I had a quick look at the defintion of Clearing the Orbit on Wikipedia[1] and it answers the question about objects like trojans, Pluto re: Neptune and Cruithne re: Earth. The controversy section doesn’t have a neutral feel it. And the IAU link is dead so, I couldn’t easily check the IAU definintion.

Having a seemingly clear term or even a catchy phrase can often be more of a curse because you end up setting false expectations and then backtracking to explain and educate. You often make your task harder.

Aside from false expectations, the other thing that happens when you try and explain this to regular people is that they aren’t going to get it. You can almost hear the sound of eyes glazing over. Frankly, the public would have been happy with two or three simple modifiers like minor, major, and giant leaving details to scientists. Actually, from a public perspective this works with Ceres. I’ve had parents and kids ask questions about asteroids get surprised when you describe Ceres because they think all asteroids are either (a) not large and round and (b) in wild orbits (okay too much Star Wars).

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[1] http://en.wikipedia.org/wiki/Cleared_the_neighbourhood

Term “clearance of orbit” is maybe unclear (heheh) and I prefer term “gravitational dominance”, but definitnion of “clearing of orbit” do NOT require clearing every little speck of dust on path of body. Yet I see some inane comments about just that, especially StevoR’s.

Especially funny is rant about Earth crashing with body that gave us moon. Someone would thought that planets was not YET formed, and all bigger bodies would be called “protoplanets”, each own with future as part of bigger body of “planet” in future (in about few tens of milions years). In other words, StevoR search for planets in bad place (protoplanetary discs) and in wrong time (young solar system in process of formation).

Next thing – trojan asteroids or Pluto crossing Neptune… these cases are reason why I prefer term “gravitational dominance”. It is just easier to explain. Pluto and trojans exists here and now ONLY because planets Jupiter and Neptun gravitationally controls them. Case: Pluto has 3:2 resonance. Orbit of Pluto and Neptune never crosses in sense that Pluto never ever will be in same place as Neptun in same time, because Neptun controls Pluto.

This is why I cannot stand “buut Pluto, buut trojans” idiocy. I repeat: clearing of orbit do NOT require eradicating every little speck of dust. Thank you for your uncooperation.

PS: oh, one more little thing. Gravitational roundness IS arbitrary. Maybe less arbitary than hardcoded size, but still. Why? Because it depends on composition of body in question. Small icy bodies will faster be round than rocky. And what composition of body have to status of planet?

The argument that we can’t have too many planets because children will be unable to memorize them is ridiculous. Memorization is not as important as is understanding concepts such as gas giant, terrestrial planet, etc. The solar system was not designed for our convenience. If it has 200 planets, then that is what it has. Arguing for eight out of convenience is like arguing we should limit the Periodic Table of the Elements because there are too many to learn. Having over 100 planets does not “degrade” the term or concept of planet any more than having billions of stars degrades the term star or having billions of galaxies degrades the term galaxy. As for the statement that we will never visit these objects–well, we already are with New Horizons and likely will do so with additional missions in the future.

The case of Ceres’ demotion is actually a good parallel because that demotion was equally wrong. However, in the mid 19th century, no astronomers knew that unlike the other objects in the asteroid belt, Ceres is in fact round, in hydrostatic equilibrium. This was first discovered in the 1990s. Now that we have this knowledge the appropriate action is to designate Ceres, Pluto, Eris and the round KBOs as planets. They can be subclassified as dwarf planets, but to state, as the IAU did, that dwarf planets are not planets at all makes no linguistic sense. In contrast, astronomy still recognizes dwarf stars as stars and dwarf galaxies as galaxies.

Dan Fischer, you are just plain wrong in stating that the opposition to Pluto’s demotion comes from a “small minority” of astronomers. Yes, Mike Brown supports the IAU definition, but 300 professional astronomers rejected it in a public petition led by Stern. Attempts to discredit them and Stern with statements such as “they never discovered a planet” are nothing more than cheap shot ad hominem attacks. These are the type of tactics used by those who cannot respond to the issues and instead attack the people making the arguments. Stern, a respected scientist, does not stand to lose anything personally, as New Horizons is already launched, and its findings will very likely receive worldwide attention regardless of what Pluto is called. However, as a planetary scientist whose specialty is Pluto and the Kuiper Belt, he is in a far better position to analyze these objects than astronomers who study other phenonmena such as black holes, cosmology, quasars, etc. The same is true for the many other planetary scientists who are not members of the IAU but whose views were ignored in the IAU’s decision making process.

