Definition of "planet"

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claerwen

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It doesn't seem that creating a definition for a planet should be that hard - just base it on mass and orbital parameters.<br /><br />For instance, min mass />= Mercury; inclination to plane of Sun's equator (not the ecliptic) <10 degrees; eccentricity <.21; not retrograde orbit; etc.<br /><br />I'm flexible on the chosen values, but I just can't see why it's so controversial. I'll go along...as long as we only end up with 8 planets <img src="/images/icons/wink.gif" />
 
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silylene old

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We did this before in a nice thread before the SDC metldown. But time to do it again...<br /><br />1) A planet circles a star or stars. If it does circle another planet, it is also considered a planet if the barycenter is above the surface of both objects ('double planets').<br /><br />2) The maximum mass is below the threshold to sustain deuterium fusion, which is about 13x mass of Jupiter.<br /><br />3) A planet's is largely composed of 'normal' matter. It is not a mini-black hole, nor a mini neutron-object, etc. (not that any of these are possible anyways). This is actually a tough definition, for as the mass exceeds about 2x Jupiter, Coulombic forces between atoms in the object core will be inadequate to prevent atomic nuclei from compressing enough to form "electron degenerate matter" (when electron degeneracy pressure exceeds coulonbic repulsion). Adding additional mass to an object between 2x Jupiter and 13x Jupiter actually makes the object's radius get smaller due to forming additional electron degenerate matter in the core. As a result, masses between 2x Jupiter and 13x Jupiter are partially composed of not 'normal' matter.<br /><br />4) A planet has enough mass to gravitationally assume a spherical or ellipsoidal shape.<br /><br />5) A planet is at least as big as Pluto. This is arbitrary and controversial. <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>
 
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Philotas

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Remove nr.5 and I agree.<br /><br />All objects that orbit a star and has enough mass to gravitationally assume a spherical shape should be called planets(for the lower limit). This the best and easiest way to define planets.<br /><br />What Pluto got to with the definition of a planet should be nothing.<br /><br />This is a nice article: http://www.space.com/scienceastronomy/planet_denitions_030227.html <div class="Discussion_UserSignature"> </div>
 
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silylene old

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<font color="yellow">What Pluto got to with the definition of a planet should be nothing.</font><br /><br />I agree with you in principle.<br /><br />However, there is a cultural component to the defintiion of a planet, like it or not. Pluto has become, culturally, a planet after 3 generations of schooling. The link to space.com artticle in your posted said the same too.<br /><br />We debated the cultural component of defining a planet in the old thread too. <br /><br /><font color="yellow">All objects that orbit a star and has enough mass to gravitationally assume a spherical shape should be called planets(for the lower limit). This the best and easiest way to define planets. </font><br /><br />Not a good definition. Your definition includes brown stars, orbiting neutron stars, and excludes double planets.<br /><br />Another simple definition I have read is that a planet is "a spherical non-fusor orbiting a fusor". However this definition allows exotic objects and excludes double planets. <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>
 
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bad_drawing

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I agree with your definitions. This would give objects that are clearly planetary but happen to orbit bigger objects (such as Ganymede or Titan etc) their proper dues. Perhaps this can be resolved by reffering to the 8 major planets that orbit on the ecliptic a title such as the Grand Planets. I can see how this can be a contentious issue!
 
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silylene old

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<font color="yellow">I agree with your definitions. This would give objects that are clearly planetary but happen to orbit bigger objects (such as Ganymede or Titan etc) their proper dues. </font><br /><br />No, it would not. The barycenter of Jupter-Ganymede, or Saturn-Titan lies below the surfaces of jupiter and Saturn respectively. Therefore, Ganymede and Titan are properly called "moons". <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>
 
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ehs40

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wouldnt it have to be spherical and i dont think that the orbital path it follows should be in the definition because we could have a planet in our solar system that orbits our sun 90degrees from the eliptic plane
 
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Philotas

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Well, that last definition of mine was supposed to replace 4) and 5) of your definitions. "All" objects was wrong, I admit.<br /><br />My point is that such an definition should easily solve the debate whether or not certain KBO`s should be called planets, wich I think they should. <br />It seems to me that people do not like the idea of that it might be 13<(could be many dozens) planets in our Solar System. The only other theory that I could support was "bigger than Pluto", the others are way to diffuse use in my opinion. <div class="Discussion_UserSignature"> </div>
 
