Note: this marks my 100th blog entry. I’m not one for artificial goalposts, but it surprised me that I’ve written this many. Maybe when I get to 1000 I’ll make more of a note of it. 
Also (added October 13, 2005), this entry was featured in the 19th Skeptics Circle at Time to Lean.
If something has a moon, does that make it a planet?
The quick answer is no. Actually, so is the longer answer. But the really long answer would entail the definition of a planet, and I’m not gonna get into that here. One of these days I’ll write the 5000 word essay bumping around in my head about that topic, but not now.
Suffice to say that there is no real definition of what a planet is; the list of planets is decided in a back (presumably cigar-smoke-filled) room of the International Astronomical Union (IAU). Currently, they have said we have 9 planets in the solar system, including Pluto.
However, the discovery of large bodies out past Pluto has some planet-definers alarmed. One is smaller than Pluto, but the other is definitely larger (I discuss them in more detail here and here). The larger one, with a catalog number of 2003 UB313 and the unofficial name of Xena, is currently nearly three times as far from the Sun as Pluto is. It’s an unresolved dot in even the biggest telescopes, but (as I describe in that first link above) astronomers know it is at least as big as Pluto, and possibly substantially bigger.
Realistically, it’s still kinda dinky though. A good guess to its size is 2700 kilometers across, which is smaller than our Moon (at 3400 km across). So we’re not even talking Mercury-size, here, and Mercury is the smallest planet no one argues about.
So is it a planet? That IAU group has been in discussion about this for a while now. Rumors have been around for weeks that they might add adjectives to the planets now, so Earth would be a major planet, say, and UB 313 would be a minor planet. And as I write this, minutes ago a news release says they’ve done just that:
Using this [new IAU] method, Earth and Venus would be known as terrestrial planets, Saturn and Jupiter as gas giants and Pluto as a Trans-Neptunian planet.
Personally, I think that’s a silly solution. No one is going to use adjectives when talking about Jupiter, so this won’t help at all.
Interestingly, though, the news was just released that UB313 has a moon (unofficially called Gabrielle, of course). Its size is unknown at present, but once it is observed enough to get an orbit, that will tell astronomers the mass of the parent body, Xena. So it’s a useful discovery.
But one thing that some people argue about planets is that they have moons. I’ve heard this from a few people (mostly non-astronomers), that something with a moon can be more properly called a real planet. This is specious at best; many small asteroids have moons (the other new object past Pluto, EL61, also has one), and two “real” planets– Mercury and Venus— don’t have moons. So the presence or lack thereof is only cause for further confusion.
I’ll note that the discoverer is calling it a planet, and though he dismisses the idea that having a moon makes it more of a planet, the way he mentions it makes it clear he doesn’t mind if some people think that having a moon helps.
As for me, do I think it’s a planet? I think the question is meaningless, because, again, our definition of planet is arbitrary. If those guys from the IAU had come out of their room and decided that the Sun is a planet, or a tree is a planet, or Scarlett Johansson is a planet, then that would be the official word. As you can imagine, I certainly have an opinion about that– what they did say wasn’t much better. But that’s an essay (and a long one) for another day.
October 2nd, 2005 at 9:22 pm
I’d like to read that essay.
October 2nd, 2005 at 10:14 pm
If a moon automatically equaled a planet. Wouldn’t Ida be a planet then?
October 2nd, 2005 at 10:18 pm
Me too!
Does the Bad Astronomer take questions from the peanut gallery? He does? Good!
It’s an unresolved dot in even the biggest telescopes, but (as I describe in that first link above) astronomers know it is at least as big as Pluto, and possibly substantially bigger
I’m confused–how is it that the “biggest telescopes” can see the Trapezium or the Crab Nebula or Sag A* with a good amount of detail but an object much closer is but “an unresolved dot”?
October 3rd, 2005 at 12:58 am
Henry: Because UB313 is *much much smaller* than the other objects you mentioned. Those are groups of stars, while UB313 is a small dirt-and-ice body. Big difference.
