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How Would You Define a Planet?
It doesn't come easy asks: "The argument over the definition of a planet continues. So far, two definitions are favored but without much consensus so far: base the definition of a planet simply on an object's size. Pluto would be near the lower limit and the newly discovered Kuiper Belt objects could also qualify, giving us 10 or 11 planets so far; or define the single dominant body in its immediate neighborhood as the only qualifying object for planetary status. If no one body dominated (such as the millions of individual asteroids in the asteroid belt) then none would qualify for planetary status. In this case Pluto would be disqualified (Neptune would be the dominant body in Pluto's region of space), and the newly discovered Kuiper Belt objects would also fail to qualify. The International Astronomical Union (IAU) working group charged with pinning down the definition of a planet may vote on the proposals within the next two weeks (or they may decide to start all over again with something new). Maybe Slashdot readers can give them some help. How would you define a planet?"
sounds good enough for me ;-)
What could possibly hurt the security of the American people more than giving our own government the ability to hide its
A planet is, like, one of those rocky things that goes around the sun... oh wait Jupiter is gas. Ok, it has to be like big and shit. It can't be a star though. Ok lets say 1,000km diameter minimum. If it's 1000km diameter or more, and it orbits a star, but it's not a star itself, it's a planet.
There. I just settled what thousands of "smart" astronomers with their "Ph. D's" can't figure out.
Pass the bong man...
The obvious conditions are round shape and orbits the sun. Size is somewhat subjective although to have a round shape it would have to be above a certain mass.
If not, it's just a rock.
But Pluto gets grandfathered in. Unless a huge planet is discovered, we're stuck at 9.
What is your favorite colour?
Oataox or whatever the hell? The guy who came up with that needs to be kicked out of the Astronomy club.
"Ain't no right way to do a wrong thing."
1. A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves. In the solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. 2. One of the seven celestial bodies, Mercury, Venus, the moon, the sun, Mars, Jupiter, and Saturn, visible to the naked eye and thought by ancient astronomers to revolve in the heavens about a fixed Earth and among fixed stars. 3. One of the seven revolving astrological celestial bodies that in conjunction with the stars are believed to influence human affairs and personalities.
But
a : any of the seven celestial bodies sun, moon, Venus, Jupiter, Mars, Mercury, and Saturn that in ancient belief have motions of their own among the fixed stars b (1) : any of the large bodies that revolve around the sun in the solar system (2) : a similar body associated with another star c : EARTH -- usually used with the
2 : a celestial body held to influence the fate of human beings
3 : a person or thing of great importance : LUMINARY
is good, too.
"How to Do Nothing," kids activities, back in print!
Personally, I think a good definition of a major planet is one that is massive enough that, given its composition, it assumes a sphere-like shape.
cb
Oooh! What does this button do!?
The public will be happy to learn of more planets -- it feels like progress. It'll be hard to convince the public we lost a planet somehow. That sounds like an unimportant consideration, but I don't want us giving the Creationists more ammo for their arguments that Science is fickle. "They used to think there were nine planets, but then they found they were WRONG!"
It's not like any serious science rests on this definition anyway.
--Greg
A nonluminous celestial body larger than an asstroid or cumbucket, illuminated by light from a star, such as Michael Jackson, around which it revolts.
Uncyclopedia: Planet
Kirstie Alley?
Reminder: Apple owns 1/255th of the internet.
If it's the size of Marvin's brain, or bigger, it's a Planet.
... well, it's just depressing.
If it's smaller
Wretched, isn't it?
Me after I eat: a Granny Smith Apple, yogurt, onions, and a Guiness or three...
Evil people don't think they're evil. - George Lucas, Making of Ep III
e.g. planetquake.net and eight billion others from that brief time period
"Planet" derives from "Plane"?
How to Measure a Planet?
... when I see it.
Blarf.
Words like "planet" are meant to "carve nature at its joints". Problems arise when historically there appeared to be joints (planets moved differently than stars in the sky) but, we are learning now that there are no useful joints here. Why bother defining the word planet at all? Is it really that useful to astronomers? And why, say, want Mercury (a small rocky body with no atmospere) to be grouped in a category with Jupiter (a large, mostly gaseous body with an atmosphere) instead of with asteroids (small, rocky bodies with no atmospere)?
I would say that the definition of what consitutes a planet would depend entirely on what such a definition is to be used for. I can't think of any decision that would hinge on a body's designation as a planet, so any system is as good as any other.
Gravity is a constant that should be used to define a planet. Any body that has enough mass to generate enough gravity to maintain a spherical shape should be a planet. Yes, even Ceres would be a planet by this definition.
There is nothing inherently safe about liberty. That's why so many people died protecting it.
While asteroid belts will vary widely by interfering gravitational effects, planets manage a consistant orbit.
Just because you can mod me down, doesn't mean you're right. Shoes for industry!
The definition is largely meaningless anyway. No science hinges on what a planet is. It's a waste of time even to argue about it.
Tell those bitches to stop with the silly arguments and get back to the telescopes. When they have a valid scientific reason to differentiate a planet from a hunk of rock that just happens to orbit the sun, then we can start arguing about definitions with some kind of actual reason for it.
- Give a man a fire and he's warm for a day, but set him on fire and he's warm for the rest of his life.
A planet has to be round, and it has to be in an orbit, in outer-space.
Maybe split the difference between the proposals? Anything orbiting the sun with enough mass to assume a spherical shape (but not enough for fusion) is a planetoid, but only the dominant body in the region gets to be a planet. That way we can avoid busting Pluto all the way down to "piece of rock", but avoid a situation in 30 years when we end up with 74 "planets" in the solar system after they map the Kuiper Belt some more.
I'm enough of a sentimentalist that I'd like to see Pluto keep some status. It deserves a bit of historic recognition just for being the first KBO to be discovered, and so much earlier than the others.
Entropy gets everyone.
As long as the official definition contains the phrase "It's bigger than a breadbox", then I'm happy.
I suggested this on www.randi.org a few weeks ago. In Pluto's case have astrologers draw up two parallel charts. One with Pluto as a planet, the other without. After a few weeks we can compare what happened in the world to the astrology charts and that'll settle it.
"The planets don't lie" as I said there. ;)
Trolling is a art,
If a rock is massive enough to have a molten core and sustain any magnetic field (and not orbiting a more massive counterpart), I'd call it a planet.
If it has a sustained atmosphere, then that's a plus, too (c.f., Mercury doesn't have one, however).
What is your quest?
with too much time on their hands....
... its fun to have the americans to blame for everything in this world: it makes every problem so easy to understand! Pluto has been, to what i can understand, qualified as a planet just so the americans could boast to have discovered a planet in the solar system... the planet definition loophole that this created is still going on today... its clearly the usa's fault!
I would define it thus: An object is a planet if it has enough gravity to form into a sphere but not large enough to ever had fusion start in its interior and has cleared its orbit of debris left over from its formation. This would allow Pluto to remain a planet, as well as "promote" Sedna to planet stus but rule out Ceres.
CDE open sourced! https://sourceforge.net/projects/cdesktopenv/
http://en.wikipedia.org/wiki/Definition_of_Planet
The problem with disqualifying Pluto because of Neptune being the more dominant body falls apart when you consider the eccentric orbit of Pluto and just how far that takes it from Neptune's "region of space".
What exactly is the definition of a region of space?
How much larger must an object be than its neighbors in order to be considered the dominant object of its neighbors? Twice as large? Four times?
wouldn't stars be "planets" as well?
Emily Lakdawalla wrote a good summary of this debate for her blog the other day. She echoes the suggestion that we should define more than just 'planets', but rather specific types of planets. Things like Jupiter and Saturn would be defnined as 'Gas Giant Planets' while planets like Earth and Venus would be 'Terrestrial Planets', Plutos would be 'Minor Planets', etc. Seeing as gas giants and terrestrial planets really are completely different things that aren't fit to be grouped together as 'planets', I support this plan. Of course, there will innevitably be arguments about the intricacies of these sub-definitions...
Will NASA get a stipend for each planet? Is there an astronomy-textbook lobby that's pushing for more chapters?
Creating arbitrary categories so we can think we know about stuff is no substitute for knowing about stuff. The map is not the territory.
Deciding "what's a planet" is a game for people who don't care about astronomy, and just want to argue over beers at the pub.
Flumph
Is a definition that would make sense in other solar systems, too.
How about, big enough that its gravity could retain an atmosphere?
If you can land on it and score with an alien chick, it's a planet.
Courtesy of the (sadly defunct) Brunching Shuttlecocks: A Brief Conversation with the Planet Pluto and Another Brief Conversation with the Planet Pluto.
When my girlfriend is deflated and on the rag. And I find the parent fucking hilarious!
Evil people don't think they're evil. - George Lucas, Making of Ep III
If there are cluster of entities in the same slot, asteroids.
See? Pluto's a planet. Nothing else is. Now move on about your daily life, citizen.
We blindly follow him on every other issue. Oops, did I say that out-loud!
A gravitationally round body (Ceres or larger) that is the dominant mass within its orbital space. Pluto has a highly elliptical orbit, so it shares an orbital space with many other large KBO's. None of the first eight planets have this problem.
Huzzah!
Something that's large enough to hold an atmosphere, any atmosphere.
