Join the planet debate

Posted: Tuesday, August 12, 2008 10:45 AM by Alan Boyle

NASA / ESA / SwRI / U. of Md.

The way some scientists see it, the asteroid Ceres (on the left) would be a planet
while the asteroid Vesta (on the right) would not. The difference? Roundness.

Does Pluto deserve a place among our solar system's main planets, or were astronomers right to demote it to second-class status? Two years ago, poor Pluto's plight touched off the dispute over the how you define a planet, but now it's about much more than one little icy world. The Great Planet Debate rises to a whole new level this week, and thanks to the Internet, you can join in the debate yourself.

The long-awaited debate takes center stage at Johns Hopkins University's Applied Physics Laboratory, which is the base of science operations for NASA's New Horizons mission to Pluto and other denizens of the solar system's outer regions.

The main event comes at 4:30 p.m. ET Thursday, when Mark Sykes of the Arizona-based Planetary Science Institute faces off against Neil deGrasse Tyson of New York's Hayden Planetarium. You can register to watch the Sykes-Tyson debate as well as other "Great Planet Debate" presentations via streaming video. The event's organizers also hope to pass along some questions for the debaters from the online audience.

Pro-Pluto? Anti-Pluto?
Tyson is often typecast as a "Pluto-hater," while Sykes is characterized as a "Pluto-hugger." However, their real views about planets are more complex - and Sykes said that the greatest benefit of the Great Planet Debate may be the opportunity to show the general public how scientists deal with such complex disagreements. It's a process that is applied to other controversies as well, ranging from climate change to particle physics.

"People get to see that there really is this process that goes on, and the process doesn't really result in a winner or a loser," Sykes told me this week. Instead, he said scientists and the general public should ideally "gain an understanding of why we think the way we do."

In Sykes' view, that is 180 degrees opposite to the process that resulted in Pluto being dubbed a dwarf planet by majority vote at the International Astronomical Union's general meeting in 2006. Sykes said scientific questions should be decided by discussion and the data, not by taking a vote. And for that reason, you shouldn't expect scorekeepers to declare a winner after Thursday's matchup.

"Even though I think I would win, I wouldn't let there be a vote," Sykes said.

12 or more planets?
So where does Sykes stand? He would get rid of the IAU's idea that a true planet would have to "clear the neighborhood around its orbit," and instead go with this seemingly simple definition: "A planet is a round object (in hydrostatic equilibrium) orbiting a star."

Why is being round a big deal? "Roundness is just an indicator that this object has undergone evolution, and that it will exhibit geological processes," Sykes explained.

Objects big enough to be gravitationally compressed into a roundish shape will usually have differentiated layers in their interiors, and could exhibit other features such as volcanism or an atmosphere. Pluto, for example, is thought to have a thin atmosphere - and its largest moon, Charon, might have ice volcanoes.

Syke's definition would put Pluto back on the list of planets that existed before the IAU's decision, but it would also add the asteroid Ceres and the recently discovered ice world Eris (which is thought to be bigger than Pluto). It would even add Charon to the list, because the two worlds trace orbits around each other even as they both orbit the sun.

Such a lineup was initially proposed to the IAU by a panel of experts but never saw the light of day - in part because of that Pluto-Charon issue. Sykes, however, didn't see that as a problem. "Why can't we have double planets?" he asked. "That's actually pretty cool."

The definition raises other tricky questions. For example, what about the not-quite-round asteroid Vesta, which is due to be studied up close along with Ceres during NASA's Dawn mission? In Sykes' view, Vesta might well have been a planet billions of years ago - but lost that status after a cosmic collision gouged a huge crater in the rock, ruining its roundness.

"It was a planet, but then it evolved," Sykes said.

Still more planets could be added to the 12 as astronomers take a closer look at the edges of our solar system. And a wealth of worlds beyond the solar system would fit the definition as well.

For more of Sykes' perspective, check out his recent article in the journal Science.

Are 'planets' passé?
Tyson is a little cagier about his strategy for Thursday's debate: "I have no platform, so what I will end up saying will depend largely on what Mark Sykes says," he told me in an e-mail.

