Thread: Now, wait a sec, How many planets are there?

> Well, if you read the link, , they're called plutonians or something.

Plutons. But that's a different thing. It's to name only objects that are near-like planets. It's part of this whole silly proposal.

> Doesn't anything with any kind of gravity have an atmosphere?

It depends on several factors. Definitely mass is the major one. But temperature plays a big role. Too much cold and the atmosphere freezes to the ground. Too much heat and it evaporates into space. "Too much" here being obviously a relative term depending on the type of atmosphere. Other factors that can take atmosphere away from a body are distance to the star and the type of star (a red giant will definitely consume the atmosphere of many of its planets).

Originally Posted by twomers

Isn't this a better definition - any celestial body (other than comets or satellites) that revolves around a star.

So every asteroid whizzing around the sun is a planet? If not, please distinguish between a planet and an asteroid.

I have two planets that.... ahhh forget it

To get technical about it, I think that the criteria should be volume (note - not
mass, due to the fact that other stellar phenomenon may prove quite dense),
"atmosphere", and primarily, eccentricity of orbit - perhaps a range of values in
which the orbit must be to be considered. Depending on the definition of
atmosphere, a nice, well defined standard could be established which acounts
for non planetary objects. Assuming that a large object with suitable volume
and orbit but no atmosphere occurs, that could be considered a planetoid.
The volume spec would rule out most asteroids, and the eccentricity
would rule out comets.

Those criteria could help standardise the naming of other bodies such as comets
and asteroids, but at the end of the day, does having a standard name for
objects that fall into specific categories really matter? When scientists talk about
a comet, it'll be refered to by the name of the discoverer or some code, same for
all objects really. When studying such objects, the type is less important than
the values of the properties mentioned.

Lastly, I think Charon is a proposed moon of Pluto? There's some ambiguity on
that matter too I think.

And then you would be removing from the definition of planet an object that, being a planet, would for instance have an eccentric orbit around the sun.

No, the definition of an object must not be based solely on it's physical properties.
Just like with the animal kingdom in which we so cleverly divided everything into ever so tighter compartments until we arrived onto the species, objects in space must probably be categorized a similar way. There won't be just one type of planet, but different "species" of planets. The same with asteroids and comets. In fact, this is what has been done with stars already.

This is probably a saner and safer way to do it. There is also a big difference between how a planet (as we know it today) was formed and a comet or asteroid. This signature, just like with the animal kingdom, should be in the basis of what belongs or not to the planet "family".

There is also the property of the planet being "alive" or not. Is there geological activity? Is there climate? Is there seasons? Is the planet evolving? Is there life? These and some other questions, individually analized or put together into groups may also help establish what is a planet, what is not, and what type of planet it is.

Aaah the human urge to categorize everything.

A rock stays a rock, I can call it a balloon, it will remain the same object with the same characteristics.

It's easy to have conventions/rules etc etc... Just like its easy to pick describing names for variables in code.

But when conventions are being rearranged that there can be discussions about then the use of these conventions get lost.

I agree partially. But I think that reasoning comes from those situation where naming conventions don't fit the objects being categorized.

The debate among the definition of a planet is old. Very old. Arguably the reason why a consensus has never been reached is because we don't know enough yet. We only know the planets of our solar system. We can theorize of how other planets may look like that are different from those in the solar system because we already kow something on how they are created and evolve until they either die "from natural causes" or are destroyed. But we certainly don't know enough yet to determine with a certain degree of precision what a planet is.

We cannot deny the need to have a clearly categorized "fauna" of solar and extra-solar objects. The advantages go well beyond astronomy. The passing of knowledge, a better understanding of what surrounds us and simply the need to properly comunicate about these matters are some of the consequences of having things categorized. It is tue though that we make mistakes.

In fact, it's never easy to name things. And the secular debate of what a planet is clearly shows this. As our undertanding grows, so does our perception of prior conventions. For the greeks a planet was simply a moving shiny object against the celestial sphere. Stars stayed put. Planets didn't. We cannot pretend we are above this need to categorize things. It is the only way, we humans know, of understanding them.

Well, it's official. As of today, Pluto's no longer a planet.

Quote from the article:
>Pluto was disqualified because its oblong orbit overlaps Neptune's.

THAT'S why it was disqualified?!?!? Psh...we've known that for ages!

I think a group of computer scientists should get together and decide if Pluto is a planet or not.

It's quiet incomplete that article from Reuters. There were several other factors. But mainly the reason was that the definition of the word "planet" was redefined. The orbit was a major and important change.

In oder to be considered a planet, the object must (along other restrictions) have cleared all other objects around its orbit path (and not be a satellite).

This does indeed offer a more clear picture of what a planet is. I'm not sure what they are going to do with proto-planets and planets that have the needed mass to clear foreign objects around their path but haven't yet (didn't collide, captured or expelled those objects yet).

But all in all, I believe this was a good thing. Especially on this age where the search for extra-solar planets is starting and thus a clear understanding of what is a planet becomes a clear necessity.