One interesting note about Mike Brown (the discoverer of UB313, EL61, Quaoar, and Sedna, among others) is that, before finding UB313, he defined “planet” as “anything which is gravitationally round and which is at least 10x as large as anything else in a similar orbit.” This would make Pluto (and Cerces, etc.) not a planet. However, after he located UB313 and found himself in the position of potentially discovering a new planet (depending on the definition of the word), he decided that “planet” should mean “anything larger than or equal to Pluto.” Not that I question his motives or anything…it’s just interesting.
And I strongly agree with Brown’s first definition of “planet.”
October 3rd, 2005 at 1:33 am
[i]I’m confused–how is it that the “biggest telescopes� can see the Trapezium or the Crab Nebula or Sag A* with a good amount of detail but an object much closer is but “an unresolved dot�?[/i]
I hope I’m not overstepping my bounds here in answering this, but it all has to do with apparent size. An object like the Crab Nebula or Sag A* takes up much more of the sky than UB313. That is, it simply looks much larger.
Imagine standing at one end of a football field and trying to see an ant on the other side. You’d be very hard pressed to do so. However, you can easily observe the Moon, even though it is about four million times more distant. It is simply because the Moon is so much bigger than an ant that you can clearly see the Moon 385,000 km away, while you you can’t see an ant that’s only about 0.1 km away.
For the math-philes in here, the equation for angular diameter is 2*arcsin(radius/(radius+altitude)) = angular diameter, assuming you’re looking at a spherical object (like a planet).
October 3rd, 2005 at 3:35 am
I hereby declare that 2003 UB313 shall be known as Planet Pagina (Uranus, Venus and Pagina)
October 3rd, 2005 at 5:18 am
I’ve thought, for a long time, that pluto should be disenfranchised as a planet. I’m quite convinced that if it had been discovered last week instead of 70 years ago we wouldnt’ call it a planet, and I don’t believe it should be called one for “historic” or “tradition” reasons.
IMO, a planet should:
1. Be gravitationally round
2. Orbit within a very few degrees of the orbital plane of the 8 major planets
3. Orbit the Sun
4. Have an orbit with low eccentricity
5. Not lose mass as it approaches the Sun on its orbit.
October 3rd, 2005 at 7:10 am
It doesn’t matter if an object is classified as a planet as long as it garners interest.
Maybe within my lifetime there will be a manned mission to Ceres.
It may never be classified as a real planet, but it may prove to be an intriguing place to visit and a manned mission may capture the public’s imagination. It probably has a stratified interior. A probe that will reach the minor planet in the year 2014 should reveal noteworthy features.
It may have concentrations of water high enough near the surface to be harvested for propellant (after electrolysis) and life support.
Send a couple of geologists and we may learn more about the formation of the solar system.
October 3rd, 2005 at 8:13 am
“IMO, a planet should:
1. Be gravitationally round
2. Orbit within a very few degrees of the orbital plane of the 8 major planets
3. Orbit the Sun
4. Have an orbit with low eccentricity
5. Not lose mass as it approaches the Sun on its orbit. ”
But how round is round? There isn’t a body known in our solar system which is doesn’t deviate from ideal sphericity. They all have some global eccentricity, and all have textured surfaces which further break their sphericity.
How many degrees may an object’s orbit deviate from the ecliptic plane? What if we discover a solar system with planets occupying orbital planes over a range of 90 degrees?
What does it mean to orbit the Sun? Every body in our solar system orbits the Sun, including the moons. Given a pair of co-orbiting bodies, it’s definitely not the case that the small one orbits the bigger one. Whatever the mass ratio of the pair, as physics is now understood they are both orbiting about their mutual mass center. For example, Earth has its own mass center, but the Moon’s orbital path isn’t centered(focused) on that point. Rather, there is a point other than Earth’s own mass center which both Earth and the Moon orbit. And given that F = GMm/r^2, the force of gravity is due to both masses in the system. Thus, the Moon orbits not just due to Earth’s mass, but also to its own. The Moon orbits partially its own mass. Additionally, the Earth/Moon system’s overall mass orbits the [Earth/Moon]/Sun mass center. So it’s not really adequate to say that a “planet” must orbit the Sun, since all gravitating bodies are orbiting each other.
How low must an orbit’s eccentricity be? Once again, suppose we discover a solar system with clearly Jupiter-like objects occupying severely eccentric orbits?