However, Tyson pointed to an article he wrote last year for the American Astronomical Society's Spark newsletter, titled "Pluto's Requiem," as an indication of where he would land - "if I were to land anywhere," he added.

In that article, Tyson says the focus on defining the word planet to the satisfaction of scientists and students has held "an irrational sway over our hearts and minds." It would be better to group celestial objects in multiple ways - for instance, studying the cyclones of Earth and Jupiter, or weighing the prospects for life on Europa and Enceladus, or comparing ring systems, or magnetic fields, or orbital characteristics.

"These classifications say much more about an object's identity than whether its self-gravity made it round, or whether it is the only one of its kind in the neighborhood," Tyson writes. "Why not rethink the solar system as multiple, overlapping families of objects? Then, the way you organize the properties is up to you. The fuss over Pluto doesn't have to play out as a death in the neighborhood. It could mark instead the birth of a whole new way of thinking about our cosmic backyard."

Is this a Solomonic solution to a scientific problem? Or is this just a way to talk around the planethood problem without solving it? The debate doesn't end on Thursday: Scientists and educators will be meeting into the weekend, and I have a feeling the issue won't be resolved in one meeting - just as it wasn't really resolved two years ago.

Feel free to add your own perspective on Pluto, planets and the scientific process as comments below - and then tune in for Thursday's debate.

Update for 2 a.m. ET Aug. 14: I want to apologize to all the commenters whose words sat in online limbo while I've been traveling. I underestimated the time and the trouble it would take me to get online in the midst of a California vacation. You may continue to see long lag times this week between your posting of a comment and my approval (and resulting publication) of that comment.

You'll find lots of great comments below, including observations from Alan Stern, the principal investigator for the New Horizons mission (and a principal instigator for today's debate); and from Dave Mosher, the science writer who hangs out at Discovery.com's Space Disco. (It looks as if the Space Telescope Science Institute's Ray Villard will be liveblogging the debate for Discovery.)

To answer one of the questions raised by commenters: Yes, in Sykes' view, the world that was recently named Makemake would rate as a planet in his book, bringing the current count to 13. Here's a news release that provides the details.

Will 13 planets (including Pluto and Charon, Ceres, Eris and Makemake) bring more fortune than eight (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune)? That's something worth musing over during this numerology-conscious Olympics.

Update for 2:30 a.m. ET Aug. 15: So the Great Planet Debate is finished ... or is it? Dave Mosher ended up doing the liveblogging for Discovery.com, and Ray Villard weighed in as well. Nature's Eric Hand summarized the debate for The Great Beyond.

Johns Hopkins University's Applied Physics Laboratory says the archived video of the debate will be available sometime in the next couple of weeks.

I'm still on vacation, but when I read the descriptions of the proceedings, I couldn't help thinking of a movie titled "The Englishman Who Went Up a Hill but Came Down a Mountain." The film was about villagers who tried piling more dirt on their local promontory so it could retain its mountain status. Similarly, it sometimes sounds as if the planetary pecking order is based on the volume of piled-up dirt (or gas, when we're talking about giant planets).

I'd be OK with roundness serving as some sort of threshold for the definition. Those spaceballs (even if they're as small as Ceres) would be more interesting to the fictional Captain Kirk as well as real-life planetary scientists. Astronomers are used to dealing with such size thresholds, even if they're a bit arbitrary. For instance, they're more interested in near-Earth asteroids that are more than a kilometer wide, because those are the biggest potential killers.

I also think scientists could figure out a rule of thumb to distinguish between planets and moons. Just as there are double-star systems, there could theoretically be double-planet systems - and perhaps the Pluto-Charon system will be the first on the list. However, that doesn't mean every world we consider a moon today (such as Pluto's Hydra and Nix) would have to be upgraded to planet status.

Because of Pluto's historical significance, I'd be OK with putting it back onto the list of nine "classical planets," even though astronomers will almost certainly continue to find bigger iceballs on the solar system's edge. Does that sound like a wishy-washy solution? Maybe so. But I do think this would give educators a teachable moment - that is, an opportunity to explain how scientists wrestle with the kinds of issues that came up during Thursday's debate.