I still think that the definition of a planet should hinge on how easily it can be viewed from Earth. Personally, I don't think that any Kuiper belt object is a planet. Pluto is not a planet. Mercury, Venus, Mars, Jupiter, and Saturn can all be easily observed with the naked eye. Uranus usually requires binoculars to be seen, and Neptune some kind of telescope (however amateur)... Pluto, and the rest of the Kuiper belt, are a very difficult thing to view. If you're an amateur astronomer, you're going to be able to discern a speck of faint light from a "medium" sized telescope.. That is, 6" reflector or so. I would love to see the definition involve being able to view the celestial object as a disc from earth's surface. Hell, even the HST barely discerns much more than a grayish disc. http://hubblesite.org/newscenter/newsdesk/archive/ releases/1996/09/
Each processor would proceed sequentially as if it had been better for them not to rise against Saul.
Flash back to 1915: hmmm...now that Einstein has published his theory of relativity, should we reconsider our definition of the term "luminiferous aether?" No, the term needs to go away completely, because it's become clear that it serves no useful scientific purpose.
Find free books.
when does a man become bald?
when do grains of sand become a pile?
no need to bicker... just keep it simple--planet: a wanderer of the heavens...
-judging another only defines yourself
and starting to look good again! At least on the latest Jenny Craig commercials ;-)
Evil people don't think they're evil. - George Lucas, Making of Ep III
Orbits a star or stars.
Enough mass so that its gravity forces it into a spherical or an ellipsoid shape.
This defintion does make large astroids like Ceres a planet. Personally, I don't necessarily have a problem with this, but I don't really care. If you want to remove these you can add:
Must be a "free standing" object (i.e. not in a belt)
If you're dead set against Pluto, you can add:
Orbital inclination must be close to the orbital plane.
I not be an astronomer or an astrophysicist, but I really don't see what's so hard about defining a planet. Whatever the Powers That Be(tm) decide, it should be based on physics and not legislation. (e.g. "mass in excess of x metric tons")
A planet is an object massive enough to become spherical under its own weight who orbits a star.
Solar Orbit Won't Work... Size Only
...what do we call them then if we're basing their "planet" status based on their orbits around a star...
..."the big round thing formerly known as a planet"??
With all those other stars out there, probably some other stellar-planetary systems have been disrupted and the planets freed from their orbits around their star...
Lets think a little beyond the here and now, so we don't have to have this discussion AGAIN in a 100 years...
Geez keep it simple.
Either add some new planets to our system, or give Pluto "honorary planet" status to keep our count above 8.
...then it's a planet. Planets have Sailor Senshi. Everyone knows this. Pluto? Sailor Pluto proves Pluto is a planet.
And don't go talking to me about Sailor Moon. The Moon isn't a planet, because Sailor Moon is really Princess Serenity.
Morans.
144l. ph34r my 133t l3g4l 5k1lz!
As I sit here, on my throne atop Mount Bitterness on the Warworld of Angst I look out upon the inky darkness of space and I know all of its planets belong to ME! If I find a planet to be worthy of my leadership I SIEZE IT AT ONCE and add it to my vast, vast Empire. If I do not find a planet fit to RULE OVER it is not a planet for long and I ELIMINATE IT so that it may not be used AGAINST ME!
Pathetic Earthlings. Undeserving of my rule. You will soon discover you do not have a planet, indeed you will soon have NO PLANET AT ALL!
Oh, but I do enjoy the villany of CAPITAL LETTERS! HA HA HA! * sigh *.
...the mice?
If you could reason with religious people, there would be no religious people
Although planets are the principal component of the solar system other than the sun, a precise definition of the term is surprisingly elusive. This article details the questions that may arise when trying to formulate a strict definition of the word.
For most astronomers the issue will be decided by the International Astronomical Union (IAU). According to a published report from Nature magazine (corresponding entry at BugMeNot), the discovery of 2003 UB313 (which is a Kuiper Belt object bigger than Pluto) has forced the issue. An IAU committee which had already been working on a definition is now expected to promulgate one soon.
Please define what life is, I've always pondered that one.
- These characters were randomly selected.
A) If you could travel from it to the system's star in less than a centar, traveling at no more than 100 hectars per centon, and;
B) It is at least 10 to the 10th power laxons in mass, and;
C) Free from intergalactic casinos run by Ovions
I'm pro-accordion and I vote
I don't know an easier way to do it than to supply a lower bound for the mass. Using Reza Lockwood as an example, we can still consider Pluto a planet, but we would dismiss most (although not all) of the trans-Neptunian objects, since Lockwood has more mass.
One thing is certain: A hard and fast definition needs to be standardized upon. This whole "Well, X is a planet because we've always considered it to be one" is unscientific, and we (as a scientific community) can do better.
Smaller than a star, bigger than its own natural sattellites? Composed of some kind of matter?
Gravity? Christmas trees?
Who did what now?
Why not just abolish the whole idea of a planet?
If it's so vague you can't find a definition for it, why have one at all?
Why not just classify objects by their mass, radius, orbit, maybe temperature and other significant information too?
-For example, you have an object of a certain mass classified as M# where # is its location on an exponential mass scale.
-Then you add R# for its size.
-Then have O# for degree of orbit, where # = 1 + (O# of the object it orbits), having something like the galactic center as the O# = 0.
Then you could use any of the characteristics as needed.
Or maybe my brain is just in parsing mode right now.
(There are monkeyboys in the facility.)
I think that at least one qualification that something needs to have to be considered a planet is that it orbits within a certain number of degrees from the ecliptic plane. This new thing out there is orbiting with an orbital tilt of something like 45 degrees! Personally I'd be in favour of narrowing the slice down to the point where we exclude pluto as well.
My suggestion
Roundish thing orbiting a start = planet.
Not round thing orbiting anything = asteroid.
Has a moon(s) = planet.
So, Pluto has been a planet "forever" (all of many of our lives). It should just STAY a planet. Now, if they want to re-define the term planet to mean "something a bit larger than Pluto", or even "most dominant thingy in its orbit" - that's all cool as long as Pluto gets grandfathered in and remains a planet.
thats right and yo moma too
When you have at least two bloggers syndicated to one point and the post interval is at least one post per day, then you have a planet.
Why do they insist on binary classifications. I'm sure that no matter what criteria you pick (and I'm sure the list would be very long), you will find bodies that seem like planets but don't fit 100% of the criteria.
We can't even agree within the limited sample size of this solar system on the definition of "planet." What happens when we get really good at finding objects that orbit other stars and have hundreds, thousands, millions, billions of potentially planet-like objects to classify. I'm sure will hit some real stumpers. What if we find two massive "planets" orbiting each other while they orbit a star? Is only the bigger one a true "planet" even if the "moon" has 10 times the mass of the Earth? Or what if we find some dwarf stellar system with a nice array of "planets" that all happen to be the same size class as our or Jupiter's moons?
The binary categories aren't even that useful. Surely some larger "non-planets" will have some planet-like properties for astronomical/terraforming/exploration purposes?
My point is that the world isn't binary. Objects around stars exist in a multidimensional continuum of sizes, compositions, histories, orbits, etc. Why not just list those properties and not waste time force-fitting the bodies into categories that only have crude predictive power.
Two wrongs don't make a right, but three lefts do.
therefore, mercury would NOT be a planet (more like a moon of the sun)
and titan, even though it orbits saturn, WOULD be a planet
i think that makes most the most sense: what an object orbits shouldn't matter, it's composition should be the largest consideration
some other nomenclature can address what it orbits ("a moon of the sun" or "a planet of saturn")
it should be considered either
REGARDLESS of what it orbits
intellectual property law is philosophically incoherent. it is your moral duty to ignore it or sabotage it
It seems to me that this could only be important or useful for professional astronomers. It makes no difference to the layman. Might as well do whatever is most convenient for the professionals.
1. Massive enough that its gravity will prevent future human occupants from reaching escape velocity with a brisk jump.
2. In orbit around a star, so that it can be easily located by interstellar travellers. A body adrift in the universe makes for a poor address.
3. Some form of aesthetic beauty. Why bother calling it a planet if you don't want to either a) live there, or b) look at it.
4. A cool name. Lila qualifies (Go Mike!), as does Clare, Krypton, and Earth. Uranus is marginal, especially when pronounce "your uh nuss" Come on.
5. Exploitable resources. We're human, after all.
I don't see why we need to have a rock solid defintion. Its not like the cosmic police are going to come get us for not encluding an orbiter in our list of "planets." We could have a hybrid rule. Why not just say it's going to be the domination rule, with a disclaimer that says Pluto is still a planet? Pluto is by far one of the coolest planets. I don't see why we can't keep it.
On the other hand, the excitement of learning a new acronym for 20 something planets would be much more fun. Imagine if we could say our solar system had 25 planets? How cool would that be? A lot cooler than 15 or however many there are.
I've heard somewhere, in the past, that Terra and Luna would really qualify as a twin-planet system. Just because Earth is larger, doesn't mean that the Moon's gravitational pull doesn't affect the Earth's orbit. They both tend to cause each other to wobble, and not have a smooth orbit. Other "moons" should qualify as planets as well. The potato-shaped objects would either be moons or just asteroids, keeping the spherical objects as Planets, regardless as to whether or not they are orbiting a star or another planet.
If it's big enough to host a game of football without the ball flying out into orbit, that'll do me fine.
It would be nice to remind ourselves that the concept of planet was something invented by humanity. It's not necessarily a real criteria in which celestial objects should be categorized. The problem is that people keep fighting trying to fit everything they see into their own manufactured preconceptions, and that will always result in failure, eventually.