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valareos

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I am also very confused on the contriversy, and had always thought the proper way to clasify what is, or isnt a planet, is how it was formed.<br /><br />For example, classic idea is the planets were formed by the dust disk that was leftover after the birth of our sun. Planets all formed in the same way, and as such all fall very close to the same plane as the suns equator. Even Uranus, which is tilted widly, still rotates around the plane of the suns equator.<br /><br />Same can be said of moons. No one has any trouble really determining a moon vs a captured asteroid. why? True moons tend to fall along the planets equator. Even, again, uranus in its tilted state, has its moons tilted so they are on the equator as well!<br /><br />Galileo used Jupiter and its moons to infer how the rest of the solar system ran, why can we not do the same?<br /><br />planets should not be classed by size, but by how they most likely formed.<br /><br />So here are possible adjusted definitions<br /><br />Moonlet: Object that rotates a planet near the plane of the equator, but does not have enough mass to assume a spherical shape.<br /><br />Moon: Object that rotates a planet near the plane of the equator, and has assumed a spherical shape<br /><br />Captured Moonlet/Moon: Moonlet or Moon that rotates a planet at an odd angle to the planet, most likely captured.<br /><br />Asteroid: Rocky object that rotates the sun at any angle, but does not have enough mass to form a spherical shape<br /><br />Kupiter: Icy object that rotates the sun at any angle, but does not have enough mass to form a spherical shape:<br /><br />Comet: Icy object that rotates the sun at any angle, and approaches close enough to develop a tail.<br /><br />Planetoid: Rocky, Icy, or Gas object that rotates the sun at any angle, and has enough mass to form a spherical shape, but can not sustain deutronium fusion<br /><br />Planet: Rocky, Icy, or Gas object that rotates the sun near the same plane of the equator, and has enough mass to form
 
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Saiph

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Well, I'm glad to see my definitions floating around again.<br /><br />One problem with an actual planet definition is to avoid being arbitrary. As such, most of those definitions outlined do so. Taking them all together, as a sort of checklist, you get a good feel for what I believe is a planet.<br /><br />The spherical nature of an object is still arbitrary (how spherical do you want to get?), but it was included to keep the likes of Phobos and Deimos, and other asteriods, from being full planets.<br /><br />So the original suggestion to to use mercury as a lower mass limit is to arbitrary, orbital elements are to arbitrary. If I see an object like jupiter in a pretty excentric orbit, it's still a planet, though a strange one.<br /><br />Note: We do not have a planet orbit at 90 degrees to the ecliptic. We have one, in the ecliptic, that is "tilted" ~90 degrees. So it's on it's side.<br /><br />The one reason why I don't favor using the formation of an object as the basis for the definitions is thus:<br /><br />We're running into snags and doubts about the details of our planet formation model. Various observations of proto-planetary disks, and hot jupiters, and other phenomena are poorly explained, and even contradictory to our model. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"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" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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silylene old

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<font color="yellow">Planet: Rocky, Icy, or Gas object that rotates the sun near the same plane of the equator, and has enough mass to form a spherical shape, but can not sustain deutronium fusion </font><br /><br />So suppose a planet which had been on the ecliptic got ejected into a new orbit tilted out of the ecliptic due to gravtiational interactions with a gas giant migrating inwards. Now is it no longer a planet? Of course it is still a planet!<br /><br />What if a large spherical body is 5 degrees off the ecliptic? Could ut still be a planet? 10 degrees? 15 degrees? 25 degrees? 50 degrees? What's your arbitrary definition?<br /><br />I would not make a rule that the planet's orbit has to be coplanar with the ecliptic. <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>
 
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cuddlyrocket

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We're actually talking about the scientific definition of a planet here - i.e. a definition that will be accepted by the scientific, principally astronomical, community and by those who'll go along with that. The IAU is a private body - it has no 'official' status - but astronomers tend to go along with what it says.<br /><br />Because it is a scientific definition, it cannot include arbitrary components (like the size of Mercury). The definition must relate to the physical nature of the objects.<br /><br />The initiation of Deuterium fusion seems a good distinguishing feature for the upper limit. And you could exclude exotic objects like mini black holes and such like. The problem is the lower limit.<br /><br />The fact that the object is big enough so that gravitational forces dominate over the inter-atomic forces of the material (so that it forms a minimum gravitational energy shape - i.e. roughly spherical) is a good lower limit. The only perceived problem is that this would include Ceres and some other objects. One possibility is that it is large enough to have undergone stratigraphical differentiation (like the Earth - which has formed inner and outer cores, the mantel and the crust). Anything that forms into a minimum energy shape but which does not undergo such differentiatio should be called a planetoid.<br /><br />It would be unclear at the lower end whether this has in fact happened. I suggest the best guess is taken at the time, and that this status remains until it has been shown to be false. So Ceres remains a planetoid, and Pluto a planet, until we find out for certain one way or another.<br /><br />Such uncertainties as to the boundaries are common in science - look at the definition of species for example. Or in real life - when do you become old. As long as the vast majority of examples are obviously one or the other, it's a workable definition.<br /><br />(I haven't gone into them, but all those other parts of the definition like orbiting a star sh
 