And that process was undemocratic, closed, and in StevoR’s words, disgraceful. What sort of scientific organization does not allow for electronic voting? Watch the planet definition session, which is on the IAU web site, to see what was essentially theater of the absurd–last minute changes that confused even those who did vote, continued attempts to get to a vote without answering serious questions from astronomers, lack of clarity over the terms of individual resolutions, and on and on. The recent plutoid decision was even worse, with major astronomers including Mike Brown never even informed that this was in the works.

This issue could largely be resolved with acceptance by the IAU that dwarf planets constitute a subclass of planets.

I intend to be at the Great Planet Debate and will blog from there on my web site at http://laurele.livejournal.com . This event is open to the public, and I encourage all who are interested to consider attending.

That’s because of the popular conception of “planetoid” to mean “little planet” when etymologically it means “planet-like” (just like “humanoid” means “human-like.” By going from etymologically broadest to finest, we can have that varying level of specificity that everyone’s arguing about.

I personally like gravitational roundness because 1) it’s non-arbitrary, like a radius limit would be, and 2) it fits the larger conception. When people think “planet,” they think of round things. An irregular object is thus not a planet.

When it comes to “well, why is Titan a moon and not a planet,” let’s go back to “moon.” “Moons” as a concept only exist because of Luna, which happens to be one of those planet-like natural satellites. From the scheme I posted earlier, you could call such things “satellite planetoids” (recognizing their self-sphericality), you could reserve “moon” for self-spherical objects orbiting planets and demote all irregular moons to “natural satellites” (which is sort of implied by my system), or any other number of systems. It doesn’t really matter so long as it’s standardized and the physical definitions have a bare minimum of arbitrariness.

It’s actually quite amusing, given how adjectival modifiers have a lot of precedent in planetary astronomy. We have inner and outer planets, divided by the Asteroid Belt. We have rocky planets, gas giant planets, terrestrial planets, ice planets, dead planets, magnetic planets, and so on and so forth. Yes, this prevents ‘neat’ single-word definitions, but when one has a flexible language and a broad audience with varying expectations, we may as well try to fit the two together rather than artificially forcing the language to be inflexible and the audience to be one-size-fits-all.

Think about aircraft. Airplanes are fixed-wing aircraft, helicopters are rotary-wing aircraft. You can also have jet aircraft, propeller aircraft, cargo aircraft, cargo airplanes, cargo helicopters, passenger airplanes, utility helicopters, so on and so forth. Where things need to be strictly defined (fixed-wing aircraft, zeppelins, blimps) they are; where things don’t need to be (fighter jets, cargo aircraft) they aren’t. Very little confusion results. Take the same philosophy and apply it to astronomy.

I’d certainly call Titan a world but the fact that it orbits Saturn to me makes it clearly a moon. The distinction seems useful because moons are strongly affected by their primary planet ie. the major Maria or lunar seas are all on the side of our Moon facing the Earth, the sulphur volcanism on Io and cyrovulcanism on Triton are driven by gravitational effects caused by their planets – Jove and Neptune respectively and so forth. Titan’s a cool place (in more ways than one ) but I wouldn’t call it a planet – even though it is bigger than Mercury and nearly as big as Mars! The planet / moon – directly orbiting its Star versus circling a planet is one distinction I do think categorically works.

Annoyingly too, this is really scraping the left-hand side of my computer screen ..

Sorry Dr Phil Plait but I really prefered the old site.

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Anyway a couple of minor corrections here since I can’t edit. Still. Sigh. :

Hot Jupiters : 51 Pegasi b, Mu Arae b, Tau Bootis b, etc ..