Were you swayed by any of the arguments aired over the past few days? Have you changed your position on the Great Planet Debate? Or are you more certain than ever that Pluto and its ilk should (or should not) be lumped together with Earth and Jupiter? As always, feel free to weigh in with your own views below.

Update for 2:30 a.m. ET Aug. 16: Additional perspectives on the debate are in from New Scientist and Space.com, among others.

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Comments

Until the 2015 mission flyby to Pluto and beyond where we will have a much better idea of what's out there, I believe it is foolish for the international scientific community to vote on what constitutes a planet in Kaiper belt region. We just don't have all the facts yet. Since we don't typically discriminate among the objects found in the asteroid belt located between Mars and Jupiter, likewise we should be content in not classifying stuff in the Kaiper belt region until the hard facts come in! The rush to classify everything prematurely also takes the fun and mystery out of it all!

Stephen Wilson, Tempe, Arizona (Sent Wednesday, August 13, 2008 12:50 PM)

Pluto should be a planet along with Charon and Eris.
Second class astronomers don't know what a planet is even after it bites them in the butt.

Dennis Walschinski, Milwaukee, WI (Sent Wednesday, August 13, 2008 1:34 PM)

I have to agree with Tyson, basically "things Happen" tryig to "fix" subjective order with a lable is certainly nothing to argue about, other classifications would be more informative to students, such as size shape and composition, and distance from earth, speed, orbit. they should be "named" with numbers only, not names like Earth, Mars, or Pluto. [Earth would be 3]

Ray Belanger, Las Vegas, N.V. (Sent Wednesday, August 13, 2008 3:03 PM)

It seems to me that we have already classified planets in our search for them around other stars. They have all these types of planets already classified:
1. Gas Giants (similar to Jupiter and Saturn)
2. Earth Like (such as Venus and of course, Earth)
3. Dwarf Planets (such as Pluto and Ceres)
4. Rogue Planets (a planet that has escaped its orbit around a star).
In all these cases, it is still called a planet. First, let us declare what a 'Moon' is. This is an object in space who’s shape does not have to be round, but can be round, that rotates around a planetary object where the center of rotation is always within the planetary object. This is called a moon of the planetary object. Also, a moon may be large enough to have an atmosphere, and a moon of its own.
The definition of a planet is that it has enough gravity to make itself round and does NOT rotate around another planetary object where the center of rotation is always within the other planetary object. But if the center of rotation between two round planetary objects is between the two objects, or oscillates from within one and goes outside of it now and then, or if it is determined that someday the center of rotation will exit the planetary object, then they are both classified as a planet, such as Pluto and Charon. A planet does not have to rotate around a star, since rogue planets will be found.

Dan, Toronto, Ohio (Sent Wednesday, August 13, 2008 4:03 PM)

With all the problems we have here on Earth, isn't it a little silly for people to get into heated arguments about whether Pluto is a planet or not?

My favorite T-shirt this month reads, "Don't worry, Pluto; I'm not a planet either."

Loren, SF Bay Area, CA (Sent Wednesday, August 13, 2008 7:27 PM)

Oh come on already... eight's enough. We need to draw the line somewhere. Satellites/Moons, Plutoids / Dwarf Planets, Asteroids shouldn't be called planets, regardless of whatever geologic / internal processes. What comes next - comets as a new class of 'planet?' Pluto isn't a planet, it was the first outer belt (ie Kuiper Belt) object to be detected. There's possibly thousands more like it out there. They can't all be considered planets.

Sam, Oakland, CA (Sent Wednesday, August 13, 2008 8:05 PM)

In my humble opinion, a planet should be defined by the following:
It's gravity should make the object round. It's gravity should clear it's orbit of debris. It should orbit around it's star (if it orbits another object around the star then it's a moon). It's orbit should be no more than 20 degrees off the planar axis (then it's an asteriod/comet).

I do like the idea of calling Pluto/Charon a binary planet as their respective masses and proximity does cause them to orbit each other as they orbit the sun.

A simple definition like this would clear the air and let scientist get back to discovering things instead of squabbling like schoolgirls.

