What is the definition of a planet?

[the_unknown]

What do you guys think should be the definition of a planet?<br /><br />The International Astronomical Union is finally going to provide a definition of planet in September. I think it'd be interesting to see our opinions before they actually come up with their definition.

 

[harmonicaman]

<br /><br />Gleaned from the web --<br /><br />POSITION STATEMENT ON THE DEFINITION OF A "PLANET"<br /><br />WORKING GROUP ON EXTRASOLAR PLANETS (WGESP) OF THE INTERNATIONAL ASTRONOMICAL UNION <br /><br />Created: February 28, 2001<br /><br />Last Modified: February 28, 2003<br /><br /><br />Rather than try to construct a detailed definition of a planet which is designed to cover all future possibilities, the WGESP has agreed to restrict itself to developing a working definition applicable to the cases where there already are claimed detections, e.g., the radial velocity surveys of companions to (mostly) solar-type stars, and the imaging surveys for free-floating objects in young star clusters. As new claims are made in the future, the WGESP will weigh their individual merits and circumstances, and will try to fit the new objects into the WGESP definition of a "planet", revising this definition as necessary. This is a gradualist approach with an evolving definition, guided by the observations that will decide all in the end. <br /><br />Emphasizing again that this is only a working definition, subject to change as we learn more about the census of low-mass companions, the WGESP has agreed to the following statements: <br /><br />1) Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System. <br /><br />2) Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located. <br /><br />3) Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is

 

[vonster]

from what ive read it seems like they are complicating it a bit more than it needs to be: IMO<br /><br />1 - Spherical bodies independently orbiting fusors (fusion based stars), regardless of orbital plane<br /><br />Upper limit ~13x Jupiter size/mass; lower limit some value slightly less than Pluto size/mass<br /><br />So this would immediately place Xena and at least one other of the newly found objects in planetary class<br /><br />2 - Sperical objects less than Pluto size/mass that are orbiting fusors are "planetoids"<br /><br />3 - Current definition for "moon" should still apply (natural objects orbiting planets or planetoids)<br /><br />4 - Current definition for "brown dwarf" should still apply<br /><br />5 - Free floating gas giants or "brown dwarf" like objects that are not burning deutronium to an extent greater than a limit (to be established) based on thermal output are "rogue planets" or "planetars"<br /><br />6 - Sperical objects orbiting either true brown dwarves or planetars should be "planemos"<br /><br />These kind of definitions are pretty broad and straightforward, of course grey areas and exceptions can and will be found .. however ...<br /><br />I see the greatest sticking point to be societal ones and not scientific ones .. for instance, objections to point #1 based on public resistance to suddenly adding 2 or 3 more planets to our own system<br /><br />But ultimately what does it matter. They are near Pluto size and orbit the sun? They are planets. It doesnt matter how many we find<br /><br />Only other alternative is to demote Pluto, which seems kind of out of the question at this point. You cant do both so pick one<br /><br />.<br /><br />

 

[observer7]

I like these but add one more condition and we can solve the "new planet" problem.<br /><br />* Must orbit the central star within 40 AU (or perhaps a ratio of the size of the star to the orbital diameter). <br /><br />40 AU keeps Pluto a planet but excludes most of the new discoveries. <br /><br />I'm a traditionalist at heart. <div class="Discussion_UserSignature"> <em><font size="2">"Time exists so that everything doesn't happen at once" </font></em><font size="2">Albert Einstein</font> </div>

 

[vogon13]

I posted a list of (IIRC) 9 characteristics, and that if a particular object had at least 5 (or was it 6?) it is to be considered a planet.<br /><br />Not to brag, but as far as I am concerned, it was the most intelligent thing I have ever seen on the subject. I'll see if I can find the list.<br /><br /><img src="/images/icons/rolleyes.gif" /><br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[kmarinas86]

The way is simple.<br /><br />Consider the following:<br /><br />Galaxies have rotation O_N.<br />Stars have rotation O₁ or less.<br />Planets have rotation O₂ or less.<br />Brown dwarfs have rotation O₂ or less.<br />Comets have rotation O₂ or less.<br />Moons have rotation O₃ or less.<br />Planetary rings are O₃ or less.<br />Asteroids have rotation O₅ or less.<br />Space Junk has rotation O₅ or less.<br /><br />Galaxies may not orbit stars.<br />Stars may not orbit planets, brown drawfs, and comets.<br />Planets, brown dwarfs, and comets may not orbit moons or planetary rings.<br />Moons or planetary rings may not orbit asteroids or space junk.<br />Asteroids may orbit space junk and space junk may orbit asteroids.<br /><br />Any object that is a planet, moon, or asteroid is a world. An object the size of some planets with rotation O₃ is both a moon, and a world.<br /><br />If an object is capable of rotation O₃ or more then it cannot be a planet.

 

[bonzelite]

not this kind of thread again <img src="/images/icons/rolleyes.gif" />

 

[enigma10]

so what would large rock with an atmosphere and 2 small moons orbiting on either side acting as a balncing force so the large rock is stationary,but no orbiting point of reference be? <div class="Discussion_UserSignature"> <em>"<font color="#333399">An organism at war with itself is a doomed organism." - Carl Sagan</font></em> </div>

 

[kmarinas86]

