Definition of "planet"

rpmath · Jul 31, 2005

To avoid an arbitrary definition of planet, I think we must use points 1 to 3. This way we'll have many planets and then we can classify them in a little arbitrary way to match the way planets are distributed somewhere like our solar system. That's the way the Minor Planet Center does... There are Major planets and Minor planets Pluto and 2003UB313 may be among the big planets for some people but they will end among other similar size objects like Ceres, Pallas, Vesta, and Juno in the 19thcentury did... Pluto and 2003UB313 are Kuiper Belt objects not because their size but because they share a region of space with objects of similar size and orbits. Edit: I had the ” char isntead of " in the links... so they didn't work

silylene old · Jul 31, 2005

To avoid an arbitrary definition of planet, I think we must use points 1 to 3. Well I tried to be comprehensive in my definition (second post on this thread). I also think it is better than any published planetary definition I have seen published (I do read a lot). I think you would want my points 1 - 4, for certain. Point 4 is a requirement for enough mass for gravity to force the object into some form of roundness (sphere or ellipsoid, depending on rotational speed). Point 5, a requirement of being at least as big as Pluto, is entirely arbitrary and cultural (as I said before). I would expect my point 5 would be the most contentuous point, and as it does seem to be. Because the issue is cultural, rather than scientific, I doubt there will ever by consensus on this point. I am surprised no one has debated my point 3. We all seem to agree a fusor (mass /> 13x Jupiter) is a non-planet. We also seem to all agree that certain exotic objects, such as mini black holes with masses less than 13x Jupiter aren't 'planets'. But do we want to include objects (partially) composed of electron degenerate matter (mass = 2x Jupiter to 13x Jupiter) as a planet? These are also rather exotic. Many of the extra-solar 'planets' recently discovered fall within this mass range. <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center">- - - - - - - - - - - - - - - - - - - - - - </div><div class="Discussion_UserSignature" align="center">I really, really, really miss the "first unread post" function. </div> </div>

vidar · Aug 1, 2005

Let’s call it 'the moon'. My main point is that if any object like Pluto (or Shanon) is called planets, the solar system will exceed 100 planets in a short time. In my view, believing Pluto was the legendary Planet X was a mistake, - it not even a planet. Pluto, and many others, are Kuiper belt objects (KBO). We do not call asteroids in the asteroid belt for planets either.

aiserou · Aug 1, 2005

I don't understand why KBO's "can't" be planets. Why is a rocky spherical body far from the sun different from a rocky spherical body close to it?(I'm just using rocky for the example) A "minor" planet sure, but still a planet. Maybe what we need is a planet classification system, something like Star Trek. "Such and such planet is a M Class planet. That one is a E Class planet" Of course, I'm sure we could easily come up with more than 26 types of planets once we start getting good at finding small planets outside of our solar system, but thats the general idea. We could classify the planet by its: 1. Size 1. Makeup (Gas, Rocky, etc) 2. Atmosphere etc etc etc

vidar · Aug 1, 2005

In our solar system there are at least two ’classes’ of planets. They are Torrential (Mercury, Venus, Earth and Mars) and Jovian (Jupiter, Saturn, Uranus and Neptune). I do not know where to place Pluto in this. I think it still is a Kuiper Belt Object (KBO), but suppose it could be further categorized as Kuiper Belt Planet (KBP).

silylene old · Aug 1, 2005

"Because the issue is cultural and not scientific, I doubt there will ever be consensus on this point." It might be easier than that. We arbitrarily define events in existence all the time. It's pretty well agreed on that the definition of green is a certain span of frequencies of visible light. The same is true for all we do in language. We arbitrarily make sounds designate meanings for events, all the time. . ..... To go slightly off-topic, the color 'green' does not necessarily look the same to every individual. How the color green is perceived depends on the ratio of "S", "M" and "L" cones in an individual's retina...and the percentage of area of the fovea occupied by rod cells, which add a gray component to the perception. These ratios do vary from individual to individual. In some extreme cases, some individuals are completely missing one or more of the types of cone cells ('color blind') and they would perceive green even more differently. Also, due to genetic variation, even all "S" cones (etc) can vary in their sensitivity due to amounts of pigments present within the cell. Point is, what is lime green to Harry might look dark green to Mary, and might look blueish green to Tom, and might look grayish green to Bill. <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center">- - - - - - - - - - - - - - - - - - - - - - </div><div class="Discussion_UserSignature" align="center">I really, really, really miss the "first unread post" function. </div> </div>