I'm sure that most celestial objects are somehow named by NASA or whoever, and they must be ranked into different levels of "significance". Why not use that? What difference does it make if Pluto or all those borderline-planets fit the definition or not? Why not admit that there is an unavoidable continuity in the size and importance of celestial object and call it a day?
Favorite quote: "
There are several different kinds of defitinion; which one do we want to go for? My choice is the verbal extensional definition. Planet := {Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto}. Period.
Attitudes make the difference between Space and Time: we want to MAX our temporal, and MIN our spatial extension.
Anything whose gravity is strong enough to give a round, rather than potato-like, shape. :-)
It is both a magazine and a breakfast cereal.
Stop Global Warming!
Just say no to irreversible processes!
Define a planet as in some way 'larger than a comet or asteroid'. Then, define an asteroid as 'smaller than a planet'. Problem solved in time for tea.
.. [syn: creek]
Observe, there is presedence for this highly useful mechanism:
http://dictionary.reference.com/search?q=river / creek/stream
river n : a large natural stream of water (larger than a creek);
creek n 1: a natural stream of water smaller than a river, plus A small stream (..)
stream n.A flow of water in a channel or bed, as a brook, rivulet, or small river.
rivulet n. A small brook or stream; a streamlet. (Logically, a stream is a rivulet is something smaller than a stream.)
brook n : a natural stream of water smaller than a river
Of course, this is all done to add a veneer of aristotlean logical classifications to things. The problem with a classification as an mentally constructed set of delineating lines overlaying a situation of infinite variety is that there will always be "borderline cases", which will always lead to the definition being called into question or seen as impractical or irrelevant. The alternative course of action could be not to define what a planet is, let people describe whichever objects they want as planets, and have data libraries sort their decisions out in a darwinian sense.
Let's make it easy. Life is when you can ponder about life. I wonder if fish can ponder.
"Beware of he who would deny you access to information, for in his heart, he dreams himself your master."
A planet needs to spin on an axis, even a wobbly one. If it's just falling over in a random pattern, it's not a planet.
Size matters as well. How big is big enough? Well I would say it has to do with its gravitational force on other celestrial bodies.
Also, is it a orbiting another body? i.e. is it a moon? Our moon spins on an axis, exerts gravitational force on our ocean, but is a satellite of Earth.
Okay, to define an object as a planet, that body needs to be more affected by the gravity of the solar body it orbits than by the gravity of any other body. If it's more affected by the gravity of some other planetary body, then it's a moon, not a planet.
Spherical shape nor mass alone should be a factor. If you're going to go JUST by spherical shape and mass then the Earth's moon could be called a planet.
But obviously, we do need to take into consideration the object's mass, or every rock in the asteroid belt could be considered a planet.
So; to recap, the object needs to be a certain size and mass -- AND -- demonstrably affected by the gravity of the primary star of the system, so that it is in a solar orbit and not overtly affected by the gravity of other celestial bodies.
So, now we're down to size and mass question again -- It's a wacky situation, as moons of Jupiter can be larger and more massive than the planet we inhabit - I wonder if we'll ever discover a moon orbiting a gas giant, that has it's own moon... A moon's moon. What will we do then I wonder?
Redefine everything all over again?
Science is a constanly changing landscape. Even if we set in stone what we're going to call a planet, we'll be having this discussion again in 20 years or so.
If telephones are outlawed, then only outlaws will have telephones.
1) Must possess an atmosphere of at least kPa.
2) Must orbit a star.
3) Must not orbit another body.
Prediction for end of Universe #42: Fencepost error in Quantum_bogosort.cpp
Is it round? :)
Is it big?
If not, call it planetoid. Ta-da!
(Or am I being too simplistic?)
No Weather, No Planet, just a rock.
If it has a moon, it's a planet. That's my definition.
Looks pretty easy to me. Pluto, being a Kuiper Belt object, is not a planet, neither are any of the asteroids in the asteriod belt. The Earth is, and so is Jupiter and Mars.
The problem comes in defining the terms, such as belt and regular.
Although I tend to believe, like Justice Stewart said a few years ago, "I know it when I see it."
I love deadlines. I like the whooshing sound they make as they fly by. - Douglas Adams
I think of two criteria here: first, a planet must be the overall center of gravity in its immediate vicinity; it's certain we're going to find Earth-size moons about some gas giant someday.
Second, I think geological activity sufficient to create a distinct core, mantle, and crust would define the solid planets. The gas giants will obviously have some strata or some differentiation, so until they produce fusion, they count.
How does that sound?
The issue perhaps is the fact that anyone who discovers a "planet" seems to accumilate all this fanfare and publicity, where as a planetoid or asteroid is pretty marginal.
Since we use Earth as our prime example of a planet, it seems that some of the base characteristics should apply.
1) The ability to store things there - lots of things.
I don't know about you, but when I used to play Yankee Trader as a kid, if the planet didn't have anything on it worth trading, it was one pathetic planet. Which perhaps brings a second point.
2) The ability to produce things, given the right colonization, etc. That is in a word, the "convenience" that having a planet offers you in the ability to manufacture and fabricate.
If you can't produce some kind of raw materials or even services there, then you don't have a corporation, let alone a planet.
So the Death Star was a planet. It had gravity too.
Stars are not planets, because although they are quite useful in the manufacture of various kinds of matter and energy, they are useless for conventional storage. Nebulas are not planets, because although they can be very helpful in the manufacture of stars, they don't offer a conventional way to store things. Asteroids and comets don't offer a "stable" environment for the storage or manufacture of things. Black Holes don't offer conventional storage - too much compression.
There are other criteria we could use. Shape, temperature range, size, etc. but none of them seem as comprehensive as these two things.
Anything the size of Mickey's dog or larger is a planet? Then there are a lot more than nine of them, there must be billions!
How ya like dat?
I believe they are all Roman gods, but probably of Greek origin.
Consequently, also big enough to be destroyed by aliens.
I think a planet should be defined as a body that formed from solar material and originally orbited the sun.
To elaborate on this definition, consider these points:
1. That the planet is relatively close to the ecliptical plane
2. That the planet is consistent with spacing of solar-system formation such as the Titius-Bode Law http://en.wikipedia.org/wiki/Bode's_law
3. That the planet is of sufficent size to have orbited the sun for millions of years
4. That the planet is affected by or affects other planets in the solar system
This would define the asteriod belt, Pluto, and Sedna as not being planets. It would also bring Neptune's planet status into question.
With the definition above, we can confidently describe some of the major events since the formation of the solar system:
A. The asteriod belt was formed by a planet that never congealed or was ripped apart by tidal gravity or by a large-foreign body
B. Venus somehow spins the wrong direction
C. Neptune seems to be out of place. Voyager's discovery that Neptune is not a giant ball of gas has implications; either (a) that Jupiter and Saturn also have very solid surfaces or (b) that Neptune is even more out of place.
D. Uranus has an extreme tilt, perhaps from Neptune's unusually close position
Without consideration of the dynamics of solar-system formation, our solar system seems like a hodge-podge of bodies that have very mysterious histories. However, a wholistic view brings meaning and richness to the definition of a planet.
Any body that contains an atmosphere and orbits a star singularly or in binary. Just what I think of traditionally as a planet.
Not since Marie-Antoinette played milkmaid has looking simple and honest been so fake and complicated.
The only way to define it is obvious:
Anything you can blow up with the Death Star!
"City hall" in German is "Rathaus" Kinda explains a few things......
I've always thought that a planet is defined as any mass that orbits a star. Simplistic, perhaps but it seems to fit.
Similiarly, I've thought that any mass that orbits a planet qualifies as a moon.
Simply say that a planet is something big, etc, but Pluto gets to be a planet by a grandfather clause.
Everyone's happy.
Fellowship 9/11
First, planets don't "contain" atmospheres, the atmospheres wrap around the planets.
Second, even guessing at what you meant, the definition is still ridiculous. Mercury doesn't have an atmosphere - at least no more than our moon has. On the other hand, there are lots of small irregular bodies that sport atmospheres at least some of the time - comets for example.
So, by your definition, we'd lose Mercury and gain hundreds of celestial chew toys.
Clear, Dark Skies
In related news.. top pro fishermen gather to decide what is a lake. When is it a pond? When does it become a lake? Why isn't it a sea? What if a river runs through it - is it then a chubby river? What do you think?
At what price learning? At what cost wisdom? The price is a man's peace of mind, and the cost is his life.
With a dictionary?
I would add the condition that it must orbit a star, (to exclude moons) and not be a star itself (to exclude binary or multiple stars). And not be part of a belt of similar objects (to exclude Ceres, Juno, Pluto/Charon, and Sedna which are all spherical).
The days of the digital watch are numbered.
It could be a creek or a stream.
Clear, Dark Skies
... if it's inhabited or if it'll feed one sheep during the summer season.
Fabio Aquotte
Rocket: A small rock
Asteroid: Performance enhancer
Sun: Male descendent
Cripes. These terms are so obvious! Don't these bozos have anything better to do?
First, you have no definition of big, but assuming you mean "large enough for gravity to pull it into a sphere" then astronomers currently estimate 40-50 such bodies exist in the Kupier belt.
So, are *you* planning to memorize the names of an extra 40-50 planets over the next several decades?
Clear, Dark Skies
Planet: a Giant lump of stuff and things as rolled up by a small green prince or one of his cousins.
Don't Crease the Weasel!
Anything requiring 50% or greater of full Death Star power to destory is a planet.
Anything requiring 25% to 50% is a moon (Wait, that's no moon!)
Anything less than 25% is a rock.