Kip Portland, Or (Sent Wednesday, August 13, 2008 8:41 PM)

Our current understanding is that Planets and Star Systems abound in the known Universe, while discovery of our own Solar System is still a work in progress. Aided initially by the early telescopes our neighboring planets presented themselves. The fainter, more distant orbs defied detection, but clever methods yielded the newer discoveries. With more observation of our planetary system, we have tracked orbits, and measured features, including axial rotation, atmospheres, magnetism, surface pressures and temperatures and their roundness. Oh the roundness!

Can we decide what is planetary by virtue of roundness, orbital inclination or elliptical orbit, when we have not fully discovered all that surrounds our own Sun, our own Galaxy and our celestial neighbors? What if such an oddity as a Plutoid, albeit in a distant Sun’s entourage, but in more temperate zone, harbored life? Would we then have the temerity to declare a foul? I think not.

Any discussion of what is, or is not a planet, should not be limited to our current understanding of this puny corner of the Milky Way. We should aim to permit acceptance of new discoveries, while allowing duplicity if practical to not discount any reasonable explanation of the condensate formations around a distant Sun. Pragmatism sucks.

Would that Sagan could elevate the discussion. Would he lapse into ‘a Pale Purple Dot’ a study of the oddity that’s Pluto? My 10 cents: decide what’s planetary if you must, but do so in a manner which is inclusive.

ken, fort worth, tx (Sent Wednesday, August 13, 2008 10:57 PM)

I’ve been watching the discussion over Pluto with quite a bit of interest. Personally, I think Pluto should be a planet. Still, I have to agree with Tyson that "an irrational sway over our hearts and minds" exists when it comes to defining the word planet. I don’t think that’s a bad thing, however. Irrationality implies passion, dedication, and drive; I would argue that most scientists entered their chosen field of study based upon some irrationality at some point in time. Tyson’s own suggestion belies this as well, when he notes “Suppose other properties are what matter to you.”

That being said, I’m very much a fan of the scheme where a planet is a round object that orbits a star. This definition still allows for the investigation of all the individual properties that fascinate Tyson, while still being simple enough to be applied to any system imaginable. The problem with defining a planet as an object that has cleared the neighbor around its orbit is that it is not a very, well, clear definition. What is the “neighborhood?” How far does one have to go? How clear does it have to be? Round could have similar problems, but let’s be reasonable (or rational?) for a moment—if you say that a planet is round and orbits a star, most fifth graders can grasp the concept and come up with a correct answer. However, most astronomers still can’t fully answer when the neighborhood is clear, at least not without quite a bit of irrelevant discussion.

Let’s face it; we all know what a planet is, more or less. We live on one, and we have several in our system. They’re round things that orbit stars. They have several other interesting characteristics, but they’re all pretty much the same in that respect. We also are learning that there are lots of planets outside of our system…we might not know for sure that they’ve “cleared their neighborhood,” but they’re big, probably round, and very interesting objects to study.

Oh, and Pluto is a planet.

Michael Wood (Sent Thursday, August 14, 2008 12:18 AM)

That does actually raise some interesting points with orbits. Double planets seem reasonable. I guess the definition would define a double planet where the cm lies outside the bodies,and a planet-moon situation where it lies inside one.

It seems to me that the solution is to first define a Planet as one of those nine bodies which were historically known as planets.These are "The Planets" This of course includes pluto. Then admit this is not really a scientific definition,but one based on the old conventions. This is similar to the way some chemicals are named. Toluene is still known by that name,rather than methy-benzene. Similarly a new system for classifying bodies in the solar system should be developed that logically classifies objects.

Off the top of my head,there are bodies like the earth and mars,like Jupiter,and like Pluto,as well as asteroids. These are all very different things and obviously should have different designations. The problem with defining a planet is that if you try to include only some of these arbitrarily,you end up with something that does not work.

Once we have abandoned the term planets you can come up with definitions that work. (using the word planet of course is allowed) You can have earth like planets ("terramorphic planet"?) ,Jupiter like planets etc.Each can be given a nice descriptive name. Objects like Pluto seem to be quite common. While Pluto is one of "The Planets" due to its historical significance,its actually a member of a class of objects that are quite common. Classification of these have not been a problem. The real problem was trying to classify these and have Pluto not be included.

Michael Kent, Ohio (Sent Thursday, August 14, 2008 9:30 AM)

The definition of a "planet" has changed before.