<font color="yellow">so what would large rock with an atmosphere and 2 small moons orbiting on either side acting as a balncing force so the large rock is stationary,but no orbiting point of reference be?</font><br /><br />The above says that a planet must have a rotation of O₂ or greater. The "or greater" part means that it does not have to belong to a sun or a galaxy as it could be a planet with a moon outside any galaxy or solarsystem.<br /><br /><font color="yellow">The way is simple.<br /><br />Consider the following:<br /><br />Galaxies have rotation O_N.<br />Stars have rotation O₁ or less.<br />Planets have rotation O₂ or less.<br />Brown dwarfs have rotation O₂ or less.<br />Comets have rotation O₂ or less.<br />Moons have rotation O₃ or less.<br />Planetary rings are O₃ or less.<br />Asteroids have rotation O₅ or less.<br />Space Junk has rotation O₅ or less.<br /><br />Galaxies may not orbit stars.<br />Stars may not orbit planets, brown drawfs, and comets.<br />Planets, brown dwarfs, and comets may not orbit moons or planetary rings.<br />Moons or planetary rings may not orbit asteroids or space junk.<br />Asteroids may orbit space junk and space junk may orbit asteroids.<br /><br />Any object that is a planet, moon, or asteroid is a world. An object the size of some planets with rotation O₃ is both a moon, and a world.<br /><br />If an object is capable of rotation O₃ or more then it cannot be a planet.</font>/safety_wrapper>

 

[enigma10]

actually, you said or less, not greater.<br /> <div class="Discussion_UserSignature"> <em>"<font color="#333399">An organism at war with itself is a doomed organism." - Carl Sagan</font></em> </div>

 

[alokmohan]

Point 1 and 3 seem to have same contents.Can you tell the difference?

 

[the_unknown]

<font color="yellow">Before reading this just remember that people used to think that the Earth was flat.</font><br /><br /><b>All planets must have all of the following characteristics:</b><br /><br />1. Spherical +/- 10% with a diameter of at least 1,000 km.<br />2. If within a stellar system it must only be orbiting the star in the stellar system.<br />3. An atmosphere, in which, has altered it's visual appearance must have been present or is currently producing 'weather' and/or it's appearance must be visually altered due to activity of plate tectonics.<br />4. Must not be sustaining fusion.<br /><br /><b>Planets containing satellites must have the following statement true to be classified as a planet:</b><br /><br />1. The mass of all satellites must be less than half the mass of the planet.

 

[qso1]

uNKNown:<br />An atmosphere, in which, has altered it's visual appearance must have been present or is currently producing 'weather'.<br /><br />Me:<br />Now Mercury joins Pluto as a potential non-planet by this criteria and Titan becomes a planet. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[the_unknown]

Ok, I changed it a little to make Titan not a planet. Still thinking of how to make Mercury still be a planet and keep it somewhat simplified.

 

[xacikhan]

"A ball among alot of different kinds and sizes of ball floating in the Universe with which we can not play like football is called planet"<br />God never wanted us to stay doing nothing thats why he created many many celestial objects on which we ll always keep on doing researches and ll never be Done.......<br /><br /><br /><br /><br /><br /><br />Improvement begins with "I"

 

[dragon04]

Fear not. Help is on the way.<br /><br />http://www.space.com/scienceastronomy/060608_planet_definition.html <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>

 

[qso1]

On your list, I think number 1 is agreeable.<br /><br />Number 2 pretty much is as well except in the case where a planet may have somehow gotten away from its host star...and being nit picky here, if its a system other than our solar system, it would be refered to as a stellar system.<br /><br />My general definition of a planet begins with distinguishing it from a star by not so much mass as by whether it generates its own energy and radiates light due to nuclear fusion processes. This could describe a brown dwarf star. If the object no longer sustains fusion, it could then be a planet by definition. Whether 13 or 30 Jupiter masses. This in part because there could be a range where nuclear fusion sustains itself. Example being some brown dwarfs at 20 Jupiter masses can be called stars because they have minimal sustained fusion powering them. Another object might be at 30 Jupiter masses with no sustained fusion reactions taking place.<br /><br />At the lower end, determining when objects such as asteroids, KBOs ect, can be considered planets...this is where I think an objects roundness comes in per your item 1 and the diameter you mention is probably right around the threshold that enough material gathers in a more circular shape in part because a 600 mile diameter rocky body when viewed at a distance shows only relatively small signs of variation in roundness.<br /><br />A small, 20 mile diameter asteroid is little more than an orbiting mountain.<br /><br />Satellite by definition is anything orbiting a planet and so far, no satellite larger than its host world has been detected although Pluto and Earths moon are large in relation to their host worlds.<br /><br />Finally, you list is a good start and as good as anything else I've seen. At this point, the astronomical community is probably wrestling with similar requirements. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[the_unknown]

Thanks qso1 for providing me with the correct term for what I was calling 'solar system.' I know that I had already learned the term stellar system once before but somehow slipped my mind. I have changed this in my ongoing list of characteristics used to classify planets.<br /><br />And I thought that anything over 13 Jupiter masses would sustain fusion which is what I was getting at, so again you have helped me with correcting this classification.

 

[qso1]

uNKNown:<br />And I thought that anything over 13 Jupiter masses would sustain fusion which is what I was getting at, so again you have helped me with correcting this classification.<br /><br />Me:<br />Glad I could help. On the 13 Jupiter mass thing, I'm not certain what the cutoff is but to clarify my comment a bit more. If we assume its 13 Jupiter masses, one day we may find something sustaining fusion at a slightly less mass. The 13 Jupiter mass is based on current understanding of stellar fusion but as we all know in science, new evidence can raise or lower the bar. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[michaelmozina]

I'm just curious how "Xena" and "Buffy" and Pluto are going to fit into this new planetary definition scheme. <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. - Kristian Birkeland </div>

 

[dragon04]

I can't remember who brought it up (maybe stevehw33?) about classifying planets by categories.<br /><br />I rather think that's what the IAU is headed towards. Rather like stars. perhaps.<br /><br />Dwarfs, Main Sequence, Giants, Super Giants. Not neccesarily that particular nomenclatiure, but you get the idea.<br /><br /> <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>