(Those of other interests can substitute "Xindi Superweapon" for "Death Star" in above definition)
If it can support the orbit of the Deathstar then it qualifies.
"planet" is a social term, not a technical one and freeze the list of planets at 9.
This leaves us free to give the things we discover designations that reflect their structure or their position.
I *do* think we will eternally regret wasting so many perfectly good names on moonlets and asteroids.
Clear, Dark Skies
Any object with it's own orbital path around a star, with it's own atmosphere as faint or distinct as it might be.
But it's an arbitrary distinction since waterways keep right on changing even after they get an official name.
In my area, we've got a Perkiomen Creek that is quite wide (and serious enough to have cut a nice valley) but the Manatawny River barely qualifies as "wet" if you ask me.
Oh, and we tend to call them "criks" in Pennsylvania, too.
Clear, Dark Skies
a comet or a leaky space probe is a planet.
Clear, Dark Skies
from a great distance
ah, mod points
#define planet 1
Is there any benefits we gain from pigeon-holing something into names such as "planet"? Vocabulary is good and defining things is important but how exactly does calling something a planet help? Given that top scientists can't figure it out, its going to confuse the rest of us.
I mean, is it worth investigating planets but not asteroids? Do planets have some divine importance that other things do not, or is it just a hangover from older times when we couldn't see these things so easily and didn't know that there was much difference between the two.
In fact, is the number of planets that an important statistic about a solar system? Wouldn't it be better to have other measures like non-star mass and the number of discrete bodies of a system and then label the bodies based on their (more specific) type: To say that our solar system has 10 planets doesn't tell you as much as saying we have 4 gas giants and 4 liquidy-rocky bodies and a number of cold stones.
It just seems like we are trying to shoe horn old lingo into a more modern era where we have found things to be not so black and white. Leave a simple definition stand that most people can live by and if people are interested we can tell them a lie that is more closer to the truth.
A large spherical object with insignificant life forms living on it that get in the way of interestellar highways.
funny++
quit asking such questions
as the dictionary describes it, which is quite relevant (with exception to our asteroid belt between the Mars/Jupiter orbits)
A non-luminous celestial *BODY* (not bodies) illuminated by the light from a star, such as our sun, around which it revolves.
Comets, I think would not fit this definition, nor asteroids, simply because their orbit does not follow the regular path of orbit that the other planets do (minus Neptune/Pluto, which have switched orbits, making Neptune once again the closer 'planet' to our sun for now.)
This definition seems to work very well in many other surveys of systems that "seem" to have 'planets' revolving around them. Why not use it until we find another definition, one that's perhaps more accurate than the one we use now, defined from the very planet we live upon?
Still waiting on Serviscope_minor to wake up to fucking reality and realize that Jessica Price isn't going to fuck him.
it's nearly impossible to convince everyone otherwise. One scientist was being interviewed on NPR after this latest discovery and said he and many scientists used to like to argue that Pluto was not a planet, but have now given up the fight due to overwhelming public opinion that it's already been a planet for many decades now, so why change our minds? And if Pluto is a planet, this newly discovered object must be one too by the same criteria. (Unless you disqualify it on the basis of its inclined orbit.) It'd probably be even more contentious than switching to the metric system. At least most of the world managed to do that though.
If you want additional caution, there is always the duck test.
What the heck do they teach kids these days?
Saturn and Uranus were titans - beings that came before the gods. Neptune was named in modern times, but they kept the roman naming tradition, same with Pluto, Roman god of the dead.
And then they proceeded to waste all the other greek and roman names on every rock, brick and crater they could find which is why we're reduced to naming moons after Shakespearean characters.
The naming of Charon was a slick trick - the discoverer specified that the name be spelled like the name of the mythical figure, but that the name be pronounced "Sharon" - which happened to be the guys wife.
Clear, Dark Skies
Right over his head...
by that definition, every single one of the tens of thousands of Kupier belt objects (of which Pluto is one) are now planets.
And I dispute your claim that asteroids have irregular orbits, Ceres and Vesta have orbits every bit as predictable as the Earth's and so do most other asteroids.
Clear, Dark Skies
Why not just admit the word has no useful technical definition and leave it at that?
Clear, Dark Skies
The center of gravity of the combined earth-moon system is beneath the crust of the earth.
Think about it - the moon is only a few percent of the earth's mass - as we orbit around the sun together, the moon does all the swinging around, the earth just wobbles a bit.
Clear, Dark Skies
Major Planet: Sufficiently large to maintain it's own atmosphere
Minor Planet: Sufficiently large to maintain spherical shape
Currently practice demands the following exception:
may not be orbiting another body except a star, or similar body.
Pluto has the problem that most of it's atmosphere is essentially frozen.
Titan maintains an atmosphere via gravity stress from Saturn generating heat.
"It is a greater offense to steal men's labor, than their clothes"
Anything comparable in size to Marvin's brain.
Stupidity... has a habit of getting its way.
1. Has or has had liquid core.
2. No natural fusion reactions going on.
3. Big enough to have energetic fluids on surface (weather or similar fluid mechanics).
Life definition: A planet is any body which is sustaining life on an indefinite or permanent basis. Under this definition, a small asteroid or comet which contains cellular life which is doing it's thing would be a planet, if such exists.
Human centric definition: A planet is any body on which humans or humanic entities are indefinitely or permanently living, whether or not using technology to enable it. Under this definition, only the Earth is a planet so far, but Mars, Venus, the moon, and even Mercury could potentially become planets.
The second definition agrees well with the Kirk test mentioned above.
...not that my opinion really matters much to the world of astronomy but...
1) Spheroid shaped.
2) Orbits a star itself directly, and not primarily orbiting another planet (i.e a satellite of that other planet), though if a pair of planets orbit each other while the pair orbits their star then I'd give them both credit.
3) Big and massive enough to have a notable about of gravity itself, probably at least a quarter of the Earth's gravity.
4) ???
5) Planet!
The existing "classic" 9 planets retain their status, even if they defy whatever definition they come up with, I don't really care what definition they use. They can always make explicit exceptions for historical reasons.
File under 'M' for 'Manic ranting'
A botanist, a landscaper and a plumber walk into the forest. The botanist says "that's a wonderful specimen of Acer rubrum!". The landscaper says "Yes, a very nice Red Maple.". The plumber says "Are you guys talking about that tree?"
The different names are useful for different contexts, but they're all correct and proper. A layman is going to use the generic term "tree" because they don't really care about what species it is. A landscaper will use the more common name because they have to be able to distinguish different types of trees. A botanist will use the scientific name, because trees in other parts of the world have different common names, but scientific names are the same.
The naming system for planets needs to work the same way. A layman can call Pluto a snowball, and they're right. A school teacher can call Pluto a planet, and they'd be right. A scientist can call Pluto a Kuiper Belt Object, and they're right.
Pluto is a ball of ice, which describes composition. Pluto is a planet, because it's been so for the last 75 years; it's like trying to say Columbus didn't discover America. Pluto is also a Kuiper Belt Object, but that doesn't describe anything unless you understand the context; most people you ask "what is the Kuiper Belt" would respond "something to hold up Kuiper's pants".
I think the IAU need to take a step back, and look at biology for a minute. Come up with something like a binomial or trinomial nomiclature system for astronomy, and stop dicking around with common terms like "planet" and "star".
I hate it too when my expectations for good types of trips are dashed - where are my tickets to Disneyland, Zonk?!?!
"A planet is an object that is massive enough to become spherical under its own gravity, yet not big enough to produce nuclear fusion in its core."
Sure it would make a few hundred objects in our own solar systems be planets, but so what ? Who's afraid of big numbers ? Is it that it would be a problem to keep up making new schoolbooks ? Just list the first 9 discovered planets and be done, who exactly expects anybody to learn entire catalogs of celestial objects and for what purpose anyway ? Use the opportunity to introduce the problem of classification of objects and you've got a good way of jumping into a description of various kinds of stary objects.
lone, dfx.
i think that there are four section for defining objects in the solar system
moon: objects which are in eccentric or non-eccentric orbit around an object which is not a star and regardless of size.
minor planets: objects in eccentric orbits of a star(s) and are equaled to or less than 3000km accross.
planets: objects greater than 3000km accross and have non-eccentric orbits around a star(s).
eccentric planets: objects of greater than 3000km and have eccentric orbits.
i don't think that pluto is a planet. it was always assumed that there was a ninth planet, way before it was "discovered", and it was looked for and when clyde w. tombaugh seen pluto on the negative, then he assumed that it was the ninth planet that everyone belived was out there and hay presto; we have a new planet. and anyways....these people didn't even know what the earth looked like....why should we be the laughing stock of the galaxie because people 100 years ago could tell a planet from a minor planet....god, the emails i get......"no no no no.....don't do that....please i need to go to bed! it's four o'clock in the morning!!!!" me to my computer..
Neptune is, on average, more in Uranus's region of space than Pluto's. (No first grade humor intended)
Then again, what do you expect from slashdot?
Wait... if Pluto doesn't qualify then why would Mercury? Thee most obvious reason something would be a dominant body would be if it had moons. Neither of those do. So what makes Neptune dominant over Pluto that doesn't make Venus dominant over Mercury? I would have guessed if they were that far apart (i.e. not interacting notably most of the time with any other non-star stuff) then it would be dominant. Which would include both Pluto and Mercury.
Cheers.
First IANAA, but for me the purpose of the definition of planet operates within the context of a Solar System. And there are a number of astronimical concepts, for most of which I do not know the technical details. But, it seems to me that the set of planets within a solar system come from the set of objects that orbit around the star(s) in question.