And what does it mean not to lose mass on approach to the Sun? I know this is meant to clarify the status of comets. But all the bodies in our solar system are both losing and gaining mass constantly. Earth loses mass due to the Sun all the time. Solar heating excites the atmosphere, and statistically some fraction of the upper atmospheric gas molecules scatter away at escape velocity. Similarly, Earth gains mass by way of meteors, and even by way of the Sun: Solar wind particles are trapped in the terrestrial B-field and are made part of Earth’s effective mass.
These criticisms are not of course meant to browbeat. They’re meant rather to point out how the notion of “planetness” really is an outmoded way to categorise objects. It was easy & convenient before the 20th century to refer to “the” planets; there were only a handful. But time has passed, and a much larger sampling of objects has been observed, forcing us now to further generalise how we organise what we know into useful paradigms.
A way to illustrate could be communities. We wander about the land, not knowing what we’ll find. Eventually we stumble upon a concentration, an anomalous cluster, of artificial structures with people living in them. We go onward, and in time discover several of these objects, and we name them as a class “cities”. At first it’s enough to just compile a running list of such objects. But if the list gets too large, it starts becoming meaningless to just keep piling on more names to the list. As our experience with cities accumulates, to understand them means to reduce them to their most elementary variables. (e.g., population, land area, proximity to rivers, etc.) We then phase from a naive knowledge that there are such things as cities to a deeper understanding of how they interact with their environment, with other cities, and so on. We aquire insight into what it means for there even to be cities in the world at all.
And so it is with “planets”. The living scientific community has now reached a stage at which it’s insufficient to refer to “the” planets. It’s becoming obvious that what we’re interested in is not “planetness”, but rather what it means for there to be the broad spectrum of object types which have been documented. It doesn’t really matter if some people persist in calling Earth a planet, or Neptune a planet, or Pluto a planet. All we need to know is that these objects are of interest to us as explorers, so that we can understand more about how to characterise the Universe.
October 3rd, 2005 at 8:32 am
[...] ON MOONS AND PLANETS: If something has a moon, does that make it a planet? … As for me, do I think it’s a planet? I think the question is meaningless, because, again, our definition of planet is arbitrary. If those guys from the IAU had come out of their room and decided that the Sun is a planet, or a tree is a planet, or Scarlett Johansson is a planet, then that would be the official word. As you can imagine, I certainly have an opinion about that– what they did say wasn’t much better. But that’s an essay (and a long one) for another day. [...]
October 3rd, 2005 at 9:04 am
I am surprised that the IAU does not see the inherent contradiction in defining a planet as ‘Trans-Neptunian’. They are immediately saying that it is both not enough of a planet to warrant being the defining boundary and enough of a planet to warrant being defined as one. Why not Trans-Plutan? Is there something about Pluto that makes it less of a planet than Neptune and all those others closer to the sun? (and i suspect if they answer that question honestly then the question of it being a planet would be moot)
October 3rd, 2005 at 9:09 am
How is this adjectival approach in the new release any different than what we actually do now? The adjectives terrestrial and gas giant are used a lot in 100-level astro courses at universities. This still doesn’t give us an actual definition of a planet, either. I don’t see the point.
October 3rd, 2005 at 9:11 am
Is it a planet?
Am I the only one who doesn’t care what it is called? I could ask, “Is it a glumphdrix?” and it would have no more or less significance than the question above.
The point here is that a very nifty discovery has been made. A not insignificant ball of rock, way the heck out of our expected orbital plane, and with its own sattelite to boot is a pretty cool thing. I find the semantic debate far less thrilling than the discovery itself.
Yes, it would be convenient if we had a simple, adjective-free, definition of the term ‘planet’, but lacking one, we can still be all kinds of excited about the news that 2003 UB313 has a moon and about the knowledge we can gain by studying this unexpected celestial treat.
Myself… I definitely think it’s a glumpdrix… for sure.
October 3rd, 2005 at 9:21 am
It’s round. It orbits the sun. If it’s paired with a large moon, it’s the more massive body. It’s a planet. Planet is an ancient word and modern astronomy never got around to formalizing a technical definition.
Actually, defining “trans-Neptunian planet” in detail is a dandy way to go.
