Celestia Forums

I've been thinking of this for a while now, but reading the latest posts in the topic about the newest "10th planet" is what got me going.

My definition:

Planet: Anything that is clearly, visibly round, is definitely a planet.

Moon: An asteroid or planet orbiting a very large body that is more than 4 times it's size.

With this, the "asteroid" Ceres would be included in the list of planets in the Sol system, because it looks pretty round to me.

I'm not sure if pluto is affected, because there is no vividly clear indication that it is round, like Earth or Luna.

what do you guys think?

Here is my definition of a "planet"

[quote]A Planet is a body that is geological active, or more precisely has a ?€?warm?€

I'm wondering about your definition of "moon". Charon is 1/2 the diameter of Pluto. What is Charon, if it is not a moon?

As for "visibly round", that is a vague definition. What does "visibly" mean here? Jupiter is noticeably oblate due to its rapid rotation; its polar diameter is 10% less than its equatorial diameter. Would Jupiter be "visibly round"?

If we were to devise a definition of a planet, we should consider definitions based on the mass of the body. Such a definition would mostly overlap with the roundness definition, but be more precise. The mass of Pluto is estimated as being 1.25 x 10^22 kg. If we defined a planet as having a mass greater than 1 x 10^22 kg and orbiting a star, then Pluto would be a planet by this definition.

Whatever be our opinion, the only dedicated body for this matter is the International Astronomical Union IAU, that has already been involved both on the object definition, and on its name, as requested by the discoverers.
IMHO the "planet" definition is a matter of taste, there is no possibility of a definitive cataloguing, I mean roundness, magnetic field, radius, distance by Sun, orbit eccentricity, orbit inclination, retrogade rotation, and so on.
We can only wait, but meanwhile be happy for this new and very important discovery.
The strange side of the thing is that it could be discovered even with amateur telescopes, and probably there are many images by them containing it.
The only problem that avoided its beforehand discovery is that 2003 UB313 moves VERY slowly, so we would need two images at least 2 days apart in order to show its movement.
But all minor Planet searchers, as I do, take or check a triplet of images with about one hour interval, and this was not sufficient.
My little cent on the topic.
Bye

Andrea

bdm wrote:As for "visibly round", that is a vague definition. What does "visibly" mean here? Jupiter is noticeably oblate due to its rapid rotation; its polar diameter is 10% less than its equatorial diameter. Would Jupiter be "visibly round"?

You're right. I forgot about oblateness...

Pluto and Charon are definitely known to be spherical, thanks to stellar occultation.

d.m.f.

d.m.falk wrote:Pluto and Charon are definitely known to be spherical, thanks to stellar occultation.

d.m.f.

Ah! Much thanks!

It seems to me that definitions based on only size or only mass, or even both, will always fail because they'll be set up in an arbitrary way to excuse some bodies.

Any definition should actually consider the body's orbit first before considering size and mass (and the orbit is what we can measure better beforehand too). Even then, it's no good checking if the eccentricity exceeds a certain threshold (many extrasolar gas giants are very eccentric, but we call them planets). What should properly be done is the plot the orbital characteristics on scattergrams, and spot 'populations' in this context. Then consider mass.

It becomes very clear then that the gas giant planets are one popluation, the terrestrials another, and the asteroids a third, with Ceres and Vesta almost forming a link between terrestrials and asteroids. It also becomes clear that Pluto is not part of the terrestrials or gas giants, but the TNO's. Call them all planets or call then planetoids or TNO's or Edgeworth-Kuiper bodies, it is clear that Pluto isn't what it was thought to be in 1930. This would also really mean that Pluto is not a ninth planet and 2003 UB313 is not really a tenth planet.

We can also see that Ceres 'marks' closely where the fifth terrestrial planet could have formed, but Pluto does not mark where the true ninth planet could have formed.

The Moon is simply the second planet in the Earh-Moon double planet system and Charon is simply the second planetoid in the Pluto-Charon double planetoid system.

My 2/100th.

I wonder what Brian Marsden thinks...

Spiff.

Spaceman Spiff, do you want to do a post called " The great planet's name debate" ?

Hmmm, I'd let PlutonianEmpire do that, but that one doesn't need to go into Purgatory. This topic just needs renaming!

Spiff.

Good to see Wiki on the ball with this.
http://en.wikipedia.org/wiki/Definition_of_planet

It's interesting to read that the classification of planets isn't a new controversy, Ceres was for a short time considered a planet.