In ancient days, the seven planets were the seven "wandering stars", those that did not seem to stay in a fixed spot on the celestial firmament. These did not include the Earth, but did include the Sun and the Moon, plus Mercury, Venus, Mars, Jupiter, and Saturn. The Pantheon ("all gods") temple in Rome was dedicated to the corresponding seven gods and goddesses.

"ken, fort worth, tx" used the term "orbs". I think this is a good catch-all term for celestial bodies, whether round or irregular, luminous or not, gaseous or icy or rocky.

Josh Levin, Marlton, NJ (Sent Thursday, August 14, 2008 9:32 AM)

<<<<<<<
Why should what is around an object affect how we define that object? Is the presence or absence of other ducks necessary to define what a duck is? Would a duck cease to be a duck if we moved it somewhere else?
>>>>>>>>
Is a doorknob still a doorknob if its mounted on drawer and used to open that drawer. In a sense it is. If it broke and we wanted an identical replacement,we would go look in doorknobs at the hardware store.However,in another sense,we would call it a drawer handle,because that describes whats its doing. If someone asked you ,does that drawer have a handle,your answer would be yes.

>>>>> The other element of a planet’s definition should be that it primarily orbits a star. Charon should be classified as a moon as it primarily orbits its larger neighbor.
If we allow Charon to be a planet, then wouldn’t the Moon also be a planet? The Moon’s orbit around the Sun never curves away from the Sun.

>>>>>>>

as someone else mentioned,the cm of the earth moon system is inside the earth,while the cm of the Pluto Charon system is inside neither. We have to define what it means to say A orbits B. If it means the cm of the AB system is closer to A then Charon is a moon. What about complex many body systems where the cm is moving.

Michael Kent, Ohio (Sent Thursday, August 14, 2008 9:41 AM)

Eric Smith,
Why would Earth be disqualified as not clearing it’s orbit?

Micheal Mealling,
Then we have picoplanets, maybe most shooting stars would be Planck Planets. All this does is describe mass. If that were to be the only part of the definition then we are not further classifying but instead unclassifying asteroids.

Josh Levin,
Quite excellent! The “surface” of a gas giant is indeed difficult to define. One way to define the size of a gas giant would be the radius at which gravity begins to decrease. Well away from the planet it’s gravitational pull increases as you get closer. Entering it’s atmosphere gravitational density continues to increase as you go down, much like with our planet. By the time you reach the center it has no gravitational density, or rather it all cancels each other out. At some point it’s at a maximum, with any planet, although there are more variations on solid planets owing to the structure provided by solid stuff. At the base of it, all mass is effecting all other mass. So to an extent they were right hundreds of years back, the entire universe does revolve around us. Just not as much as it revolves around our sun, and that not as much as it revolves around the local black hole, and that not as much as it revolves around our galaxy, … With increasing mass the effect is more evident, it is a popularity contest. Saying what orbits what is, to a degree, arbitrary. It’s kind of like saying when dinner is hot and when it got cold because you wouldn’t get off the computer. By midnight dinner is definitely cold, Mercury definitely orbits the Sun. Whether it a planet/moons, double planets/moons, triple planets/moons, etc., is less important, to me, than the fact that it’s a barycenter in orbit around something else. You can identify a system. All the planets could be said to orbit the moving, relative, barycenter of the whole system. In the ‘80’s(?) all the planets lined up on one side of the Sun, the barycenter would be as far from the sun’s cetner as it will ever be, at some point the planets may align so that it’s right at the sun’s center. Clearly the most dominant contributor over time is the sun.

Vanderghast,
Absolutely. Or not. It will forever have an asterisk in it’s stats. *orbits the Earth as part of it’s system.

Wayne,
I hate to think that classification as a planet or not would depend on our ability to exploit (read ruin) it. But there is some despicable truth to that. The importance of correctly classifying things for purposes of scientific study / understanding / advancement of knowledge: Is that pet you have a mammal or a bird or a plant? Screw it, doesn’t matter, who cares, what difference do those things make?