October 3rd, 2005 at 9:21 am
After having listened to both arguments, both pro and con, I’m in agreement that the variables of these orbital bodies we study are far more important than any use of the “planet” terminology. The only real dividing line apparent between a planet and any other orbital body is a distinction of mass, and wherever this line is drawn it will be arbitrary, and arguments will arrise about any body near that golden value. I feel that without any such naturally apparent boundary between them, we should reserve the term planet for the original “wanderers”, Mercury, Venus, Mars, Jupiter and Saturn. The term planet way coined to refer to those rouge “stars” and I think the distinction should remain there. So there are my two coppers woth:)
October 3rd, 2005 at 9:24 am
I agree – this doesn’t answer the question at all. Is Xena a Trans-Neptunian planet like Pluto? Seems to me that makes it a “planet”.
Of course, calling it a KBO runs into the inherent question of why Pluto deserves to be a “planet” if a larger body that is very similar (presumed for now) is not a planet.
Now ponder this question – we have sent out a couple of different messages to ETs that attempt to identify and describe our Solar System. They include descriptions showing 9 planets. Clearly the data we are now collecting makes those previous descriptions awkward, if not obsolete.
Maybe in the distant future that’s what saves us from the invading aliens – they keep looking for that SS that has 9 planets.
October 3rd, 2005 at 9:26 am
I was agreeing with Tobin Dax.
October 3rd, 2005 at 9:28 am
In the great, grand scheme of things, it probably doesn’t matter what a planet is defined as.
What I would like to see is a naming convention that prevents sharing the solar system with celestial objects named Britney, Christina, Xena, Vanilla Ice, Santa, Easterbunny and so forth. I think that sticking to Greek and Roman mythology is a good call, or maybe expanding potential names to other mythologies (Chinese, and Persian since both those people have or had a long history of astronomy).
October 3rd, 2005 at 9:43 am
The only criteria I use is how much would we have to change the “My Very Energetic Mother…” Pnemonic device. I think if I can come up with something about pretty xylophones we’ll be all set.
October 3rd, 2005 at 10:01 am
I think it’s exciting to see these discoveries waaaaaay out at the (current) edge of the solar system. I don’t really care too much what we call these new “planet-like” objects, as long as we learn more about them and hopefully find more of the same.
—–
“Scarlett Johansson is a planet”
Is it rude and crass to say that I’d moon over her any day? Boy, I sure hope it is.
October 3rd, 2005 at 10:28 am
“A good guess to its size is 2700 kilometers across, which is only marginally bigger than our own Moon.”
Our Moon was bigger last time I saw it…
October 3rd, 2005 at 10:38 am
Oops! Somehow, I took the radius of the Moon (1700 km), doubled it in my head, and got 2400 km. Sigh. I had been doing a LOT of math that day, and was probably fried. I’ll fix it, thanks.
October 3rd, 2005 at 10:58 am
Is Jupiter a minor brown dwarf now?
October 3rd, 2005 at 11:11 am
OK, “planet” comes from the Greek “planetes” meaning “wanderer”. If you’re going to count everything that “wanders” in our solar system, then you’d have to count every single thing. Even the Sun, since Jupiter makes it wobble around. The solar system would have thousands (possibly up to billions, counting every little speck of dust) of planets.
But, I do agree that it really doesn’t matter if we call an object a “planet”, “minor planet”, “asteroid”, “planetoid”, etc. If you want to talk about MAJOR bodies in the solar system, then there are two: the Sun and Jupiter. The other seven undisputed planets are “assorted debris”. Remember: the universe is under no obligation to sort itself into little boxes for us.
October 3rd, 2005 at 11:14 am
“Xena” and “Gabrielle”?
I think I just threw up in my mouth a little…
October 3rd, 2005 at 11:23 am
This is a direct quote from Brown’s website: “There is no chance whatsoever that these will become the permanent names of these objects!” He is directly referring to “Xena” and “Gabrielle,” and also other newly discovered objects with nicknames like “Santa” and “Rudolph.” So no need to vomit!
Of course, if the IAU takes long enough to assign official names, those could just stick. I mean, are we ever going to think of it as something other than “Sedna?”
