I Suggest a revised definition of "planet"...

[MKruer]

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

[Scorpiove]

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 .

[d.m.falk]

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.

[MKruer]

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

[Spaceman Spiff]

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

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

[Spaceman Spiff]

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

Hmm, yes, read that somewhere. It's no good though. Again, it's impractical when it comes to deciding whether a newly discover point-like object or extrasolar 'planet' is a planet. How can you tell it's shape? How can you tell if the heat is from fusion, primordial (Brown Dwarves!), tidal or recent collision?

Is Ceres rounded or not?

Spiff.

[adamnieman]

The fact is, there is a wide variety of objects in orbit around the Sun. This makes theoretical distinctions between categories of objects difficult and contingent. Rather than trying to shoe-horn a theoretical distinction between planets and asteroids into their definition, I advocate doing it as we always have: How do we decide what to call a planet and what to call an asteroid? Planets are bigger than asteroids!

The cut-off point is arbitrary and Pluto is as good a size as anything. So if it's Pluto-sized or larger, it's a planet; if it's smaller than Pluto, it's an asteroid. Really big asteroids can be called 'planetoids', really small ones can be called 'meteroids'.

This definition makes no ontological distinction between planets and asteroids (essentially, planets are asteroids and asteroids are planets) which keeps the solar system more straightforward conceptually.

Adam

[Spaceman Spiff]

Yes, but...

The division line being set to let Pluto in as a planet and not smaller TNOs is also vulnerable to quibbles. There's a real possibility that there are dozens, hundreds of undiscovered TNOs larger than Pluto, or Mercury, maybe even as large as Mars. In twenty years time, who's going to insist on naming all of the 256 planets?

A reason why I'd like to get the number of planets down to eight is that it's better for schoolchildren who learn astronomy to recite something achievable such as listing eight names in order rather than 256. If they add to teh eight "... and there's Pluto, and Ceres is the largest asteroid, and there's one called Pallas ... " that's extra brownie points. But when there's 256 planets, there's just no such fun and opportunity for a sense of achievement. "Think of the Children!" .

Spiff.

[selden]

Pluto is still called a planet for historical and social reasons, not for technical ones. The IAU argued about this for years and finally decided it wasn't worth the pain involved in changing what it's called. There's no point in arguing about it here: they aren't going to change how Pluto is classified in the popular literature. Anyone who spends any time studying astronomy quickly learns about the different types of bodies orbiting the sun.

[ajtribick]

If they find something the size of Mars in the Kuiper Belt then things will get interesting.

Maybe the minor/major (or über/unter) planet distinction is a good one - reserve the term "planet" for those objects large enough that their shapes are determined primarily by gravity (so they are spherical), then divide by dynamical considerations - Pluto remains a planet, but it is an unterplanet. Though I wonder if this system wouldn't make Mu Arae's gas giants unterplanets...

I guess this is what happens when you dive in with a classification system too early in the game... you're nice ABCDEF system ends up as OBAFGK...

[Spaceman Spiff]

Pluto is still called a planet for historical and social reasons, not for technical ones.

Selden, yes, the IAU doesn't have a rule or a definition, just a list. But, don't you want to see the astrologers shown up? They'd blame astronomers naturally, but why didn't they foresee this problem???

Spiff.

[Spaceman Spiff]

I guess this is what happens when you dive in with a classification system too early in the game... you're nice ABCDEF system ends up as OBAFGK...

Quite! One should be able to wait until enough data is gathered to study how they relate to each other. Using orbits and masses lets you do that more quickly that surface shape or features, but some people are just so impatient! "Is it a planet or what? Tell me now! I have to write something for the readers!! You're an expert!!! Why won't you tell me!!!!"*

Spiff.

* OK, I exaggerate the hysteria...

[Ryan McReynolds]

Hmm, yes, read that somewhere. It's no good though. Again, it's impractical when it comes to deciding whether a newly discover point-like object or extrasolar 'planet' is a planet. How can you tell it's shape? How can you tell if the heat is from fusion, primordial (Brown Dwarves!), tidal or recent collision?

But you can tell all of these things, by estimating mass. Relatively simple math will tell you if the object's gravity is sufficient to overcome mechanical forces and allow it to flow into an equillibrium ellipsoid--to "round" it. Likewise, either the object is massive enough that the conditions for deuterium fusion can occur or it isn't.

But for the sake of argument, let's pretend that it takes a bit (or a lot) of study to determine if something is a planet or not. So what? Why is it crucial to classify an object immediately upon its discovery? And why can't the classification simply be changed if it turns out the initial speculation was wrong? This is science, not dogma: it evolves. There's no compelling reason that an object has to be permenantly assigned a classification before studying it. You discover an extrasolar planet and later find out it's a little more massive than you thought and probably fused deuterium. Big deal, you start calling it a brown dwarf instead.

Is Ceres rounded or not?

Of course it is. So are Vesta, Pallas, Varuna, Pluto, Sedna, Quaoar, 2003 UB313, etc. They are all minor planets in this classification scheme. There could be thousands of minor planets found in the Kuiper Belt and Oort Cloud. Using gravitational roundness as a criterion basically says that they were right to call Ceres a planet when they first discovered it, because it is not fundamentally different from the other planets. Despite the confusion of late, planets have always been round objects that shine with reflected sunlight, and in my scheme they still are. No other scheme seems to preserve this classical definition.

[MKruer]

Here is my definition of a "planet"

A Planet is a body that is geological active, or more precisely has a “warmâ€

[MKruer]

Yes, but...

The division line being set to let Pluto in as a planet and not smaller TNOs is also vulnerable to quibbles. There's a real possibility that there are dozens, hundreds of undiscovered TNOs larger than Pluto, or Mercury, maybe even as large as Mars. In twenty years time, who's going to insist on naming all of the 256 planets?

A reason why I'd like to get the number of planets down to eight is that it's better for schoolchildren who learn astronomy to recite something achievable such as listing eight names in order rather than 256. If they add to teh eight "... and there's Pluto, and Ceres is the largest asteroid, and there's one called Pallas ... " that's extra brownie points. But when there's 256 planets, there's just no such fun and opportunity for a sense of achievement. "Think of the Children!" :).

Spiff.

This is such a lame reason not to classify a new body as a planet. Oh no I have to remember another few names. Hell most people don’t know the names of all the planets anyway why would adding more “planetsâ€