Hot Neptunes / Gliese 581 b

Hot Ice planets : Gliese 436 b

Super-Earth’s (or more likely super-Venus’es!) : 7. OGLE-05-390 L b or “Hoth”, possibly Gliese 581 c, Gliese 876 d

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Gliese 581c :, Gliese 581c orbits near the stars habitable zone in 13 days, could be rocky and is among lowest mass exoplanets yet found – 5 earth masses. The star also has a 15 earth-mass Hot Neptune orbiting in 5 days (Gl581-b) & an outer 8 earth-mass exoplanet orbiting in 84 days. (Gl-581-d)
Despite considerable initial hype, later studies suggest Gliese 581c is more likely to be a hostile Neptune-Venus cross than anything resembling an “earth-like” habitable planet.

Gliese 876 : Gliese 876 located 15 ly off is the first red dwarf discovered to have planets, boasting 3 worlds : a pair of Hot Jupiters in a protective 2 :1 resonance pattern and an inner 7 Earth-mass “luciferean” exoplanet. (Gl-876 d)

OGLE-05-390 L b or “Hoth” : A small and – quite probably – rocky 5-earth mass exoplanet that was discovered by microlensing around a red dwarf 21,000 ly distant – the furthest known exoplanet. It’s “year” is our decade! (10 year orbit.)

Preview, pretty please! Pray tell if Discover can afford one simple button? It could even be *orange*. [Shudders.]

notably Pluto expert Alan S. Stern, head of the New Horizons mission

Seems to me a referent who stands to loose kudos wouldn’t be a good one. (“And now, finally, New Horizons reaches our last unexplored planet…, ehrm, excuse me, one of the very many plutoids that litter our backyard.” Yes, I can see how that went down well among the politicians who opened the purse.)

@ Mithril:

if Earth had cleared it’s orbit, the dinosaurs would still be alive.

I think you need to look on the definition of “clearing orbit again”. AFAIU neither of your two claims bear against it.

(The result is ~ 26 different species definitions – though AFAIU one can make a case that two will suffice, if one wants to keep customary species besides something that suits asexual populations.)

I think the ‘clearing orbit’ aspect of the definition is really interesting: I see it as an assertion by dynamicists that we won’t ever find something worthy of the name of planet that wouldn’t fit the definition. In other words, some of the standard ‘what-if’ questions that people ask to challenge the definition describe situations that are unstable dynamically and so don’t last on typical solar system timescales (billion years +).

I find it interesting and beautiful too. Albeit I don’t understand why a rather arbitrary dynamical criteria is chosen. AFAIU solar systems aren’t dynamically stable over their lifetimes (planets rotation axis precess, it tips, planets can be disturbed, et cetera) so the most relevant could be to satisfy human lifetimes. It is a man made category after all.

The other option is to allow even shorter update times, say allow all those expected trans-neptunians. After a while, who cares? It’s all going into a database onto the web anyway. This is what biologists do with all those genes they discover in addition to having different names in different species.

And it must be awfully difficult to use such a dynamical criteria on exoplanets. The “stable” criteria would look at our planets from a neighboring star, so comparing over the board, pretending some future highest economical/practical resolution on optical detection. (Famous last words: “640 k will be enough.”) Dunno if Mercury would be a planet then.

Why this exception is justified? Is “planet” an astrophysical concept or an orbital one? From an astrophysical or planetological point of view Titan is a planet, and so it is truly treated by scientists from Cassini-Huygens probes.

“By that criteria an Earth or Neptune or Jupiter discovered out in the Oort cloud wouldn’t be a planet based solely on its location. I mean come on this is just too silly for words!”

I’llalso note that Pluto dominate sitslocal regionof space boats three moons -Charon, Hydra & Nix, has an atmosphere and in every way other than the nonsensical ‘orbital clearance joke-of-a-criteria’ is a planet.

A better definition exists that restores Pluto to full planetary status plus Ceres,Eris and its ice dwarf planet kin. This definition is even very much like the IAU version but without the confusing, ill-considered and ill-defined third criterion based on orbital clearing.

Under this better definition a ‘planet’ is a natural object that is non-nuclear fusing (& never-nuclear fusing) directly orbiting its primary star (or for ‘double planets’ a barycentre that directly orbits that primary) and has sufficient gravity to compel a spheroical or nearly spherical shape. (As Earth is actually an oblate spheroid, Jupiter etc ..are somewhat flattened by their rapid rotation.)