October 3rd, 2005 at 11:41 am
I would hope we have very clear definitions of what a planet is as we do for geography. Not sure if it would accomplish much but would save a lot of bickering.
Great work Phil… but um… where’s your review of Serenity?
October 3rd, 2005 at 12:34 pm
If an object orbits a star and has a more spherical orbit/shape, wouldn’t it be a planet? Oh, yes, it would also have to be bigger than most comets/asteroids.
I don’t know why I just thought of this, but why is it that steroids is in the word asteroids? Maybe I just have a spelling error or something.
October 3rd, 2005 at 12:41 pm
pumpkinpie, According to the IAU Minor Planet Center, Sedna is now the official name of object 2003 VB12.
http://cfa-www.harvard.edu/ps/MPDes
Also, Sedna was always an official recommendation for the name. Brown has stated that Xena is NOT an official recommendation, just a nickname.
October 3rd, 2005 at 1:01 pm
Mike Brown’s team’s discussion of “planet” definition.
http://www.gps.caltech.edu/~mbrown/sedna/
Steroids vs asteroids – unintentional coincidence.
Asteroid is from the Greek – asteroeides = star like.
http://en.wikipedia.org/wiki/Asteroid
Steroid is from Greek – stereos = solid. Sterols are fat-soluble compounds, such as cholesterol.
http://www.answers.com/topic/cholesterol
http://www.answers.com/main/ntquery?method=4&dsid=2040&dekey=steroids&gwp=8&curtab=2040_1&linktext=steroid
The base from Greek in both includes “oid” for form or shape. One is “star shaped”, the other is “solid form”. It is only coincidence that aster and ster are so similar.
October 3rd, 2005 at 2:37 pm
Hi.
Thought I’d give my two cents on this subject. Reading about whether Pluto and ‘Xena’ are planets or not has my head spinning. Will someone please come up with something that works and everyone can agree on. When it comes to stars, we have a great classification system based on temperature, elemental abundance, and size. I do not hear anyone complaining about our sun being classified as a G2V. This system has even been amended to include brown dwarfs (class L and T) But when it comes to what is or is not a planet… couldn’t someone adapt the stellar classification system to planets? Well… enough rambling, there is an unofficial system and it comes from Star Trek.
There is a book called Star Trek: Star Charts and it has a system for planets. Its a little more extensive than the star system but then planets can come in many flavors. The book classifies a planet by age, diameter, location, surface, and atmosphere. Earth for instance is a Class M world. Its 4.5 billion years old, has a diameter almost 13,000 km, its located in a region of space where water can exist in all three phases and has a Nitrogen/Oxygen atmosphere. BTW according to the book, Pluto is a Class C planet. The book is a great read, maybe someone should mention it to the IAU.
October 3rd, 2005 at 3:38 pm
These discoveries of planets (or whatever you want to call them) is interesting, for sure. It is true that terminology is arbitrary, however, if one wants to “carve the world at its joints” in a useful way, then one can start looking for useful classes to name. In that respect we have learned that many properties previously thought to be dicotomous now come in degrees; subsequently it stands to reason that (if there was a useful hypothesis that made use of such a notion, anyway) degrees of planethood could be a useful notion.
October 3rd, 2005 at 4:19 pm
BJN, a moon does not make a planet. (Or were you reading something else above.) Besides, what is a “large moon”? This object isn’t very dense, and it’s smaller than Charon. The size or mass ration of Ida & Dactyl is probably similar, so is Ida a planet since in has a relatively large moon? Your comments are a grey area, and are in no way inherently correct.
Doug, the stellar class description in ST Star Charts have enough common errors in them to make my take everything else with a grain (or 100) of salt in the realn of scientific accuracy. I enjoy the book, but Mr. Mandel obviously does not have a good grasp of freshman-level astronomy.
October 3rd, 2005 at 4:41 pm
Inaccuries aside though, ST approach to planet classification is a lot better than what is being tossed around in the halls of IAU. Single parameter definitions like size or location by themselves just do not cut it. (Its like three blind men trying to describe an elephant just by touch, its not a complete picture). The more parameters used, the better. If stars are defined by temperature, element abundance and size then at least planets could be defined by the parameters used in the Star Charts book.