FYI:
Round here means hydrostatic equilibrium. It’s just easier to spell. If we were to find a liquid planet with a high rate of spin so that it’s long axis (at the equator) was 10x it’s short axis (pole to pole) it would still be our “round.”
Material composition then comes into play because of structure. A huge potato shaped chunk of iron has way more mass than enough moon dust that is able to pull itself into round. The round moon dust would be “round” and the iron potato would not be. Half the iron potato with enough liquid around it, less mass than the full iron potato, falls into “round.”
If we sent a robot into the asteroid belt to chisel a large asteroid into a perfect sphere and then expel all the other asteroids from that orbital neighborhood we should not call that a planet. If we sent a robot into the asteroid belt to assemble all the asteroids into one body, and that body has enough mass to pull into round, and enough mass to (be able to) clear it’s local orbit, then it’s a(n artificial) planet.
Clearing the neighborhood is reminiscent of whether or not you’re sick. All kinds of bad ju running through my system right now, but I’m pretty clear. I think essentially, if the object would capture the debris upon crossing it, or if the debris is locked by the objects gravity then it’s a no brainer to call the orbit cleared. Fast movers, while in the orbital area, that are in more elliptical orbits or in different orbital planes that cross discretely should probably not count, but then where do we draw those lines?

Tim Rommes, Washington, UT (Sent Thursday, August 14, 2008 11:09 AM)

Why the fascination with orbits? Only one commenter (AFAICT) has mentioned Rogue Planets. Since we're all throwing our personal definitions around I'll offer mine.

Planet: an object in hydrostatic equilibrium with a spherical or oblate shape that is insufficiently massive to induce nuclear fusion due to self-gravitation.

Classes of Planets:
Class 1: A planet that doesn't orbit another discrete body. AKA a "Rogue Planet"
Class 2: A planet in orbit around a star.
Class 3: A planet in orbit around another planet.
Class 3a: A planet in orbit around a Class 1 Planet.

That's pretty much it. Objects that are no longer undergoing fusion might be classified as Post Stellar Objects (White Dwarfs, Black Holes, Planetary Nebulae, etc).

Jason Bontrager, Brenham, TX (Sent Thursday, August 14, 2008 2:00 PM)

Interesting Rorschach test!

The real issue is what's the point of definitions - or rather, concepts.

We need to ask that of the concept "planet".

Does it have any scientific utility whatsoever?
(I don't know.)

...or are we confounding that with colloquial speech..

..and worse, going on from there to see scientists as just another kind of authority, and therefore expecting them to come out with a stone tablet on the matter

..and /worse/, their accepting that!

Children of a certain age span, who have ~learned~ that Pluto is a planet, need not be subjected to the emotional traumatization of being told that it is so no longer; people older than that should learn enough about epistemology to understand the above; children younger than that need never care (just as most of us don't spend time pining about the demise of the dinosaurs - and, I suggest, psychologically healthy people in the somewhat-distant future won't tear down the quality of their lives by bemoaning the extinction of the blue-spotted redbellied swamp crawler, or whatever.

P. Michael Hutchins, Carlisle, MA (Sent Thursday, August 14, 2008 5:16 PM)

What about Eris? It is considered to also be a dwarf planet. As it should be. As far as I'm concerned Triton and Enceladus would be dwarf planets as well since they seem to have similar chemical compositions, round... captured sphered or not.

[The object found to orbit between the Kuiper Belt and the Oort Cloud? The dwarf planet is the most distant object ever seen in orbit around the sun, even more distant than Sedna, the planetoid discovered almost 2 years ago. It is almost 10 billion miles from the sun and more than 3 times more distant than the next closest planet, Pluto and takes more than twice as long to orbit the sun as Pluto.]

http://www.gps.caltech.edu/~mbrown/planetlila/index.html

Amazingly cool.

Roundness doesn't always count, but it's where they came from and even if they were captured objects as Triton and Enceladus seem to be.

MHO

Marcia (Sent Friday, August 15, 2008 12:40 AM)

I think it'd be really kewl to have 13 planets and counting. The big thing that the IAU fails to understand is that "new" discoveries are exciting. Subtraction or reclassification is not exciting, it's boring. So while the IAU may be great scientists, their PR skills, well, they ain't so good.

Look, at the end of the day, we just need a basic term for these bodies with a very basic name. Think animal, vegetable, mineral.

