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TheThorn
2004-May-11, 04:50 PM
You'd think if anyone knew what the definition of the word "Planet" was, it would be The Planetary Society. Apparently, they don't either, and they'd like your opinion. Follow this link to The Planetary Society's (http://planetary.org/news/2004/planet_vote.html) poll on whether Pluto or Sedna should be called planets.

We get to vote on it!

StarLab
2004-May-11, 06:01 PM
Based on my lack of knowledge and information on Sedna, I voted that it is a planet but that Pluto is not.

Spacemad
2004-May-11, 07:36 PM
:unsure: It seems to me rather a strange way of going about things - I mean asking the general public for their opinion when they can't made their minds up themselves! Surely it would have been better to present a couple of opinions to the public & then poll them on their opinion.

We have been brought up to consider that Pluto is a planet - they canīt take away its status as such now. As for Sedna I donīt think it should be considered a planet - perhaps a planetoid but not a planet! Perhaps an inferior measurement should be established - but where? I read, about the time of Sednaīs discovery, that perhaps the lowest limit should be when a conglomeration of material has reached the point where the gravitational forces constrict it to a sphere, in that case Sedna would be considered a planet & perhaps many of the asteroids would have to be upgraded, too!

Then we come to the other extreme - where do we establish the upper limits? Gas giants hundreds of times more massive than Jupiter have been discovered orbiting stars thousands of light years from Earth - could they be considered failed stars - where would a brown star fit in - is it a star or a massive gas giant?

It seems the discovery of more "objects" that donīt fit our standard method of thinking are constantly being found -Whatīs a Planet - Whatīs a Star - Whenīs a planet a planet & when isnīt it a planet? Whenīs a star a star & when isnīt it a star?

Plenty of headaches for astronomers & the like for many years to come!!!! :P

Tinaa
2004-May-11, 09:42 PM
My astronomy professor and my astronomy textbook claimed pluto was not really a planet. So I'd vote no on Sedna.

Fraser
2004-May-11, 10:36 PM
The best description for a "planet" is: one of the nine objects in the solar system people generally consider to be planets. I think that makes things perfectly clear, and nicely ends the age old "is Pluto a planet" problem.

StarLab
2004-May-11, 10:57 PM
I like that; instead of a general term, we have a nickname for the nine things orbiting the sun that were discovered first! Fraser, you da' man! ;) :D :P B) :)

Algenon the mouse
2004-May-12, 01:03 AM
I think Pluto is a planet. It is rather small, but it does have a moon that is rather large in size compared to Pluto.

Did you know that for awhile Pluto was closer to the sun than Neptune? Neptune and its ecentric orbit..hehe.

TheThorn
2004-May-12, 02:34 AM
I'd agree with Fraser's suggestion (in fact, I've made the same suggestion myself in other threads) except for one problem. Extra-solar planets. That definition falls down when we start looking at things that orbit other stars.

My vote was that neither Pluto nor Sedna are planets. There are just too many things out in the Kuiper Belt that are almost as big as Pluto to continue thinking of it as something other than the largest (currently known) member of a very big family, the Kuiper Belt Objects. I wouldn't be surprised if they find another object (or objects) slightly bigger than Pluto out there too.

StarLab
2004-May-12, 02:35 AM
I think Pluto is a planet. It is rather small, but it does have a moon that is rather large in size compared to Pluto.
Soo...Algenon....what you are saying is that an object that orbits a classified star, which is orbited around by another body, is considered a planet.

So here is a list (from larger to smaller):

Stars orbit galaxies
Planets orbit Stars
Satellites orbit planets
Stuff orbits moons

Y'know, Algy, I like that idea: that celestial objects that orbit stars that have satellites orbiting them should be considered planets...that's not a bad definition.
In this scenario, Pluto is a planet, Sedna is not.