Then I think we add a proviso that the object must nver have a retrograde movement in relation to its orbit around the star. This would eliminate any object that orbits an object that orbits the star(s) in question. (I think)
Then perhaps we can use the lagrange points to help eliminate the small and/or the eccentric. This is probably some function of the radius of the Hill sphere since that radius is a function of the mass of the objects (the object to be classified and the mass of the start(s)) and the distance from the object from the planet.
Why is this definition useful? well, perhaps it isn't however in my view if we ever get of this rock then objects with somehow conveniently sized Hill spheres might have some suitable orbital features that make them qualitatively different to those objects that do not have such conveniently size Hill spheres. In fact, it is this issue that is at the source of my "astronomical feature" definition since objects that fall into the category (however it is defined) are inherently more useful than ones that do not. they would be the planets.
Just a thought
"The first thing to do when you find yourself in a hole is stop digging."
The largest celestial body in a group of celestial bodies orbiting a large celestial thermonuclear fire (a "star"). "Group of celestial bodies" means what we perceive as a planet as well as the things in orbit around it (moons, rings, etc.).
Maybe add "and whose group's barycenter (basically the center of gravity of the entire group, moons and all) remains inside the celestial body", which would count out Pluto as well as the largest asteroid in the asteroid belt.
Does it really matter to scientists? I mean, really, regardless of how we define "planet", it is only semantics. The definition is not going to change a single scientific property of a single object in the universe. So I'd say from a purely scientific standpoint, it is irrelevant. On the other hand, from a PR standpoint, it could end up being very important. If I was an astronomer, I'd vote for a definition that is as broad as possible without over-doing it. Think of it - you are asking a funding agency, congress, etc for money to study object X. You are much more likely to capture the agency's (and the public's) attention if you say you are studying a planet. Should any intelligent society be ignorant of the planets in its own solar system? What will our galactic neighbors say! On the other hand, try getting funding to study "one of the several thousand objects beyond the orbit of Neptune". Just doesn't drum up the same level of excitement, does it?
i'd say a planet is anything that occurs naturally in space that can maintain a signifigant amount of life
A planet would be any object or group of objects orbiting a star that cannot be readily mistaken as a common member of a belt of objects in said orbit.
The implications are straightfoward.
Any well known object in a belt can be considered a planet. A planet is simply a landmark in a field of debris. Be it Earth, Pluto, or the best little truck stop in the entire asteroid belt. Its a landmark convieniently named for reference.
The term planet is not scientific. Further, there is no scientific need to differentiate between an object which is a planet and one which is not. Scientific dialog must be in terms of characteristics of objects.
Any debate within the scientific community over this particular point is merely an excuse used to defend an individuals or groups lack of rigor or diligence in their professional communication.
There's a your-mama-is-so-fat-joke just waiting to be written. In fact, your mama's so fat, that I don't even need to write the 'your mama's' joke about defining planets, because even the geeks on slashdot already been there, done that and got the t-shirt. It should be on thinkgeek by tomorrow.
...and other related terms
Planet: Any natural roughly spherical object (a) orbiting a star or stars (b), with no other natural objects in the same orbit around (b) which are not orbiting said object (a), and which is in a stable orbit around said star or stars (b)
Planetoid: Any object which would be a planet as defined above, other than not being in a stable orbit
Rouge Planet: Any object which was once a planet, and is no longer in an orbit around a star or stars
Moon: Any natural object in stable orbit around a planet
- Just because we CAN do a thing, does not mean we SHOULD do that thing.
What the heck. I'm an ignorant layman who'd probably look in the wrong end of the telescope, so of course I'll weigh in with an opinion. It's the Slashdot Way.
:-)
My goal: to be conservative and preserve the status quo. Nine planets was good enough for me, dammit, and it should be good enough for all of humanity until the end of time.
After five entire minutes of thought (and man do I need a nap now), I came up with this:
1. Orbits the sun.
2. Is not a star. Planets are fusion-free zones.
3. Gas giants count, as long as there's none of that nasty fusion stuff going on.
4. Has enough size/gravity to hold an atmosphere of X% of the pressure of Earth's, if it were in the same orbit Earth is. Initial value of X is 10, subject to change. Actual PRESENCE of atmosphere is irrelevant. It's the mass that matters. This means that small, very dense objects could be planets, and things made of duffuse materials would have to be exceedingly large.
5. Tweak the number in 4 (10%) up or down to the minimum required to let the existing 9 planets remain as planets. Ideally, tweak it high enough to shut out all these silly pretend planets people keep coming up with.
Everyone knows there are nine planets, so by golly, we rejigger the rules until we have nine planets.
Because the International Astronomical Union always turns to the Slashdot community for answers to the really tough questions.
I think it should be based on the gravity power level it produces. black holes should be planets not filled with stuff yet. lol
planet Audio pronunciation of "planet" ( P ) Pronunciation Key (plnt) n. 1. A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves. In the solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. 2. One of the seven celestial bodies, Mercury, Venus, the moon, the sun, Mars, Jupiter, and Saturn, visible to the naked eye and thought by ancient astronomers to revolve in the heavens about a fixed Earth and among fixed stars. 3. One of the seven revolving astrological celestial bodies that in conjunction with the stars are believed to influence human affairs and personalities. http://dictionary.reference.com/search?q=planet I think if we need to define something that we already know what it is, we have some serious issues on hand.
I would suggest that the definition of a planet be an object of sufficiently large size that it became rounded by its own gravity. PLUS, it must have been formed through accumulation in the accretion disk of the planet it currently orbits. Thus, objects like Pluto would not be considered planets (as they were not formed inside of our solar system).
No comment.
What label you put on an object does not change its physical properties. Come on, people, how many good scientists are wasting their time arguing over useless titles? I couldn't care less about the definition of the word "planet."
A Planet is an object that orbits a Star NOT belonging to any other well-defined category according to region, size and composition besides gaseous-giant or terrestial-tectonic. Conversely, and for clarity, any object that orbits a Star, either independently or in conjuction, readily belonging to an additional well-defined category is called a Planetoid. In so far as a particular usage may be considered irregular, not of a well-defined cataegory, or referring to the superset of all Star-orbiting objects, the term Planetary Object is used in conjunction with the specified capacity, although colloqually the term Planet may be used in it's place.
So...
Pluto is a not a Plant, but rather a Planetoid, b/c it can be readily categorized along with the other Kepler Bodies (as a matter of region, size and composition). And Jupiter is a Planet b/c it cannot be readily categoized with anything else besides the other gaseous giants. Likewise for the Earth, being terrestial and tectonically active. But not the Earth's Moon b/c it's not tectonic. Nonetheless, Pluto can be referred to as a Planet casually, as may the Moon, as a corse synonym for Planetary Object.
So the terms take on a percise meaning, but one that is also somewhat flexible according to usage.
Anyhow, that's my 2 pennies.
:T:R:A:N:S:
That a publication entitled "New Scientist" should publish this article is an irony, because to invent and dispute definitions is to carry on the scientific tradition of Aristotle.
* Must orbit a star, in any form (i.e., brown dwarf, red giant, typical main sequence, etc). While I suspect there may be planets out there that are not orbiting stars, my bet is that they are rare enough to warrant their own title. Anything else is most likely not something that might be considered a planet (i.e., comets and the like, which may not be in orbits around stars).
:) It's the planet, the smaller body is a moon. Call me when we find a system where the objects are identical in size!
:) I don't want to see a bunch of asteroids being called planets because they meet the other criteria. I'm sure we'll discover some interesting exceptions, but for the most part I think this will wind up being true.
:)
* Must *not* orbit another body that itself would qualify as a planet (yes, I realize that technically our moon orbits the Sun just like Earth does, but the distinction is which body exerts the most direct gravitational effect). It seems reasonable that if an object is orbiting something we call a planet, it probably should not be a planet itself, it should be a moon. A problem here is of course when we discover a binary planet system where both objects would otherwise be called a planet. I say tie goes to the larger object
* Must be larger than our own moon by at least 10%. This is the most arbitrary of the criteria, but I think of it like this... we need a known reference point for comparison, and since no one wants to call the moon a planet, I'll add a little and call it a selection criteria!
* Its orbital path must be something close to circular, perhaps to within 25% or something... all the planets in our own solar system are someting close to circular orbits (I don't know by what percentage they vary away from a perfect circle, but I'm guessing it's no more than 25%). Anything in a more eccentric orbit is probably a comet or meteor or something like that, and this rule should help keep it that way.
* It must posess some atmosphere. A body with no atmosphere probably isn't large enough anyway, but I think its important to state it. It can be as thin an atmosphere as you like, so long as it has something.
Eh, I'm sure there's all kinds of flaws with those criteria, but no one else seems to be coming up with the obvious right answer either
If a pion (n-) collides with a proton in the woods & noone is there to hear it, does lamdba decay into the source pa
I like the definition being:
Planet: a body large enough to be be pulled into a roughly spherical shape due to gravity. Note: not nessacarily orbiting a star, there could be rogue planets wandering our galaxy, in the spaces between stellar systems.
Then you can have
Minor Planet: a planet not large enough to hold onto an apreciable atmosphere. This would make mercury a planet, but make it a minor planet. Ceres would also be a planet, but a minor planet. Pluto gets to remain a planet.
Major planet: a planet large enough to hold onto an apreciable atmosphere. Mars, Venus, Earth.