Simple definitions:

If it creates energy through nuclear fusion and anchors a solar system, it's a star.

If it's made up of anything, orbits a star, and has hydrostatic equilibrium, it's a planet.

If it's made up of anything and orbits a planet, it's a moon.

If it's made of rock or ore, orbits a star, and doesn't have hydrostatic equilibrium, it's an asteroid.

If it's made up of ice, orbits a star, and doesn't have hydrostatic equilibrium, it's a comet.

This idea of having different classifications for planets is all fine for people who really get into this subject (like me). I have no problem with calling a planet a terrestrial planet, a gas giant, an ice giant, etc. But give the public a new discovery, not a new genus.

Last time I checked, astronomers make their living off grants and donations. Isn't it smarter to make the people with the money get excited about being a part of new discoveries than to have them to realize their money's going towards "reclassifications"?

Another Brad from St. Louis (Sent Friday, August 15, 2008 5:09 PM)

Another Brad,
Very true, but very much a political answer to a scientific problem. For the sake of the science stuff like this should be handled in house, shielded from public view so as not to distract Daddy Warbucks so he can stay focused on those new discoveries. Of course, there's not much of a house to keep this in.

Tim Rommes, Washington, UT (Sent Saturday, August 16, 2008 4:26 AM)

I really don't care what the final definition is as long as it is not specific to our solar system. The current definition means that if mars orbited abother star, it would not be defined as a planet. The whole plutiod definition only applies to our one solar system, and so is unworkable.

Dennis, Richmond VA (Sent Sunday, August 17, 2008 12:53 PM)

We already call Jupiter, Saturn, Uranus and Neptune "Gas Giants". We call the four inner planets "Rocky Planets". I suppose Pluto just got the short end of the stick!

Will Davis (Sent Monday, August 18, 2008 2:12 AM)

What's the difference if we humans choose to call Pluto a planet or a dwarf planet? It certainly doesn't make any difference to Pluto, and takes nothing away from those who discovered it and have studied it.

The term "planet" has been used for thousands of years to describe a "star" that wanders, as opposed to stars that do not. With our modern understanding of the solar system and the universe, it might be time to come up with some new terminology. Earth and Jupiter are both classified as planets because they both fit the techinical definition, but apart from both orbiting the sun, they don't have much else in common. I think it's high time that we came up with a totally new system for classifying celestial bodies within our solar system, and stop arguing about things that don't really matter.

Kirk Nichols, Traverse City, MI (Sent Monday, August 18, 2008 1:11 PM)

was there not a way to determine distance for solar? like AU, why not use the same determination for planets...example earth would be a 1.0 planet based on mass or roundness or habitablity. then you could determine that the next planet would be + on - percentage wise of whatever was chosen as the new standard.just a thought...

jerry starkey, muskogee, oklahoma (Sent Tuesday, August 19, 2008 11:51 AM)

The gravity-clearing concept should be excluded from the definition of a planet, because that concept is absolutely biased towards near-sun objects at the expense of far-sun objects.

In our own solar system, it would appear obvious that part of the reason there are a number of dwarf planets (or whatever you want to call them) outside of the Neptune/Uranus orbit pattern is because the density of mass in such an outer layer is so much lower than at the orbital distance of, say, Venus. In theory, there should be a "solid mass" in the Pluto orbital comparable to any of the other solid planet orbitals, but because the volume of space that this mass is enclosed in is SO MUCH larger than in an inner orbital, there is not enough gravity associated with the larger clumps of mass in that orbital to make all the solid mass congeal as one planet.

On the subject of planets versus moons, it would seem that the issue is one of orbits, and not just definitions. Would you consider a round moon (Titan, Triton, our Moon, etc) a planet, or a moon. Triton would be particularly interesting, because I think it would be easy to make an argument that Triton is/was a planet, or at least a dwarf planet/KBO, which got caught-up in the gravitational pull of a larger object (Neptune). Does this make it a moon, or a captured planet?