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Saiph

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my case exacly, and the reason why I posed those rules.<br /><br />Unfortunately, they'll likely never be accepted, as planets will likely always be a "I know it when I see it" definition. Otherwise there will be a flap over pluto. heck, the differentiation definition may even kill mercury. (Maybe, not sure though). <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"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" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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vidar

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What are planets, moons and objects?<br /><br />At early ages, planets were thought to be moving stars. Now it seems that some comets and asteroids orbit planets, and that some moons orbit the sun.<br /><br />The moons Ganymede and Titan are larger than the planet Mercury. The moons Callisto, Io, Månen, Europa, Triton are larger than the planet Pluto. The moon Phoebe seems to have the consistence of a comet. These facts put the pervious century’s idea of what are planets, moons and objects to a test.<br />http://www.solarviews.com/browse/misc/plntmoon.jpg <br /><br />A definition for moons and planets could be: Moons have cold cores and no atmosphere. Planets have warm cores and atmosphere. <br />But that would not be quite right either.<br />
 
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najab

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But how do we <i>know</i> if differentiation has occurred without drilling into the body in question?
 
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yevaud

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Spectrographic analysis of reflected light from the body. This gives you an idea of what the average surface material is comprised of. Then compare that to the overall mass and dimensions, hence density of the body.<br /><br />If the surface material is largely comprised of, say, frozen hydrogen, but the mass is too large for the entire planet to be comprised of H, it's a good indication that there is much denser material deeper down.<br /><br />Mars was easy, as the presence of all of those pyrites at the surface showed the effects of a faster differentiation than Earth. More dense material is present at the surface. <div class="Discussion_UserSignature"> <p><em>Differential Diagnosis:  </em>"<strong><em>I am both amused and annoyed that you think I should be less stubborn than you are</em></strong>."<br /> </p> </div>
 
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najab

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Being deliberately obtuse - how do you know that there's not just a surface coating?
 
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yevaud

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Initially you don't. But that goes back to determining the overall mass and density of the body.<br /><br />Example: if you did this with the Earth, it'd suggest that it's entire surface was comprised of various minerals, dirt, and water. Together which is not dense enough to reach the mass of the Earth, if you assumed that the Earth was comprised of nothing but. Has to be denser material underlying.<br /><br />Same thing with the new planet. <div class="Discussion_UserSignature"> <p><em>Differential Diagnosis:  </em>"<strong><em>I am both amused and annoyed that you think I should be less stubborn than you are</em></strong>."<br /> </p> </div>
 
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cuddlyrocket

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Basically, the same method we use to find the gender of an unborn baby - sonic scans. In geology, otherwise known as seismology. Just track the path of sound waves following earthquakes and other such events. Worked on both the Moon and Earth, which are both differentiated with a core etc.<br /><br />Another method is careful analysis of the path of an orbiting satellite. As planets and planetoids that rotate are not spherical (centrifugal force etc.), the path is different depending on the exact structure of the planet. But seismology is best.<br /><br />I suspect we wouldn't know whether Ceres is differentiated for a few decades yet. And quite a few more after that before we found Pluto wasn't.
 
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Philotas

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"The only perceived problem is that this would include Ceres and some other objects."<br /><br />What people think of when they hear the word “planet”, is a circular shaped object of either rock or gas. Hence I think no one would have trouble of calling Ceres a planet when they see the close-up pictures due to come when Dawn reaches it. Same is for Pluto when New Horizons reaches it. It`s just too much contraversy(sp?) swirling around in the corridors.<br /><br />Why make everything so difficult when it can be done so easy? <div class="Discussion_UserSignature"> </div>
 
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aerogi

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I think as long it is spherical, and orbits the sun, it should be called a planet. <br /><br />But they can have the following like:<br /><br />*Inner (rocky) planets: Mercurius, Venus, Earth and Mars<br /><br />*outer (gas)planets: Jupiter, Saturn, Uranus and Neptune<br /><br />*KBO planets (or something else): Pluto and all the other 'spherical' objects from that area...
 
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Philotas

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Norwegian for "the Moon" <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> </div>
 
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lunatio_gordin

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lol. Ah. I'll stick with Luna, thanks. <img src="/images/icons/tongue.gif" />
 
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