GOURDHEAD
2004-May-12, 12:25 PM
The word "planet" means wanderer. Any object orbiting any star as seen from another object orbiting that same star will appear to wander against the background of stars. I'm sticking with my suggested description posted in another thread. I'm willing to let comets retain their special category. but asteroids, even at the micro-gram mass level, become planets as noted. We could consider eliminating objects with masses at or below what would be totally consumed in the atmospheres of objects with equal to or greater than one earth atmosphere to simplify the bookkeeping.


For those who are interested in whether a stellar orbiting body is a planet or not I suggest categories of planets with earth as a standard as follows:

Hyper planet Above 10 jovian masses and below 100 jovian masses or
the mass at which sustained fusion of hydrogen begins

Super planet one to <10 jovian masses

Large planet 10 earth masses to < 1 jovian mass

Planet 0.5 to < 10 earth masses

Hypo-planet 0.01 earth mass to < 0.5 mass

Micro-planet < 0.01 earth mass

The above categories will provide a useful set of handles and preserve the meaning of the word planet as wanderer which each stellar orbiting object satisfies.

Asteroids would become non-stellar objects in their own galactic orbit.

Planetwatcher
2004-May-12, 03:50 PM
GOURDHEAD makes a valid point here.
The word "planet" means wanderer. Any object orbiting any star as seen from another object orbiting that same star will appear to wander against the background of stars. This is one of the few things I seem to remember from school, back in the old days when we had to fight off the teradactals and rodans with spears, and fire. :lol:

I think Sedna should have the same duo status as Pluto. If for no other reason then to have a tenth planet, and an overlapping means to distinguish the Kupier objects such as Varum and other bodies simular in size.

TheThorn
2004-May-12, 04:34 PM
Originally posted by StarLab@May 12 2004, 02:35 AM
Y&#39;know, Algy, I like that idea: that celestial objects that orbit stars that have satellites orbiting them should be considered planets...that&#39;s not a bad definition.
In this scenario, Pluto is a planet, Sedna is not.
The problem is that under this definition, several dozen asteroids and KBOs (including some that are so small they aren&#39;t spherical) would be classified as planets, but Mercury and Venus wouldn&#39;t make the list.

DippyHippy
2004-May-14, 09:23 PM
I think Pluto&#39;s a planet and Sedna&#39;s not, if only because we didn&#39;t know about KBO&#39;s et al when we discovered Pluto and that for 60 years it was known as a planet. If Pluto were discovered today, there wouldn&#39;t be any debate - it wouldn&#39;t be considered a planet at all.

I say leave it well alone - Pluto should stay a planet but Sedna&#39;s definitely not.

Fraser
2004-May-15, 04:53 AM
A more scientific description would be: a planet is an object which is very different from its orbital environment. In our case, Pluto wouldn&#39;t qualify because it&#39;s only a larger Kuiper Belt Object, not a dramatically larger object like the other 8 planets. Sedna wouldn&#39;t qualify either.

But with our own Solar System, I&#39;d say we give Pluto a pass: it&#39;s a planet.

zrice03
2004-May-20, 05:52 PM
Think that Pluto will remain designated a planet simply bcause of tradition and inertia. Like, we say "Newton&#39;s Law of Gravity" but "Einstein&#39;s Theory of Relativiy" even though they&#39;re both physical laws and both pretty much proven.

Of course, what will really shake things up is if we find another KBO that&#39;s bigger than Pluto.

Greg
2004-May-23, 08:20 AM
A new classification system is desperately needed. Especially since we will need it to make some sense out of all of the extrasolar planets we are about to find in the future. I never believed that pluto should have been classified a planet. I do not believe Sedna should be classified as one either. There are 3 main characteristics in my opinion that define a planet, and to be called a planet a chunk of rock should meet all 3 criteria. One is its diameter. I personally have a problem naming something a planet which is smaller than our own moon. A minimum diameter should be set, say 5000 miles. Perhaps a mass fudge figure can be thrown in as well to help objects such as Mercury. Second is its orbit. The orbit of the planet should be within the planetary disc and similar to other planets in the system. This would work against an object like pluto. Similarly the object cannot orbit another object, otherwise it would be a moon. Lastly is composition. There is a big difference in the composition of the surface of objects in the kuiper belt vs inner terrestrial planets and gas giants.