October 3rd, 2005 at 5:02 pm
I have heard that Charon is actually larger than Pluto. If that is the case, why would Charon not be the planet and Pluto the moon?
Does anyone have a link to a good reference? Or perhaps BA can comment?
October 3rd, 2005 at 5:25 pm
Do any moons have moons?
October 3rd, 2005 at 7:20 pm
I note that the IAU still has not resolved the issue of whether or not 2003 UB313 is a planet.
IMO, if Pluto is to be called a planet, then certainly, UB313 should be as well. Size does matter. Adding adjectives simply muddies the waters and the ordinary folk will make up their own minds.
Also remember, that “astrologers” consider “Chiron” to be a planet and its only a small icy dirt ball, but then again “astrologers” arent the brightest peas in the pod.
Lets hope the IAU can come up with something ordinary folk will agree with.
Personally, we could collectively call all KBOs, SDOs, Plutinos and TNOs, “Ice Dwarfs”. Yet still include Pluto and UB313 as “Planets”, simply because the masses (ordinary folk) will make it so, anyway.
Folcrom
October 3rd, 2005 at 8:08 pm
Charon is much smaller then Pluto. The Nine Planets is your friend.
October 3rd, 2005 at 10:19 pm
Hi BA
If your last post was with regards mine, then I was mentioning “chiron”, the centaur, not “Charon”, moon of pluto. Yes Chiron is a terrible small chunk of ice. Yet astrologers call it a Planet!!
We of course (ordinary folk), no better. Chiron is a ice ball.
If it isn’t at least as big a Pluto, it shouldn’t get a gernsey. UB313 fits the mark.
Folcrom
October 4th, 2005 at 2:32 am
Doug’s idea of having the IAU adopt Star Trek designations for planets is the best idea I’ve heard all day. Granted, I’m on French time and I just woke up, but still, it might be the best thing I **get to hear** all day. Of course, the “canonical” Trek movies and shows only give a couple classifications — class M for Earth-type and class D for airless lumps of rock (see Wrath of Khan) — so we’d have to invent the others as we went along. How about class J for hot Jupiters like 51 Pegasi 1? And class X for planets orbiting pulsars, because they’re just that cool? ::wink::
Oh, and let’s not forget what the narrator in **Fight Club** says about naming astronomical objects: “When deep space exploration ramps up, it’ll be the corporations that name everything. The IBM Stellar Sphere, the Microsoft Galaxy, Planet Starbucks.” (The novel mentions “Planet Denny’s” too.)
Blake
October 4th, 2005 at 2:47 am
Actually Blake, the book Star Trek: Star Charts has classification from A to Z. The “Demon” planet from Star Trek Voyager is for instance a class Y. A water planet would be a class P for Pelagic. Pluto is a class C, so would “Xena”, etc…
October 4th, 2005 at 4:12 am
The star trek literature is probably very logically consistent too… it has to be. There are literally millions of nerds around the world who will nitpick at any inconsistency. I’d wager it has the accuracy and logical consistency of any doctoral thesis (though not the scientific background).
October 4th, 2005 at 8:26 am
Irishman, I can’t connect to the link you provided. I was going by the IAU website which says “The name Sedna has been proposed after an Inuit ocean goddess, but the name has not yet been endorsed by the IAU and needs to be reviewed first by the IAU Committee on Small Bodies Nomenclature.”
Of course, it hasn’t been updated for over a year! But I just don’t remember any announcement that Sedna was the official name. If you could provide a different link, I’d be happy to read it!
Also, I don’t think it matters to the general public if Sedna was the “official” recommendation, but Xena and Gabrielle are not, even if Brown et al explicitly say it. The fact is they are calling them by those names, it’s everywhere on the internet and news, so that’s what people are going to remember. And the longer it is before the “official” names are released, the harder it’s going to be for the public to accept the new names.
October 4th, 2005 at 8:35 am
Thanks BA. great link for quick info!
October 4th, 2005 at 9:39 am
“Do any moons have moons?”
I’m going to answer this since no one else seems interested. In theory, there is no reason why an object can’t orbit a moon (technically, when the Apollo Service Module went into lunar orbit, it became the moon of our moon). However, every object has to orbit within their primary’s Hill Sphere (basically, the area around an object where its gravity dominates). The Hill Sphere of moons tend to be rather small because they are either very light or near a massive planet, so its hard to have an object naturally orbit in one.