Gas Giant: a major planet whose majority of mass is in its atmosphere. Neptune, Uranus, Saturn. I believe these are thought to have rocky cores, but they are mostly thier gases.
Brown Dwarf: A gas giant, which is nothing but atmosphere. Some theories say Jupiter doesn't have a core, it's all gas to its very center. Some refer to it as a failed star rather than a planet. Hey I'm all for bumping Jupiter up in status.
If Mr. Edison had thought smarter he wouldn't sweat as much. --Nikola Tesla
I don't care what is decided, so long as Earth is an M-class planet.
At the very least, a planet meets these criteria:
1. Mass is too low to allow for thermonuclear reactions.
2. Large enough that they are formed into a spherircal shape by the force of gravity.
3. Rotates a star.
Other than that, though, the definition of a planet tends to lose meaning. Is size a relevant criterion? Maybe, but there are dozens of Kepler belt objects that meet the definition. Bigger than Pluto? Okay, but it's still rather arbitrary.
You also get funky objects like Triton (one of Neptune's moons) that was once a Kepler belt object around the size of Pluto, but was captured by Neptune's orbit.
Sounds like plenty of good reasons to just abandon the term "planet" in scientific discussion altogether.
Before answering this, perhaps one should answer 'What would the definition of a planet be used for'
Burn the land and boil the sea, you can't take the sky from me
Well the definition must classify a body as planet based on atleast the following facts : 1) Mass (there should be a lower limit to mass) 3) Density (both upper and lower limit should be applied to density) 2) Shape (this should eliminate most of the asteroids) 3) Size (The lower limit should be pluto) 4) The Orbit. (The orbit must centre about a star, satellites will be eliminated) 5) Composition (ie. % solid % gaseous and % liquid) All the limits should be set such that the existing objects classified as planets should fit in. This is necessary to avoid unnecessary confusion in public.
who the hell cares?
Is there anything at stake here? Does a hunk of rock get more representatives in congress if it's called a planet? Is labelling something a planet going to change our knowledge of its shape, composition, orbit, history, appearance, or anything else meaningful? Is the word "planet" a natural phenomenon, the definition of which we can discover through scientific investigation? Is there any realistic circumstance in which agreeing to a formal definition of planet is going to help scientific communication any more than simply being explicit when it's important? (Bearing in mind that agreeing to a formal definition means we'll have to annotate all uses of the word, formal and informal, for the next fifty years.) And most importantly, if some committee settles on a definition that excludes Earth, will we all have to leave and go find a real planet to live on?
There's nothing more irritating than a debate about terminology masquerading as a scientific or philosophical debate. Can someone convince me this isn't just yet another example of this annoying pastime?
I'm gonna let out my inner geek on this one.
I think "planet" can be defined in a fairly simple way that would be satisfy nearly everybody
A planet is
I think the second point is the key one. I think to be called a planet, the object has to obey the "what goes up must come down" rule for the range of human-powered activities. If you set a golf ball on the surface and take a whack at it, there should be no doubt that the ball will follow a suborbital trajectory.
The bottom line here is that most professional astronomers don't care about these objective definitions. When astronomers are doing research, none of it hangs precariously on the definition of planet or asteroid or something else. They specify what they mean -- Jupiter, or the major gas giants, or the Earth-crossing asteroids, or transneptunian objects, or plutinos.
These names (planet, asteroid, comet, etc.) are just arbitrary labels invented by people, after all. They have no special significance, and they never have. After all, planet comes from the Greek word for "wanderer," a reference to the fact that planets appeared to be stars in the sky that moved. Asteroid means "star-like," a reference to the fact (as astronomical observations improved) they appeared to be moving objects that didn't have observable disks like the other planets (because they are too small).
The IAU, the international organization responsible for such names, has never given them any objective definitions. Why? Because they don't need any. Sorting out terminology like this is almost completely ancillary to getting actual astronomy and astrophysics done. The very reason that those interested in establishing definitions can't agree on objective definitions underlines the point: because they are totally arbitrary and not very important.
Almost all of the furor about redefining terms, recategorizing objects, demoting planets and promoting asteroids, has come from amateurs and the popular media. Don't you think that if professional astronomers thought that this was such a crucial issue that they wouldn't have taken care of it handily? They haven't because it's not nearly as important as amateurs seem to think. That amateurs and the popular media are seemingly fixated on such trivialities indicates strongly to me that they're missing something: namely that these classifications have no external significance.
The map is not the territory. Give it a rest, already. I know, why doesn't everyone concentrate their energies on doing actual astronomy?
If the name is essentially something that is going to be useful then I'd say over a certain mass, or perhaps over a certain GRAVITY. So if someone says - hey there's a planet over there, you know you can land and go for a walk without floating off into space. :) So solid form (?) and reasonable gravity.
To follow up my own post, even the IAU is stumped on what to call objects orbiting brown dwarfs.
Amen.
My theory: A child can point to Pluto on a map of the solar system though he cannot at all understand Newton's abstract conjectures. His intellectual relationship with the natural world begins when he learns about the various animals and, say, the reptiles---that is, when he discovers various concrete things and is taught to classify them.
It seems that long after such children grow up, they yet expect scientific inquiry to proceed along those lines. But science grew up long ago.
The definetion will change with changes in scientific instruments, which change with technology, and ultimately from science. Pluto was classified as a planet when discovered because the instruments used could find nothing larger in space at that distance. When the instruments became more detailed in thier findings, suddenly there were more objects then Pluto at that distance. Nature is seldom ever the black and white contrast that reductionist science likes to catergorize.
"What has weighed down everything?
What is most extensive?
What is the one thing that has
All under its control?
Name has weighed down everything;
Nothing is more extensive than name.
Name is the one thing that has
All under its control."
Buddha
Why is there a committee to decide this? What purpose does it serve?
-- Tom Rathborne
Since it doesn't really matter, surely we should just come up with a definition that leaves us with the existing 9 planets. Then we won't need to rewrite any books, or cause stupid arguments.
In my opinion size doesn't matter. Anything that orbits the sun should be a planet, and anything that orbits a planet is a moon and so forth.
A planet is any non-star whose immediate orbit is around a star.
Yes, this makes for tiny planets. Too bad; deal with it. Anything else is arbitrary.
If the definition of "planet" isn't specific enough that astronomers can use it, then I don't think it should be used when teaching children either. Astronomers still need to be able to communicate with the general public.
I agree. Now if the schools and media would just start using parsecs instead of light years, and, for that matter, grams and liters, instead of pounds and gallons, we would be getting somewhere.
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There are exactly two types of planets. Terrestrial, or rocky, planets and gas giants. Terrestrial planets consist of heavy metals, rocks, etc. Gas giants are mostly gas. A planet must fall within one of these categories. In addition, it must be large enough to be a sphereoid and it must not be part of a debris field. This gives us eight planets. Four inner terrestrial planets and four outer gas giants. The rest is destritus. Pluto is not a terrestrial planet because it is severely lacking in heavy metals and rocks, so it is not a planet. Same with the new "planet" and Sedna and any other large KBO we find.
It does sound as though "planet" is a bit of arbitrary distinction. Does science need it, or is is really only useful as a layman's word?
Still, if I had to, the one I'd choose is: "non-fusioning sun-orbiting object made spherical by the force of its own gravity".
This would only be marginally useful in our own solar system, but as we discover more and more planets around other star systems we can have a mathematical basis in which to designate an object a planet or just some orbiting crap, plus it instantly give a relationship to our own.
The sun:earth ratio is 333,000:1
Make that the S:E ratio (for arguments sake).
Jupiter (I think) is 33,000:1
You could call that a deci-SE etc (obviously better terminology needs to be used) We compare the gravity of other planets to Earth's (g) so we can always use the Sun:Earth ratio as a gauge for all other planet discoveries
By their shape we shall know them - An asteroid could be odd shaped - a planet is always round.
Now if it has an atmosphere - and some water - and ain't to far - I might just even visit one day.
- Julian Woolford - Kimachi Media - www.kimachi.com
http://www.google.co.uk/search?q=define:planet
Khyber: "Comets, I think would not fit this definition, nor asteroids, simply because their orbit does not follow the regular path of orbit that the other planets do"
Porkchop: "I dispute your claim that asteroids have irregular orbits, Ceres and Vesta have orbits every bit as predictable as the Earth's and so do most other asteroids."
Khyber: "Did you not read the part which I specifically said excluded the asteroids orbiting between Mars and Jupiter??"
Now, which of us can't read?
BTW, "common sense" has no place at all in the scientific method. No does our "CURRENT DEFINITION" of a planet since, in fact, we don't have one.
Clear, Dark Skies
The term "planet" has no real technical meaning, we should develop descriptive terms for types of celestial bodies (rocky, icy, gassy, dopey, etc..)
Clear, Dark Skies
What is the air-speed velocity of an unladen swallow?
I'd go back to the original definition: a celestial body, visible with the naked eye from Earth, which (phenomenologically) 'wanders' or 'moves' relative to the backdrop of stars. This would include comets, the moon, and the Sun, but it would exclude Kuiper belt objects, because they can't be seen without the aid of telescopes. It might exclude Uranus and Neptune as well, for the same reason.
HTH.HAND.
Cut that out, or I will ship you to Norilsk in a box.
except for the huge PR backlash when they tried to "demote" Pluto.
I think we should abandon "planet" as a technical term and simply adopt terms that reflect reality better - "rocky planet", "minor planet", etc..
Clear, Dark Skies
is probably larger than pluto.