TheThorn
2004-May-24, 12:58 AM
I don&#39;t think it will be long before people stop referring to Pluto as a planet. Ceres was referred to as a planet at first, as were the next three asteroids. But a half a century later, when they started discovering dozens more, they realized that these things weren&#39;t really planets. History is just repeating itself.

I don&#39;t think it will take a ruling of some authority to make the change. People will just gradually realize that Pluto is not really a planet. In fact, I think it&#39;s already starting to happen. The collective name for the Plutinos, Cubewanos, and Scattered Disk Objects (and "Inner Oort Cloud" objects <G>) is "TNOs" for "Trans Neptunian Objects". It seems like we already know that Neptune is really the furthest planet.

But we certainly do need a more precise definition.

Here&#39;s mine. Big, but not big enough to start a fusion reaction of its own, following a relatively circular orbit close to the plane of the equator of the star that it orbits.

Draw the "big" line using mass at, say, 10^23 kg. Mercury is over three times that big, but Pluto misses by a factor of ten.

Draw the "relatively circular" line to include Mercury, and it will have to be e < .25 or so which would not, by itself, exclude Pluto.

Draw the "close to the plane" line at 10 degrees, and Mercury is in (7 degrees) but Pluto is out (17 degrees).

I don&#39;t know if the orbit criteria are all that important. What if we find a Jupiter sized object in an eccentric polar orbit around some star somewhere?

But the size criterion is key. There&#39;s a big gap in size between Mercury and Pluto - Mercury is 30 times as massive as Pluto. Pluto is about twice as massive (as best we can estimate) as the Sedna or Quaoar. If we&#39;re going to draw the line somewhere, it makes sense to draw it where there&#39;s a big gap.

BTW, there is another place where it might make sense to draw that line: 10^25 kg. There&#39;s a "gap" there, too. Uranus is 9 times that big, but the Earth misses by a factor of two. If we didn&#39;t live on the Earth, we&#39;d probably say there are really only 4 planets orbiting Sol, and the rest is just minor rocks. ;)

But being terra-centric, myself, I vote for 10^23 kg. ;)

zephyr46
2004-May-24, 01:38 AM
As much as I wish I could call Sedna a planet, It&#39;s orbit is more Cometary than Planetary. Objects in the outer have many catagories, Trans-Neptunian Objects, Centaurs, Kuiper Belt Objects, Plutinos, Cubewanos??

I think stars, Juipter emits more radiation than it recieves, I think that sets it apart from planets. Remenber stars can be small as well, White Dwarves, Neutron Stars etc, very small objects, still called stars though. I think with juipter it is worth noting if an object emits more energy than it recieves. My theory is the gas giants in our solar system were the potential companion star to Sol, so we could have been in a Alpha Centauri system, for some reason, we didn&#39;t or, haven&#39;t yet.

GOURDHEAD, think your onto somthing there, but don&#39;t through the baby out with the bathwater. We have a center dedicated to monitoring and recording Minor Planets. I think Anything round, not emiting more radiation than it recieves, not in a stable orbit with no other objects of similar size should fit minor planet status.

Pluto, KBO and Minor Binary Planet?

How difinate do our catagories have to be? Plutions, share an orbital resonance, Trojans, for Mars, Juipiter and Neptune, co-orbitals, Quasi-Satelites.

I Agree the Asteroids should be defined by the irrigular nature of their structure.

I think this debate and these defintions, we have to accept, will evolve over time, as we observe the interactions and evolution of these objects.

John L
2004-May-24, 03:27 PM
The definition of a Planet should be:

A Planet is an object that orbits a star, that does not share its orbit with similarly sized objects, that does not fuse dueterium in its core, that is round due to its own gravitation, and has a differentiated interior (core, mantel, crust).