For example, Earth’s Hill Sphere’s diameter is over a hundred times the diameter of Earth itself (about 1.5 million km). However, the Moon’s is only about eighteen times (about 61,700 km).
So far, we have seen no natural occuring examples, but I wouldn’t be surprized if we eventually found some somewhere in the universe.
October 4th, 2005 at 10:18 am
BA, I notice that the Nine Planets title page has changed to Ten Planets (although, it is just with the word Nine crossed out and Ten written beside it, just in case).
Congratulations on reaching a century of blog entries!
Since the definition of a planet seems so arbitrary at the moment, I don’t think it really matters. Maybe they should just say “well, if it’s bigger than Pluto we can call it a planet as long as its primary is Sol (or at least a star as opposed to another planet)”. I think, even if Pluto gets downgraded (and what’s that going to do to all the poor astrologers? More importantly, who cares?), Uranus and Neptune should still count as planets, even though they were not identified by the ancient civilizations in which astronomy began.
October 4th, 2005 at 10:44 am
I agree with Scott McLean, I don’t really care what size or criteria that make an object a planet. So I suggest that we use his word ‘glumphdrix’ about all objects in the solar system, from every pebble up to jupiter.
Of course the discusion will now change into whether the newly discovered glumphdrix is a planet-size glumphdrix or not
October 4th, 2005 at 2:52 pm
pumpkinpie, yeah I tried the link and it didn’t work for me, either. I think it’s actually a results from a search page. I went to the IAU main site and followed links to the Committee on Small Bodies Nomenclature page, then followed to the MPC link to the Search page, and then searched for the designation 2003 VB12.
http://cfa-www.harvard.edu/iau/MPDes.html
I am having trouble using the search page to find information regarding the declaration of the name, but I did find this:
http://cfa-www.harvard.edu/mpec/K04/K04S73.html
“Among the new names of minor planets published in this batch of Minor
Planet Circulars is (90377) Sedna = 2003 VB12. ”
That article actually discusses some controversy over the process that occurred, but does not invalidate the naming.
Anyone who understands the IAU and MPC databases and circulars better than I is free do dig up the announcement. My efforts are not stellar, but adequate to prove the point.
The disclaimer you mention is definitely out of date. I wish they updated that page, too, because it mislead me. I had seen the comment on Mike Brown’s page that the designation was official, so that’s why I didn’t trust the first prominent notice on the IAU page and dug deeper.
October 4th, 2005 at 3:02 pm
Regarding the Star Trek type classification system for planets, I think that it is irrelevant to the discussion at hand. I think the IAU is leaning in that direction with their new pronouncment on planet types, but I don’t think it really matters what level of detail one goes to in describing the types of planets, that still leaves the big question unanswered – is this new object a “Planet”?
And actually, I don’t think astronomers are necessarily the only ones who get a vote, because the word “planet” is a much older than the IAU. We’re really stuck on definitions – what does “planet” mean? From that context, if astronomers can’t come up with something sensible and consistent and reasonably clear (and do it quickly), people will make their own determination and then we’ll have another case of a technical term that doesn’t match the layperson’s use – the astronomical version of “theory”.
Then again most people will end up stating whatever they’re taught – nobody really worries too much about why Ceres isn’t a planet, they just know MVEMJSUNP. That’s if they’ve even heard of Ceres.
October 4th, 2005 at 3:04 pm
You know, we already have that to some extent. Witness the “Minor Planet Center”, and tracking asteroids and comets and such in the Minor Planet database. Laypeople don’t call them “planets”, but astronomers do.
October 4th, 2005 at 5:19 pm
Zeb…thanks. i thought the idea of apallo being a moon was a good one. It all just seems so natural. The galaxy orbits a black hole, the planets orbit the sun, the moons orbit the planets…the next logical step is……
and i guess i should call them satalites instead of moons.
October 5th, 2005 at 6:18 am
Thanks, Irishman.
