun. The former is a moon; the latter is a planet.
I, and many others, have already proposed a definition that excludes dominance: a Sun-orbiting object whose mean diameter equals or exceeds Pluto’s.
What is so hard about that?
And how do you know the Earth-anti-Earth scenario is impossible? There are moons of Saturn that share the same orbit around Saturn.
You can read about the orbital relationship between Saturn’s moons – Janus and Epimetheus – at http://www.answers.com/topic/epimetheus-moon. Epimetheus means hindsight in Greek, and in hindsight, it looks like the phrase “clearing the orbital zone” opened a big can of worms!
In reading all of these most interesting comments, I am struck by the fact that we have two groups of well-informed, intelligent people who are talking right past each other, neither quite able to grasp where the other is coming from.
On the one side are the traditionalist scientists, who believe that Pluto (and by extension, Eris) should be labeled as “planets” because that’s the way we’ve always done it, so why change now? After all, doing it this way gives us a nice, tidy list, and it is very easy even for novices to understand. If you’re an enthusiast, you’ll soon learn about asteroids, comets and Kuiper Belt Objects, and you’ll begin to grasp the more subtle nuances of our solar system. But the layman won’t really care about all this.
On the other side are the rigid scientists, who want everything completely quantified, with precise definitions and cut-offs, because that’s how they do their work.
As a computer programmer myself, with a four-year degree in mathematics, I can understand wanting rigorous taxonomic precision, but I also have to call myself a traditionalist in that I doubt I’ll ever stop thinking of Pluto and Eris as planets no matter what the IAU decrees. I have a traditional nine-planet orrery on my filing cabinet, and the only way I’ll ever consider replacing it is if “The Noble Collection” comes out with a ten-planet version.
Unfortunately, the universe itself does not always oblige us with having everything neatly pigeon-holed. The data release program I manage at work tends to crash at unpredictable intervals because some unexpected data configuration comes along that it wasn’t set up to handle gracefully. In astronomy, even “hydrostatic equilibrium” is somewhat fuzzy; I recall seeing a chart, perhaps on Wikipedia, listing the largest asteroids in descending order of size. As you work your way down the list from Ceres, officially recognized as a “dwarf planet”, you come to a point where the mathematically “round” objects are interspersed with ones visibly irregular. Where, then, do you draw the line between “small solar system object” and “dwarf planet”?
As for orbital dominance, Magenta points out a certain arbitrariness in determining this, though I’ve read that it wouldn’t much matter to the IAU’s current definition of “planet” if we made the mass ratio cutoff a factor of 10, 100 or 1000.
Ultimately, I don’t think the two camps are ever going to be completely in agreement over the definition of “planet”, because they have two different world views. I’ve resigned myself to watching the astronomical community bitterly wrangle over the matter for years to come, while I’ll have to content myself, for the most part, with sitting on the sidelines. After all, I’m only an enthusiast.
I still think the only workable solution to this conundrum is to agree to disagree: let’s define a cultural/historical “Traditional Planet” category for public consumption, in parallel with the more rigorous scientific definition. But I fear I’m very much a voice in the wilderness. So be it.
Thank you, Deborah Byrd, for giving us this forum for discussion. It has been most illuminating.
Thank YOU, Bob, and thanks to Magenta, Thought Shaman, Laurel and others who have made this discussion so interesting.
I agree that different world views will make the Pluto controversy hard to resolve!
Bob you are quite right in that there are two camps.
However, it is not the case that they are talking across each other. One camp has a typical case of what Stephen Colbert terms “truthiness.” They want to hold on to their positions regardless of what the facts are.
I disagree with your implication that it is the traditionalists that are comfortable with ambiguity. In fact, it is those who are willing to consider Pluto et al., to not be planets, who are attempting to look at the facts of the domain, and are comfortable with a not so precise “formulation” for classification (insofar as the “principles” are sound) who are being open.
It is sad that some people do not seem to understand their own principles for classification.
I have pointed out that hydrostatic equilibrium is simply a formulation for an object being sufficiently big, yet people keep complaining about it. The size criterion is a carry over from the traditional way of thinking.
The other point of dominance is also a carry over from the traditional way of thinking. The reason Titan and Ganymede are not considered planets is because Saturn and Jupiter respectively so dominate their orbits that they have captured the two smaller bodies and forced them into orbit around themselves.
Note that “orbits” is a transitive property. If Y orbits X, and if Z orbits Y, Z also orbits X.
Location obiviously matters regardless of size in traditional classification. If Titan and Ganymede were elsewhere in space they would likely be planets even in the traditional view.
Therefore, size and dominance are the two traditional criteria that form the basis for the classification of a planet.
All I ask is that people apply the classifier to the entire domain consistently.
It is inconsistent to reduce the domain to only those bodies that directly orbit the star(s) and then claim there is no concept of dominance. Especially since the concept of dominance was applied to reduce the domain by eliminating the satellites (moons) in the first place.
As far as the view of Pluto’s size being proposed as the cut-off, it is simply classifying by fiat, and is not good science. As I mentioned in a previous post, in science a better classification tends to supplant an earlier one with arbitrary formulations. Further, if we are using this view then we can equally well pick Mercury, or the much smaller Varuna.
The historical argument for Pluto being the cut-off doesn’t hold either simply because it is the historical “inaccuracy” of Pluto that is point people are trying to correct.
I agree with Bob that it may be possible to have two definitions, one colloquial and one scientific. However, this could be a little strange as we’d have to teach them both to the populace. I can imagine a quiz question on the number of planets being answerable both ways causing some mayhem.
The other approach is to simply designate that a planet is a cultural body of interest (from a scientific perspective, the absence of definition of planet will not affect the process of enquiry). In this case, we can simply vote as to which bodies we consider to be planets periodically, say every 5 years or so. Just imagine the fun people can have gathering votes for Pluto or poor little Varuna :).
Bob, I don’t think one has to be an astronomer or a member of the IAU to present views and thoughts. I thank you for yours and thanks to Magenta, Bruce, and others as well.
Pluto isn’t a planet? Oh NO! Somebody get the wambulance! Next thing you now, the Salt Lake will be re-classified as a dwarf lake! What will I tell me children?! Stupid scientists are ruining my life! And I don’t even have one yet.