Clear, Dark Skies
Forget all this rubbish about being spherical. What the hell is "spherical" anyway? Most planets are NOT spheres.
Use gravity. Have the definition of a planet as being a body whose normal surface gravity at any point on its surface is not below some minimum value of X gals, where X gals is some practical value chosen for its ability to support adequately people walking and/or living for a short period on the object.
The nice thing about such a definition is that very small objects with sufficient gravity will be planets (unlikely), very large objects with low gravity will not be planets (unlikely), and best of all, even objects that are not very "round" can still be planets as they are suitable for "expeditions".
I hearby patent this method of naming planets ON A COMPUTER. The USPTO should therfore grant me a 20 year domination of the galaxy shortly.
May the Maths Be with you!
Where do you think the Kupier belt came from?
Pluto, comets, asteroids, little frozen knickknacks a quarter of a light year away, all formed from the "accretion disk".
Clear, Dark Skies
Thee most obvious reason something would be a dominant body would be if it had moons. Neither of those do.
You fail it.
Clear, Dark Skies
The best part is, Pluto will continue to be Pluto regardless of what the IAU (I Am Uptight?) decides. It was there before us, and will continue after we've been long forgotten.
"Give a man fire, and he'll be warm for a day; set a man on fire, and he'll be warm for the rest of his life
Pride and bragging rights. The guy who discovered Neptune and Pluto became celebrities, and everyone still knows what they found even if few still remember their names. The guys who hunt comets (for example) can find dozens over a career and few will know their names, or the things they found.. Discovering a "planet" is a much bigger deal than discovering "yet another rock"
Clear, Dark Skies
Pluto's orbit is eccentric enough that it passes closer to the sun than Neptune (but never gets close to Neptune - think "spirograph") It's also significantly tilted with respect to the rest of the solar system.
All the asteroids, comets and Kupier belt object exist in the context of the Solar System and were formed through the same mechanisms as the major planets.
Clear, Dark Skies
so does the Moon.
Clear, Dark Skies
* Pluto's orbit is retrograde, eccentric and tipped.
* Our moon, and several others, are larger than Pluto. In fact, our moon is three times larger than Pluto.
* Mercury has less "atmosphere" than many comets
As a word "planet" has no technical value, it's original meaning was "a star that moves" and that's probably as good a definition as any. There's no point in trying to force it into some rules-based definition when we could just as easily use modifiers to explain what we're talking about - "rocky planet", "icy planet", "gas planet", "minor planet", etc..
Clear, Dark Skies
1) Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, or Pluto.
2) Any non-star outside the solar system which can be observed orbiting a star.
Definition 2 might need some work, though.
Sedna, Quaoar, 2003UB13, 2003EL61, and 2004DW are all planets?
Clear, Dark Skies
I'd have to say that the best definition of a planet would be a spherical object that orbits a star. Would it have to be a perfect sphere? No, but it would have to look like one from a distance. For example, although Earth is not perfectly spherical (it has an equitorial bulge), it does look spherical when seen from the moon.
I have gas, but my car uses petrol.
Find in page > "Platon" > mod +1 educated
One could argue that the moon + Earth are a dual planet.
Do I get my Nobel Prize now?
You have two hands and one brain, so always code twice as much as you think!
a) They orbit a star (system).
b) Same inclination with respect to Sun, except Pluto.
c) Low orbit eccentricity, except Pluto
d) Members of a phase.
Off the top of my head, a "phase" is a series of orbiting bodies that share characteristics and whose members only exceptionally (less than half) cross over into other phases.
Hence, Phase I: rocky, iron core: Mercury - Mars.
Then comes an intervening asteroid belt, call it its own phase if you want to.
Phase II: gas giants with significant numbers of sattelites: Jupiter - Neptune.
Phase III: Kuiper-belt objects of irregular size, orbital eccentricity & inclination: Pluto, Sedna & the gang.
Hence, we live in a solar system with 4 Phase I planets, 4 Phase II planets, and a flagship member of the Kuiper belt, Pluto, which we may call a "cultural" planet, and count as a planet if we like to, not getting too emotional about it.
The advantage of this system is it's testable: when our technology matures and we can begin to observe systems around single sun-type stars without a huge failed-binary sister, then we can determine whether the phases outlined above apply to these new systems. It also predicts that a in a single-star system with orbiting rocky bodies that don't share the same plane of inclination at all, those orbiting rocky bodies aren't planets, but need a new word.
Thought experiment: an alien drops by, asking how many planets there are in my solar system, and I don't want to embarass myself. I say, "eight main phase planets."
increasing the number of planets to 40 or 50? Because that's the estimated number of Pluto-sized objects waiting to be discovered.
Clear, Dark Skies
define: planet
The existing "classic" 9 planets retain their status, even if they defy whatever definition they come up with, I don't really care what definition they use. They can always make explicit exceptions for historical reasons.
This is certainly the most common attitude, but I don't understand it - why do you care whether or not Pluto is a planet or a just one of the biggest Kuiper Belt Objects?
Clear, Dark Skies
I don't understand why there's enough controversy on this topic to merit multiple Slashdot articles over the past year.
Why all the hubbub?
Can't people just accept the fact that these celestial bodies have different qualities that defy ones ability to create a simple binary "is a planet, is not a planet" classification scheme?
Why are you letting these clowns ruin our country?
Our brain wants to label things. It's all artificial. Call it Xena and get over it.
This is *so* a perfect slashie bike shed thing to discuss...
Will make some group of astrologists mad. Why not define planet loosely, and then define classes of planets based on that definition, eg:
Planet Class 0: Any round object (surface +- 3% of flat) orbiting the sun with a minimum diameter of 2000 kilometers.
Planet Class 1: Must be orbiting its sun(s) and only its sun(s) in its planetary system.
Planet Class 2: The classic 9 planets.
Planet Class 3: Must have an atmosphere.
etc.
I don't know. How about something big and wide and round, like Liza Minelli's ass?
An object orbiting the Sun and visible by reflected sunlight. There is no official lower limit to the size of a planet, but the name has not been applied to small objects such as comets or asteroids. The name comes from the Greek word for "wanderer" and was applied to the visible planets that were observed to move relative to the fixed stars. In contrast to comets and asteroids, planets tend to be in quite regular orbits that are usually close to a single plane.
What do we want a definition of planet to be, or more importantly not to be? Knowing what we want is the first step towards a new definition... The problem is that, even though our notion of planet comes from a historical, humanocentric background, we are trying, in the prospect of our quest of absoluteness, to create a definition that is compatible with both the historical and the scientific views. But no "simple" definition will manage to do that and satisfy a majority.
If we want to do both, the historical part forces us to create a definition that categorizes the 9 objects we call planets nowadays as planets, but labels no, or only a very small number of objects, as planets. As of wether Pluto should be considered a planet or not, rewriting books and declassifying Pluto as a planet may, arguably, be a more or less reasonable idea in theory, but is definitly not in practice, for there is an incompressible and large number of people that are attached to the current classification.
Usually, we associate the notion of planet with an object that orbits around the sun and is "rather large". But in fact, "rather large" depends on the distance of the object to the Sun. For example, if Mercury was 2 times more distant from the Sun than Pluto, we would probably not call it a planet. But if Jupiter was 3 times more distant from the Sun than Pluto, its sole size would assure it the designation of planet. Besides, we could base our classification only on the size of objects, but it would allow any object the size of Pluto at the very improbable distance from the Sun of 10 times that of Pluto to be called a planet. And we do not want that, because we could one day discover a whole bunch of those and we want to limit the number of new planets to only a very small amount.
All this indicates that we have to create a classification that takes distance from the Sun in account, because it is the only means of limiting efficiently the number of eventual new planets: we can be reasonably sure that we will not discover new objects bigger than Pluto that is less distant from the Sun.
So a new definition would specify that a planet's orbit's center is the Sun, that it is "large enough" and "close enough" from the Sun.
We could formalize the "large enough" and "close enough" by an empirical coefficient, a "planetness" coefficient that would disqualify an object as a planet under an arbitrary threshold.
A purely illustrative and quick example would be something like:
Equatorial diameter: ED(planet); Semi-major axis: SMA(planet); Size factor: SF; Distance factor: DF;
Size coefficient: SC(planet) = arctan(ED(planet)/SF)*2/Pi;
Distance Coefficient: DC(planet) = arctan(DF*1/SMA(planet))*2/Pi;
Planetness coefficient: PNC(planet) = (SC(planet)+DC(planet))/2. With ED and SMA in km, SF=2 350, DF=6 000 000 000 and no care for consistent units, we could define a planet as:
A planet is an object which orbit's center is a Star, and which Planetness coefficient is greater than 50%.
Pluto would be in and 2002 UB313 would be out in this example. Mercury:85%, Venus:93%, Earth:93%, Mars:88%, Jupiter:95%, Saturn:92%, Uranus:84%, Neptune:78%, Pluto:50,5%, UB313:47%.
An object the size of Jupiter would virtually be guaranteed the name of planet, for its size coefficient is 99%, and it would have to be more than 2440UA from the sun to be disqualified as a planet. Tweaking relations would allow to have the desired behavior, like increasing the planetness gap between Pluto and UB313, or including UB313 in, or rejecting an object the size of Jupiter from a smaller distance. Finally, to take in account the notion of size of the system, we could have the size factor and distance factor depend on the system's star's equatorial diameter.
Wouldn't it be acceptable?
When I was a kid, I read a book that described Planets as large bodies orbitng the Sun, planetoids as slightly smaller bodies like the moon, and planetesimals as objects like asteroids and such.