Saiph · Aug 1, 2005

The rocky planets are "Terrestrial" not torrential. But yeah, those are the two types we have. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

yurkin · Aug 1, 2005

The problem is cultural and not scientific. I like the definition that anything big enough to become spherical under its own gravity is a planet. I like this because it gives a solid definition that isn’t open to discussion or debate. I don’t like using Mercury or Pluto as the limiting size because those definition are biased towards our solar system. This same definition can also be used to differentiate between moons and moonlets, quasi-moons and quasi-moonlets.

rpmath · Aug 2, 2005

I think you would want my points 1 - 4, for certain. Point 4 is a requirement for enough mass for gravity to force the object into some form of roundness (sphere or ellipsoid, depending on rotational speed). The roundness may be a good factor for a lower limit... It can be affected by the planet history: if it formed close or far from a star, if it had a big impact, the amount of some isotopes when it formed, etc. I remember reading something here at SDC about “onion shaped asteroids” that were melted and have layers. May be one of that can be classified as planet, but there are fragmented ones among them, and some fragments may be bigger than the round ones. I just was trying to keep minor planets as planets, and be free to classify a planet as a major one based in the way it is placed in its solar system. The roundness limit may let the most asteroids out of the “Minor Planet” group, because they are not round enough to be called Planet. I am surprised no one has debated my point 3. We all seem to agree a fusor (mass /> 13x Jupiter) is a non-planet. We also seem to all agree that certain exotic objects, such as mini black holes with masses less than 13x Jupiter aren't 'planets'. But do we want to include objects (partially) composed of electron degenerate matter (mass = 2x Jupiter to 13x Jupiter) as a planet? These are also rather exotic. Many of the extra-solar 'planets' recently discovered fall within this mass range. It seems as a good point: if it supports fusion is a star, if not is a planet. I wasn't aware of degenerate matter in big planets... but they still look as planets. I think planets from mass = 2x Jupiter to mass = 13x Jupiter look almost the same size outside, they just have more degenerated mass inside. Perhaps brown dwarfs may look as planets too... but the definition of this limit seems easy enough. To ca

ather · Aug 2, 2005

Perosnaly, I think size plays a big role. I mean, seen a small boulder's called a moon. I think 1000 Miles is a good planet Limit. Pluto fits in, Sedna, doesn't. we also need distance, too. The solar system, with all rocky bodies out there, seems to extend about 2 light years out. I'm sure we can all agree, any planet out there shouldn't count. I also think planet is no good, without qulaifiers. we have 4 Terrestrial Planets, 4 Gas Giants, and 2 Kupier Belt Planets. We do not ever say we have 10 Planets (or 9 or 8.) You have Rogue Planets for starless Planets roaming merrily through space. Maybe a Planet at the evrye dges if the System is picked up from another system that passed by. If that's possible, I say we call that an Adopted Planet. By makign descritpive names, we avoid problems of a Planetray count. We have so many of such and such planets, and so many such and other, etc. And shape matters too. Round. Orbitla degree? No. What if a close passerby knocked Pluto itno a weird orbit (asumign it would normlly have met the Planet definition scientist want to use.) I also will neevr stop recognizzing Pluto as a Planet. It mgith nott echcnially be one by any proper definition, but It's been promoted. Kinda like how Columbus never actually discovered America, but still gets the crediteebcause he lead to their discovery by America Vespucca(Sp?). Pluto elad to whole new worlds of some kind out there. Give it some propes. Also, notice how Pluto has always been dfiferent from the KBO's. Bigger, atmosphere etc. I don't know about 10thie, but previosuly, Pluto was closer to planet. And really, size wise, why does Mercury coutn, but ot Pluto? A previous generally accepted size limit above Pluto? Or maybe we of the Erath should vote on membership. Majority wants Pluto. Now, let's decide Sedna and 10thie. Ironically, in the end, we'll probbaly lsoe Earth as a planet. Why? Most Planets do not have life on the

