How is a Comet Defined?

[DaveC426913]

This seems a rather ignorant thing to say:

http://www.physorg.com/news91889915.html

They're talking about originally defining Pluto as a planet, but see bold:

"...At the time, there was a debate about what to call this thing. There weren't that many options. Can you call it an asteroid? People knew about asteroids at this point, but the asteroids were all in this little band between Mars and Jupiter. A comet? Comets are known to have orbits that loop way out and then come back in, so it kind of looks like a comet, but comets are defined by the fact that they have tails -- the gas expanding out from it. So it clearly didn't fit the bill of comet. So by default, "planet" was really the only thing it could be..."

 

[cesiumfrog]

This seems a rather ignorant thing to say: [.."..]comets are defined by the fact that they have tails[.."]

Could you explain why you think that is ignorant?

The word "comet" http://www.etymonline.com/index.php?search=comet&searchmode=none" means "long haired star"; Pluto has much more in common with the wanderers.

 

[DaveC426913]

Because comets are NOT defined by the fact that they have tails. Comets live quite happily without tails a billion miles or more from the Sun. The origin of the name is antiquated now that we're studying them in the Oort cloud.

 

[cesiumfrog]

I still think "ignorant" is far too harsh.

Whilst we may study particular objects in the Oort cloud, I don't believe it's common modern usage to call those objects "comets" unless we also think they periodically exhibited tails of some extent (whilst passing close to the sun).

As for the paragraph on Pluto, it clearly refers to language "at the time" before these terms had developed their modern technical meaning. The literal meaning of a word is extremely relevant to "a debate about what to call this thing". I didn't interpret it as an issue of whether the things now called comets are composed the same as the composition of the thing Pluto.

Do you have some "authoritive" modern (technical) definition for comet? To me, the tail seems like the natural distinguishing characteristic, not just historically but also in order to (mostly) avoid the same slippery slope that pains us in distinguishing little mountainous planets from big round asteroids.