My own limited thoughts on this are that more of these objects will be discovered and the IAU simply won't class them as planets. A classification system of sorts is already in place for them. See http://en.wikipedia.org/wiki/Trans-Neptunian_object
(Plutinos, scattered disc objects and one I haven't heard of before cubewanos)

I prefer a definition synthesized from those proposed by Stern & Levinson, Buie, Basri, and Brown. That is:

planet: an object that orbits a star or stars and is large enough to be shaped primarily by gravity ("rounded") but not large enough to have ever undergone fusion in its interior

A planet that can dynamically clear its orbit--that is not part of a population, belt, or cloud--is a "major planet," while a planet that cannot clear its orbit is a "minor planet." There are 8 major planets in the solar system, and an unknown number of minor planets, of which Pluto is the most notorious. The four largest asteroids are also minor planets, as are all of the big KBOs and Sedna. The 8 major planets plus Pluto comprise the "historical planets," freeing anybody from fretting about Pluto's classification. It is always a planet, but its significance depends entirely on what you're talking about.

Objects which meet the mass requirements but to not orbit stars are simply "planetary bodies." A planetary body that orbits another planet is a "planetary-scale satellite." A planetary body that does not orbit a star or planet is an "unbound planet."

For comparison, Stern & Levinson use the same definition, but call major planets "??berplanets" and minor planets "unterplanets," which strike me as unneccessary neologisms. Basri calls spherical non-fusing bodies "planemos" (planetary mass objects) with planemos that orbit stars known as planets. Again, I don't see the need for new words. Buie uses the gravity-fusion definition without caring if a planet orbits another planet, so all the big moons are planets, too. Brown used the "not part of a population" argument alone, disregarding mass--though he seems to have abandoned that idea by suggesting 2003 UB313 is the tenth planet.

I blended all those together into one that made the most sense to me. Just thought I'd share.

Ryan McReynolds wrote:For comparison, Stern & Levinson use the same definition, but call major planets "??berplanets" and minor planets "unterplanets," which strike me as unneccessary neologisms.

But they sound cool.

I still think my definition is fundamentally better, for the simple reason is that factors such as size, mass, and distance for the sun all contribute to the fundamental idea that ALL planets have to have ?€?active?€

That would eliminate Mercury and Mars, doesn't it?

d.m.f.

Nope, because both planets have magnetic fields, which to the best of our understanding, says that underneath, at the core, there is a hot gelatinous piece of iron.

For me if Pluto has a hot core, then yes its a planet, no matter what the composition or orbit is. same goes for other KBOs

How about we just make a new class of planets? Kuiper belt planets? The inner 4 planets of our solar system are very different from the outer four gas giants. Yet they all are equally considered to be planets. The first four fits the defintion of planet a little different than the last four.

Now all these "bigger kbos" that we have discovered kind of fit the definition of "planet" in their own way. Couldn't we just make them into a new class of planets? After all, I don't think the rules for how planets are formed in our solar system are gonna be exactly followed in another, yet we still consider them planets as well.

Looking in each part of the solar system, you find small and big objects. In the inner solar system you find asteroids and the bigger planets, and then you also find this in the outer solar system where the 4 gas planets reside. Why coudln't we find this exact thing in the outer fringes of our solar system or in the kuiper belt? Also the kuiper belt has a lot more area than lets say the asteroid belt found between mars and jupiter.

Coudln't this be reason enough to say "hey, thats why these planetoids haven't knocked each other out of orbit, there is simply to much room!" I don't know if my ideas make any sense but eh its my two cents .

You sure about that?

Mercury has a large iron core, yes, but its mantle and crus are solid, meaning the planet is technically cold. I'm unaware of any Mercurian magnetic field.

Mars' core has also gone cold, similarly, and any magnetic fields are localised near the surface. There have been no geologic (areologic?) activity for the good part of a billion years.

d.m.f.

MERCURY: MAGNETIC FIELD AND MAGNETOSPHERE
http://www-ssc.igpp.ucla.edu/personnel/ ... /merc_mag/

Re: Why is planet Mercury's core considered to be iron?
http://www.madsci.org/posts/archives/ja ... .As.r.html

Scientists Say Mars Has Liquid Iron Core
http://www.aig.asn.au/mars_core.htm

nuff said.

and even it the cores did cool, the fact that in the historical past the planets had hot cores that cool, that would still classify them as planets.

The cores of the planets are both hot, it?€™s just that the planets have cooled so much that the crust is for the most part geologically inactive, but yet their would still be quakes from mantel shifts within the planet. The moon even has these

[quote="MKruer"]
Here is my definition of a "planet"

[quote]
A Planet is a body that is geological active, or more precisely has a ?€?warm?€