October 5th, 2005 at 7:15 am
I think that the Star Trek approach is an excellent one; it takes seriously the prospect that our definitions will have to accommodate many factors as we discover and visit more of the universe. Carl Sagan has something similar in Ch. 12 of “Cosmos”, Encyclopedia Galactica. It classifies worlds with a number, a “civilization type,” “society code,” star type, etc. One world is classified as “Planet = sixth,” with diameter and size and other data listed. That already seems a little quaint: the sixth large body out from a star might not remain constant (I’m thinking of how Neptune and Pluto briefly “switch” their order). But the idea of this kind of catalog is a good one.
October 6th, 2005 at 7:22 am
A planetary classification scheme may or may not be a good idea, but it is irrelevant to the question.
It’s like a guide to different countries. Each country can be classified by language, ethnic groups, geography, religion, etc. But that still leaves undefined the word “country”.
Stars can be classified by size, color, spectrum, age, distance from us, distance from the galactic center, spin, whatever. But that doesn’t define “star”.
Planets can be classified as “rocky bodies”, “gas giants”, “ice lumps”, “atmospheric or non-atmospheric”, “liquid water zone”, or any other labels you want to conjure. But that still leaves unanswered the fundamental question on everyone’s mind – the definition of “planet”.
October 6th, 2005 at 10:38 pm
I gave it a go:
http://catchingthesky.blogspot.com/2005/10/what-are-planets-and-moons.html
October 14th, 2005 at 8:05 am
What is a mountain?
What is a hill?
What is a planet?
What is not a planet?
At some point, an arbitrary designations system will be required.
And BTW, a planet does not have to orbit the sun. That would eliminate all of the extrasolar planets.
October 17th, 2005 at 11:20 am
Kevin, with regards to your definition, where do asteroids and comets fit? There most certainly are a large number of asteroids of various sizes that directly orbit a stellar body – the Sun. Halley’s Comet is in a stable (for now) if very elliptical orbit of the Sun. By your definition they would all be planets. Even a lone rock the size of your fist would be a planet if orbiting the Sun. You discount small objects from orbiting the Sun, but I don’t see your justification for doing so.
October 17th, 2005 at 10:15 pm
As for me, I guess I’m in the minority that wants the definition of planet changed to smaller rather than bigger. You see, I say Pluto is a planet. And Charon too. This is beacause the center of thier combined gravity is a point in space somewhere between the pair, therefore calling one a moon and the other a planet seem silly to me. And yes, I’m aware that this brings the lower bound on planets to a smaller level than anyone else’s definition. That’s ok, I think we need a few more planets.
Oh, and any definition of planet based on orbital eccentricies or plane is the astronomical equivilant of gerrymandering.
As for Jupiter and the Sun being the only major objects in the solar system, I object. It seems to me that Saturn should make the short list as well. As for rest, yep, just minor space debries.
October 18th, 2005 at 2:42 pm
Where do you want to draw the line of significance? Using The Nine Planets for data and converting all numbers to the same exponential base, I get
Sun = 1989
Jup = 1.900
Sat = 0.568
Ura = 0.0868
Nep = 0.1025
Thats using E^27, or 9 sets of three zeros (000), in kg.
The Sun is 3 orders of magnitude more massive than Jupiter – 1000 times. Saturn is about 1/4 of Jupiter. Neptune is about 1/20th of Jupiter’s mass. Uranus is a little smaller than Neptune.
So if we’re going to include Jupiter, I suppose it’s reasonable to include 1/4 Jupiter, but not 1/20th Jupiter. So Saturn is in.
February 1st, 2006 at 10:31 pm
[...] The most commonly discussed issue on the ‘net has centered upon whether or not 2003 UB313 will officially be deemed a planet by the IAU (and Mike Brown has commented on those ongoings, here). Personally, I’ve never really grasped the wrangling over nomenclature, examples of which can be found on many popular astronomy message boards (and some are most passionate, pro and con). A planetary declaration would certainly serve as quite a distinction for the discoverers from CalTech and Yale who’ve invested so much time and effort in the observations. It should be noted, though, that size does not a planet make. I’m exponentially more interested in the object’s properties and characteristics, and what its study can reveal about our cosmic neighborhood. [...]
February 2nd, 2006 at 1:09 pm
Hello World.
February 2nd, 2006 at 1:12 pm
I’m doing a science fair project of the planets and this website did not help. ;o