That’s clear, simple, straight-forward and effective.

That broad definition is then usefully sub-divided into a spectrum of planetary types from ice dwarfs and the very largest asteroids like Pluto and Ceres at one end through Earth-like worlds and Gliese 581c super-Earth’s up to Neptune-like Ice Giants, Jupiter-like gas giants and finally superjovians like the largest exoplanets.

Sub-categories then include :

Terestrial or rocky planets : Mercury, Venus, Mars, Earth

Jovian planets : Jupiter, Saturn, Ouranos, Neptune

Ice dwarf planets luto, Eris, Sedna, Quaoar

Asteroidal planets : Ceres, Juno, Pallas, Vesta

Hot Jupiter’s : 51 Pegasi b, Mu Arae b, tauBootis b, etc ..

Hot Neptune’s / Gliese 581 b

Hot Ice planets :Gliese 436 b

Super-Earth’s (or more likely super-Venus’es!) : Gliese 581 b, Gliese 876 d

Pulsar Planets (those orbiting pulsars – PSR B 1257+12 b, c,d,e & the “Methuselah” or “Genesis” planet orbiting PSR B 1620-26 in M4

& so forth..

A planet’s a planet wherever it orbits! (With the sole exception of orbiting another planet in which case its a moon ie. Titan, Ganymede, Europa, Luna!)

I agree with Stern’s conclusion.

The IAU had already come up with a good definition that included Pluto along with Eris and Ceres. I think that first proposal should have been the official definition not the later one with its ridiculous and unscientific “orbital clearance” criteria which was deliberately selected for no good reason other than to exclude Pluto.

Plus the IAU’s definition meant a silly shift of the term “classical planets” from the useful one of the originally known unaided eye visible planets – Mercury, Venus, Mars, Jupiter & Saturn to the IAU’s “Classical eight” a term with little use or meaning besides excluding legitimate planets like Pluto and Eris.

In so doing so the IAU disgraced themselves and brought astronomy into public disrepute. It is a decision that cries out for reversal. Note the inconsistencies and illogical fallacies easily demonstrated by a ‘reductio ad absurdum’ argument. Note that “orbital clearance” is an utter nonsense term – vague, unclear and a matter of interpretation. Pluto &Neptune cross orbits, Jupiter has Trojan asteroids in its path, all planets have comets and asteroids cross their path and by this definition, planets cannot collide or be double planets -by “definition.” A strict adherence to this criterion would mean NO planets exist in our solar system!

Among the ludicrous results of the stupid “orbital clearance” criteria; our Earth was NOT a planet when the Mars-sized object that created our Moon was on collision course with Earth but was one immediately afterwards. By that criteria an Earth or Neptune or Jupiter discovered our in the Oort cloud wouldn’t be a planet based solely on its location. I mean come on this is just too silly for words!

Note that the smallest of the pulsar planets (PSR B 1257+12 e) is only 1/5th of Pluto’s mass yet still counts as an exoplanet.

Note that if a dwarf star is still counted as a star why should a dwarf planet be counted as less a planet?

So Pluto is_ a planet. Period. So is Eris. Any definition that denies this has consequences that render it an absurdity. The sooner the IAU comes to its senses and accepts that reality the better. The longer they leave this correction, the more of a laughing stock they appear & the more harm they do to astronomy’s reputation among the public.

Now getting back to life …

And man – ditch the old “they voted while everyone was out of town” bit. That’s just sour grapes and is not an entirely accurate description of how the IAU votes.

For example, let’s say that something exactly like Vesta is found in a debris free orbit around the sun. Even though this object is in a clear orbit, it is not round so this object is not a planet. Ok, no problems here.

Now let’s say that another object is found, this one exactly like Mimas. It too, is in a debris free orbit around the sun. This object is (unless I’ve missed something) a planet. It has a clear orbit around a sun and is round, fulfilling all the planet requirements. However, it is smaller, and less massive then the Vesta like object I was talking about earlier.

Now, maybe its just me, but having a planet thats smaller and less massive then a planetoid seems wrong.