Yvan Schmidt, Placentia, CA (Sent Tuesday, August 19, 2008 4:19 PM)

"Planet" is an anachronism from the ancient Greeks that means "wanderer", and was meant to apply only to the wandering celestial bodies that are visible with an unaided eye. We need a whole new word to define another world. How about "world"? "Globe" could be a good one if the roundness thing sticks. Maybe we just need to be more organized in classification - planet, globe, world, body, could be terms sort of like genus, phylum, species, order... well, you can see I'm not a biologist!

I think Pluto never should have been a planet in terms of being a planet like the other eight - its orbit is too eccentric.

Frank, Dallas, TX (Sent Tuesday, August 19, 2008 6:29 PM)

Trying to put a group of heavenly bodies in a catagory by themselves is "trying".

Just give a name to each "object" and be done with it. There will be times when an object will not fit in the molds we design, so we have to shelve it and call it "something else" while it sits there. If everything is an object, we can get on with discovering more "Objects".

In our future endeavors, one may hear on the news or read in a paper, "We have an object named YYYY revolving around the sun XXXX at "such and such" distance in galaxy AAAA. It is ZZZZ big and may (or not) be in the position to where our type life can be supported."
Planets, comets and other Objects in space were named in the past because we were too small to see the big picture. Now we are big boys (& girls)so we will be finding more than what our minds can collate.

Delmar Fairchild, Barron, WI (Sent Tuesday, August 19, 2008 7:40 PM)

Yvan Schmidt,
Point, or actually points, well taken. Here are my non-authoritative replies.
“at the expense of far-sun objects” This is an argument against calling Michael Jordan a great basketball player because we don’t call me one. And nobody calls me one. What about another object in a different area of space that just didn’t have enough mass available to bulk up to planetary standards? Or one that had enough mass available but didn’t get it before a competing object did? They’d be planets if they had more mass available, but they didn’t. This has to be accomplishment based. Maybe at such slow orbital speeds the far out regions are in the planet forming process. Maybe in other systems with large stars that is the case also but those planets won’t have time to form. The definition of band clearing should be closely looked at, though.

IMHO the planet determination should come first. Then primary orbit. That way it’s either a planet or not, then is that planet a moon, a moon just meaning it’s in a secondary (or tertiary, …) orbit. And I don’t go for the “rouge planet” idea. Planets are objects captured in a star system, not interstellar objects. Whether it’s a planet or a moon carries less intrigue for me as whether or not the other body has a tidal influence. Our moon presents the same face to our planet all the time. A moon that spins relative to it’s host planet is a much more exciting thing to me.

Tim Rommes, Washington, UT (Sent Wednesday, August 20, 2008 8:39 AM)

Tim, the problem with the dynamical definition is that an object of the same mass and size can in one location (closer to the sun) be considered a planet and in another location not be considered a planet (eg, if it orbits in the Kuiper Belt). A good definition must take into account both where an object is and what that object is. The comparison with calling Michael Jordan a great basketball player doesn't hold, mainly because in that instance the sole criteria for comparison are the two individuals themselves, not where the two individuals (you and Jordan)are. That's in addition to the fact that the phrase "great basketball player" is itself far more subjective than the term planet. I do agree that round moons of planets should be considered secondary planets, a term often used during the 19th century to define them. In fact, when Galileo first spotted the moons of Jupiter, he referred to them as planets. The issue with a rogue planet is this: these are objects that were part of some system--either a star system or a pulsar,etc.--and then were somehow ejected from their orbits. Do they stop being planets once they are thrown out of their orbits? Again, that means defining them solely by where they are as opposed to what they are. Since at this point in time we do not have the technology to detect the composition of rogue planets due to their distance, I don't think we are in a position to make any more than tentative classifications of these objects.

Laurel Kornfeld, Highland Park, NJ (Sent Thursday, August 21, 2008 3:04 PM)

Laurel,
I think the term planet is very subjective. For instance(s): I think a planet is an object that is part of a system so, yes, we stop calling rogue planets planets. I also think that clearing their neighborhood is important. If it would be a planet in our orbit but it's a KBO then it's not finished. For me a planet is an object that only interacts with rogues, things on crossing orbits, or things in it's gravitational hold. Obviously, other people feel quite differently on both counts.

Tim Rommes, Washington, UT (Sent Friday, August 22, 2008 5:32 AM)

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