With this definition Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune are all Planets. Pluto is disqualified because it shares its orbit with several similarly sized objects such as Varuna, Ixion, and 2004DW. The asteroids are not Planets even though some are round and differentiated because they also share their orbit with several similarly sized objects. The Trojan Asteroids that share the orbits with some of the gas giants don&#39;t count, but because they are not similar in size to the gas giants, the gas giants still count. And anything of 13 Jupiter masses or more can fuse dueterium at its core, thus being labeled a Brown Dwarf, not a Planet.

SkyBoard
2004-May-24, 10:26 PM
I dunno. I think the Ida-Dactyl scenario is unique...I wouldn&#39;t mind making Ida a planet out of pure respect for its asteroidal power...

zephyr46
2004-May-25, 05:06 AM
Both Ida and Dactyl are irregularly shaped, Binary Asteroid.

There are a few Binary KBOs A/CC Topic Binary Objects (http://www.hohmanntransfer.com/topics.htm)

Fraser_Abel
2004-May-25, 05:07 AM
Yeah. True, but the I-D duo seems like a planet-moon duo, something rare (I&#39;m sure) among asteroids...

zephyr46
2004-May-25, 05:14 AM
Sorry I ammended that post;

http://www.hohmanntransfer.com/topics.htm

Yeah Asteroid with satelite asteroid? Though Phobos and Demos get no critisim for being anything other than moons&#33;

A lot of moons are very irregular.
particularly past the asteroids.

http://neo.jpl.nasa.gov/images/ida2_s.jpg
NASA NEO (http://neo.jpl.nasa.gov/images/ida.html)

John L
2004-May-25, 02:19 PM
Ida is just an asteroid belt asteroid. If you wanted to call any of them a planet you&#39;d have to give props to Ceres and Vesta first. Those are the two that are believed to be differentiated bodies.

Spacemad
2004-May-25, 04:53 PM
Originally posted by John L@May 24 2004, 03:27 PM
The definition of a Planet should be:

A Planet is an object that orbits a star, that does not share its orbit with similarly sized objects, that does not fuse dueterium in its core, that is round due to its own gravitation, and has a differentiated interior (core, mantel, crust).


With this definition Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune are all Planets.


I don&#39;t think your definition of a planet works very well with the gas giants as they don&#39;t have a crust - as far as we know at present - they may have a core but I donīt think they have a mantle. :unsure: The mantle comes between the core & crust, therefore if they donīt have a crust they canīt have a mantle, donīt you agree?

Therefore your definition only works for the 4 rocky planets - if Mars has a mantle any longer, if it doesnīt then the list of planets is reduced to only three&#33; :)

John L
2004-May-25, 06:00 PM
I gave those as an example of differentiation. Even though Mars may have cooled to the point that it is solid throughout now, it was still differentiated. It has a core that is different than the solid or liquid mantel, which is still different from the out crust. The asteroids Ceres and Vespa are differentiated, but because they share their orbit with hundreds of thousands of similarly sized objects (them being only the largest of that group) they are not planets. Pluto, sharing its orbit with hundreds of known and hundreds of thousands of suspected similar objects is also not a planet.

Jupiter is believed to have a solid or liquid hydrogen core with various layers of gases above that could be considered analogous to the mantel and crust. They&#39;re just in a gaseous rather than solid state. The important thing about gas giants is that they are not massive enough to begin dueterium fusion in their cores. That is where brown dwarves begin.

Fraser_Abel
2004-May-25, 06:03 PM
yeah, John, I think you&#39;re right...isn&#39;t Vesta the one with the core (or mantle)?

TheThorn
2004-May-25, 10:27 PM
Originally posted by Fraser_Abel@May 25 2004, 05:07 AM
Yeah. True, but the I-D duo seems like a planet-moon duo, something rare (I&#39;m sure) among asteroids...
Actually, asteroids with moons are pretty common. For instance, Robert Johnson (http://www.johnstonsarchive.net/astro/asteroidmoons.html) lists over 50 on this page, as does . Denis Denissenko (http://hea.iki.rssi.ru/~denis/doublemp.html) on this page.

Some of these are even "contact binaries" - two asteroids that are actually touching each other.