Philotas · Aug 2, 2005

<blockquote>In reply to:<hr />I'm sure we can all agree, any planet out there shouldn't count.<hr /></blockquote> I strongly disagree. <blockquote>In reply to:<hr />Oh, I also will not recognize 24 planets. But I will recognize Pluto.<hr /></blockquote> Why is that?? As I said before we really need to remove all this contraversy. <div class="Discussion_UserSignature"> </div>

vidar · Aug 3, 2005

For all we know, Pluto could be Neptune’s moon, pulled or kicked out of orbit. I think it was a mistake to name it a planet, when what they looked for should be 3 times as massive as Neptune. I think it is best to correct that error as early as possible. It can keep its name. Frankly, I think it is a good idea to put the Earth in a special category, - it is special, - it supports life. Venus and Mercury are not terrestrial in my understanding. They can not be like Terra, - or Earth. Mars might be terraformed in a thousand years, which classifies it as Terrestrial. What about the term M-class planets? Is that just science-fiction?

majornature · Aug 3, 2005

The simple definition for the word Planet is a nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves. I describe it as a Massive ball of rock(depending on size, density and gravitational force) with a dense core that orbits a star at a time period interval. Do not make the word planet seem so difficult to define because after all, it is an object... a beautiful object I might add. <div class="Discussion_UserSignature"> We are born. We live. We experiment. We rot. We die. and the whole process starts all over again! Imagine That! <img id="6e5c6b4c-0657-47dd-9476-1fbb47938264" style="width:176px;height:247px" src="http://sitelife.space.com/ver1.0/Content/images/store/14/4/6e5c6b4c-0657-47dd-9476-1fbb47938264.Large.jpg" alt="blog post photo" width="276" height="440" /> </div>

vidar · Aug 3, 2005

I figured out myself, M-class is stars, not planets. Read all about it: http://en.wikipedia.org/wiki/Stellar_classification It might be a good idea to refer planets according to its star and distance, i.e. Earth T1G2, (Terrestrial planet, 1 AU distance to the star, G2-class star), Saturn (J10G2) (Jovian planet, 10 AU distance to the star, G2-class star). The main thing would not be whether they circle a planet or a star. In that case the ‘moon’ Europe could be (T5G2) and Titan (T10G2). Two more categories could be helpful; Moons (M) and objects (O) like Asteroids, comets, KBOs and other rocks. That would make Ganymede (M5G2), our Moon (M1G2), Sedna (M50G2) and Comet Halley (OG2R), where ‘R’ means rough, undefined distance. For objects in other star systems, like Alpha Centauri. T1G2 is a Terrestrial planet like ours orbiting the first star. J5K1 is a Jovian planet orbiting the second star at 5 AU. M10M3 is a moon orbiting the third star (Proxima) at 10A. Comments to this? (Oh, by the way. M-class planet is science-fiction http://en.wikipedia.org/wiki/Planetary_classification_nomenclature )

lunatio_gordin · Aug 3, 2005

I don't really like that. Could get too confusing with M class Planets and M class stars... EDIT: oops, you already addressed it. I really got to finish reading threads before i reply...

vidar · Aug 3, 2005

Yes I can see a problem. The star classification is official and not too well known. The planet classification is unofficial (StarTrek) and well known. So we are living on an M class planet orbiting a G class star. (M class stars G-class planets are quite different.)

Philotas · Aug 3, 2005

Before we classify planets we better define what a planet is. A planet classification that would work in our Solar System at least, although quite simple: Terrestrial Planets(the inner, rocky planets): Mercury, Venus, Earth, Mars and Ceres Gas Giants: Jupiter and Saturn Cold Giants(these are icy, whille the GG`s are 'hot'): Uranus and Neptune Icy planets(these consist mostly of ice, while the terrestrial mainly cosist of rock): Pluto, Quaoar(sp?), Sedna and 2003 UB313 Then you just make other classifications when needed, like 'hot-Jupiters'. Not sure if it smart to copy the star-classification system. <div class="Discussion_UserSignature"> </div>

vidar · Aug 3, 2005

Classification of planets has been simple until now. We relate them in our neighbourhood. Soon we will find planets in other star systems. Then the classification will be more complex. Until now, classification of the stars has been more advanced than planets, because the observations of stars are far more than planets. I think planets should be referred to their star somehow, or else we will loose track when there will be 10 planets for each star. It is not that complicated. But if we start out too simple, we will be caught up with that error and eventually have to change it. There are already problems with the definition of some planets, moons and object in our solar system. Such problems will multiply with the discovery of extra solar planets.