Sounds like a cool hierarchy to me...
"Sic Semper Path of Least Resistance"
...is Urantia
I'm a sceptic when it comes to "spiritual organizations" and I have yet to find fault with this one.
some amazing ideas, and you can read it all online for free
http://www.urantia.org/
The first 8 planets fit in a relatively consistent model; when Pluto was discovered, it was far enough away that we didn't realize how different it was from the other planets. We didn't even get a handle on how small it was until (IIRC) the 1970s, which is when we realized that what we thought was "Pluto" was really "Pluto and Charon."
Pluto is really just the closest of a whole cluster of similar bodies which may be orbiting the sun as far as a light year out.
Clear, Dark Skies
And, actually, that's about where I'm at as well; there are just so many people running around waving their dictionaries or trying to come up with ridiculously complicated rules that preserve the first 9 as planets but exclude everything else.
To me, it seems simpler just to let the public have their 9 and let the astronomers use more technical terms that describe the objects rather than lumping them together.
Clear, Dark Skies
So that explains the whole WWII nonsense? And no this is not a time to invoke Godwin's Law. I am joking.
Joke:
1) attempt at creating a mirthful response.
2) Something a Slashdot Moderator will mislabel as troll or flamebait.
The definition I'd use would be: An object that has gravitational rounding, has as the center of its primary orbit the sun (or, for complex cases, orbits no more planets than suns*), and has non-uniform, non-random composition with a single core.
*This is a cheat. I was going to say "orbits at least one sun" to take into account binary star systems where the center of the planet's orbit may not actually lie inside either sun, and also binary systems where a massive gas giant takes the place of one of the suns, as far as orbits are concerned. However, that doesn't allow for extra-solar planets with no suns at all. It also doesn't allow for a case where you've a moon that orbits two or more planets, but may occasionally also orbit a sun. The Universe is complex, so complex cases will arise, so we need a definition that won't need modifying much later on.
My idea here is to approach this from two directions. The first is to look at all of the bodies in the solar system that are definitely NOT planets, and identify what it is about them that gives them this non-planetness. For example, comets are not planets. Comets have multiple rocky cores - gravity has been insufficient to combine the object into a single mass, it has merely been enough for adhesion to take place. All planets have a single core, including the gas giants. Ergo, this allows us to uniquely define comets and thus filter them out of the bodies of the solar system.
Asteroids are the next ones. It's very likely that most don't have a core at all, but some may. If they do, it'll be a single core. So, we have to look for something else to exclude these. Asteroids are very uniform in composition, as far as anyone can tell, and meteorite fragments that have reached Earth from the asteroid belt would indicate only a very limited subset of elements being present. My guess is that the accretion disk started fairly uniform but eventually acted as a gravitational centrifuge, separating out the elements by mass. The asteroids probably formed from whatever had the right characteristics to be at that distance from the centrifuge.
In comparison, Earth has everything from the heaviest to the lightest elements, in roughly the proportions you'd expect from observation of the table of the abundances of the elements. This would seem to be the case for Mars, Venus, Jupiter and Saturn. We don't know enough about the other planets to be certain, but I'd be surprised if it was any different. So, composition seems to distinguish planets from asteroids.
The problem with too simple a definition is we're going to be forever updating it, then arguing over what should keep the title and what should be downgraded/upgraded. Only, as time goes on, there will be more at stake - politically, economically, etc. Right now, we've only got a few egos and some textbooks to contend with, but that won't be true forever, so we want a definition that will need as little changing as humanly possible.
It's a small world and it smells funny; I'd buy another if it wasn't for the money; Take back what I paid (SoM)
Will make some group of astrologists mad
Not as angry as they get when you call them "astrologists." Astronomers study the sky, "astrologists" fleece the yokels at county fairs.
Clear, Dark Skies
If a planet gets kicked out of its orbit (unlikely now, but during planet formation it's surely possible), it would stop being a planet?
"It's too bad that stupidity isn't painful." - Anton LaVey
What if we define a planet as a "separate sun-orbitting object". Pluto would be OK, as are the other planets. But not those things in the Kuiper Belt.
Mercury wouldn't qualify under that definition unless you take atmosphere to mean thinner than many vacuum chambers on Earth. Many other posters have requited orbit around a star, which seems unjustified - you have the right idea in distinguishing a planet from a moon by whether it orbits another non-stellar object. Wandering planets should still be called planets. But I also think that two planets of similar size orbiting one another should both be planets - a reasonable rule would be if neither one is at least twice the mass of the other, then they're both planets. The spherical requrement is wrong since a spinning object will be an oblate spheroid; most or all planets including Earth are oblate to some extent. Futher, an iron planet formed by collision after a large impact could be very non-spherical and asymmetrical if it had already cooled before the impact. In order to have a definite, objective test that is as simple as possible, size should be the only criterion for calling an object not orbiting another planet a planet. The size requirement should be expansive enough to cover all the traditional planets, with the possible exception of Pluto. In borderline cases, and to be clear, the size should be expressed as volume, or equivalently in terms of diameter of a sphere of that volume. To make the notion of volume more precise and observable, some test for what counts as volume should be given, such as "absorbing or reflecting more than x% of visible light" or "having a density of at least 0.y g/cc".
If we want to choose a round number for the radius and/or keep Pluto as a planet:
0.419E10 km^3 - 1000km radius
Pluto 0.715E10 km^3- 1190km radius
If we want to choose a very round number for the volume and/or stop calling Pluto a planet:
1E10 km^3 - 1337km radius,
which is just too leet not to use.
"Is life so dear, or peace so sweet, as to be purchased at the price of chains and slavery?" - Patrick Henry
This week's issue of Nature has an article
;-)
titled "Astronomers reject the term 'planet'". (Subscription required for full article.)
It seems that an "expert panel" was assembled by the IAU last year to settle this and other questions. Their current proposal is to eliminate the use of the unmodified term "planet" altogether. If their suggestion is adopted, you will have to include an adjective with the term.
The remaining question to be settled is whether acceptable modifiers may include location-related terms (orbit, distance to primary, etc.), or whether only adjectives describing a property of the planet are accepted.
Thus, one suggestion is that UB313, Pluto and Charon be classified as "Trans-Neptunian planets". But some panel members object to this on the grounds that "Trans-Neptunian" isn't a property of the objects themselves; it's a property of their orbits.
The current concensus seems to be that the panel may list a few acceptable adjectives, and leave the full list as a topic for further IAU discussion.
I suspect that the issue of people being upset by the planet count being other than 9 is something that the panel would consider a joke. Neither the media nor textbook publishers should be the ones deciding technical astronomical terminology.
It does seem to some people that objects like Ganymede and Titan should qualify as planets, though their primary isn't the Sun. And the Earth/Luna pair is often described as a "binary planet", as is the Pluto/Charon pair.
OTOH, there's a certain elegance to a definition that says planets must orbit the Sun. This makes the question of extrasolar planets quite trivial to answer: There clearly can't be any. Of course, that just means we'll have to think up another name for them. It's more economical to call them all "planets", with one or more adjectives attached, to point out what sort of physical object they are.
If a Luna-sized rock orbiting a white dwarf is a planet, why wouldn't a similar-sized rock orbiting Jupiter or Saturn be a planet?
And why would people outside the atronomical community presume to pontificate on the topic? (The good old First Ammendment, I suppose.
Those who do study history are doomed to stand helplessly by while everyone else repeats it.
The only defendable definition I can think of is that of given an exhaustive list. That list should include at least nine items (Mercury, Venus, Earth, Mars, Saturn, Uranus, Neptune, and Pluto), and not include anything else in our solar system. For now and quite possibly forever, I would not include any other object.
Yes, it is ugly, but I do not see any way out of this. One always runs into questions such as:
Oh my God, that is so fucking funny! I'm laughing and my scrotum is bouncing. Oh Lord, my scrotum is bouncing!
Cyric Zndovzny at your service.
Note that above a certain size, gravity tends to compel a sperical shape. Note that Earth out by Pluto would have a frozen atmosphere as well. Possibly the best definition would be mass.
Major planet = with mass sufficient that it retains an atmosphere, baring exceptions for frozen atmospheres, etc.
minor planet = merely maintians sherical shape. Or else maybe something with enough gravity that a human cannot achieve escape velocity or orbit by merely jumping, or even by throwing something into orbit.
The escape velocity on the Moon is about 1.47 miles per second = 5292 MPH
The escape velocity on Ceres is almost 1478 feet per second = 1007 MPH
Let's assume a typical top flight major League pitcher can pitch a ball up to 100 MPH, giving us a practical lower limit to minor planet.
"It is a greater offense to steal men's labor, than their clothes"
I exploited this topic 7 years ago!
http://www.joz3d.net/html/planet-meteor.html
The first is if you can use a pogo stick on the body, but can't escape the gravitational pull with it, then it's a planet. If that "planet" is the minor influence orbiting another planet (massive or otherwise) then it's downgraded to moon.
If such a planetary or moon sized body is part of a field, then due to gravitational forces you may be able to escape one to get to another pretty easily. This would change its classification to not a planet or moon, since due to its surrounding influences, you can escape its surface (those properties would then label it as an asteroid, or simple debris if it is orbiting some category of planet).
Second: If you can't use a pogo stick then it's a massive planet.
Third: If the pogo stick (composition of the stick needs to be standardised) melts then you're getting into the category of stars...
Ok, so you lot come up with something better! :p