Philotas · Aug 15, 2005

What I posted in the poll: Objects to be 'upgraded' to planet status with the definition “A planet is a object orbiting a star with enough mass to gravitationally assume a spherical or ellipsoidal shape”: Ceres, Varuna, Quaoar, Sedna, 2003 EL61 and 2003 UB313; wich gives 15 planets(I forgot to count 2003 EL61 and Sedna earlier). To make it easier for the public to learn the names on the planets one can divide planets into two groups: ‘the major planets’ and ‘the minor planets’. The major planets are the biggest and most important planets in our solar system. They count 8 or 7, depending whether you want to call Mercury major. A new major planet must be bigger than Mecury or Mars, that also depends on ones definition. These are the planets one have to learn about at school and such, because they are the biggest, and so far have their orbits relatively for themselves and doesn`t make up a group, hence they are worthy indiuvidual facts, not like asteroids or KBO`s wich share composition etc. Still minor planets are planets, and not asteroids or comets. This way, the biggest planets can have a worthy planet status for themselves, while we still have a sharp, good scientifically definition for a planet. Once out of our solar system we don`t need major or minor planets, but just planets. Now, this defintion only work well in our solar system, so it can`t stand alone. The point with that post was to avoid what most people dislike with that simple defnition: too many planets. Alternatively one could make a special definition for our our Solar system such that one end up with 8 or 10 planets with todays discoveries. When it comes to Brown Dwarfs, I completely agree with silylene that objects bigger than 13x Jupiter aren`t planets. That definition also relates to size and mass, as the spherical one does. <div class="Discussion_UserSignature"> </div>

CalliArcale · Aug 15, 2005

One problem -- by redefining "minor planet", you are bumping tens of thousands of objects out of that category. <img src="/images/icons/wink.gif" /> Currently, asteroids and comets are called minor planets. <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

Philotas · Aug 15, 2005

Yeah, I am somewhat aware of that problem. Maybe we should call asteroids and such for 'micro planets'. <img src="/images/icons/laugh.gif" /> Minor planet can`t be used I see, but I doubt finding a new name is the most difficult problem to solve in this debate. <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> </div>

Philotas · Aug 18, 2005

<blockquote>In reply to:<hr />My view is that it’s not the position that determines whether it is a planet or moon, but rather its properties. A moon can be knocked out of its orbit. That’s what I think has happened to Pluto and Mercury.<hr /></blockquote> But that will make it very difficult to define planets: how to find out whether Pluto or Mercury was once satelites? And the only controversy about defining moons that I`ve heard about is how small a moon can be; the term 'moon' is very good and easy to define as it is. I agree that position doesn`t define a planet, but with moons that seems as the best solution. <div class="Discussion_UserSignature"> </div>

vidar · Aug 18, 2005

I think Pluto and Mercury are moons knocked out of their planetary orbit, and that they consequently are orbiting the sun. Orbiting a planet, and orbiting a star is not that different paths. A moon does basically follow a planet’s orbit around the star. There are two major problems I see with the traditional definitions. First we have found many, and will find even more, KBOs (Kuiper Belt Objects). These will be characterised as planets if not redefined. We might end up with 100 planets instead of 10 in our solar system. Secondly, soon we will map other star system with planets, moons and objects. Imagine what mess it will be for the Alpha Centauri system with it’s three stars.

Philotas · Aug 19, 2005

Well, if you read my earlier post, you`ll see that by dividing the planets into two different classes, there isn`t really more problems than those you make yourself with 100 planets in our solar system. And I don`t see why there`s going to be a problem with the Centauri System or any other double/triple star system. <blockquote>In reply to:<hr />I think Pluto and Mercury are moons knocked out of their planetary orbit, and that they consequently are orbiting the sun. <hr /></blockquote> Yes, maybe, but one cannot know, wich is important when defining. <div class="Discussion_UserSignature"> </div>

vidar · Aug 19, 2005

There are two classes of planets. There are the Terrestrial and the Jovian planets. In addition there are KBO (Kuiper Belt Object). These are official classes. I can agree that planets should be categorized into more groups. It is not hard to imagine that there are planets with properties like the moons IO, Europa and Titan. Other star system can be very different than our own. Alpha Centaury is the closest and will be the first to make us realise that. It is not hard to imagine that a planet can orbit both main stars. There might even be planets zig-zaging inbetween them. The dwarf star is likely to orbit the stars like a KBO, and have its own planets. Surely this won’t fit into traditional definitions of planets.

Definition of "planet"

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