Is it a Star or a Planet?

astroron (Ron) · #1 05-06-2007, 11:36 AM

The coldest star ever found!http://news.bbc.co.uk/2/hi/science/nature/6720831.stm

erick (Eric) · #2 05-06-2007, 11:45 AM

Doesn't a "planet", by definition, orbit a "star"?

astroron (Ron) · #3 05-06-2007, 12:00 PM

Quote:

Originally Posted by erick

Doesn't a "planet", by definition, orbit a "star"?

The answer is No, they have found many Planetary type objects in the Orion nebula nowhere near stars.

erick (Eric) · #4 05-06-2007, 12:19 PM

How about that! Thanks.

bojan · #5 05-06-2007, 01:33 PM

Quote:

Originally Posted by erick

Doesn't a "planet", by definition, orbit a "star"?

Erick, I agree.
Planet (I think the word has Greek origin, and means "wandering star") is an object orbiting a star, if we stick to a historic definition - and I strongly support that idea.
For lone objects that are not quite stars but they emit their own light (or radiation in a broader sense) there are more suitable names like "failed stars" or "brown dwarfs" or whatever...
Now there is a possibility that there are "planets" - dark, cold, rocky or ice bodies - that are not orbiting (any more) any other, more massive object, and perhaps in the future we will be able to detect them.
But for now, lets not confuse the things.... Planets orbit their stars. Even that is quite the departure from original definition (because old Greeks meant only bodies that are orbiting our star, Sol)

ballaratdragons (Ken) · #6 05-06-2007, 01:35 PM

Stars also orbit Stars

bojan · #7 05-06-2007, 01:39 PM

Yes, that is true, but companion stars do emit their own light.
Planet does not, majority of its light emission comes from central star (I think the only exception in our solar system is Jupiter). But this still does not make it a star :-)

higginsdj · #8 08-06-2007, 08:03 AM

Hmmmm.. Define 'emit their own light'. Jupiter, Saturn and Nepune all radiate in the infrared and emit more energy than they re-radiate from the sun.

Cheers

bojan · #9 08-06-2007, 08:42 AM

Quote:

Originally Posted by higginsdj

Hmmmm.. Define 'emit their own light'. Jupiter, Saturn and Nepune all radiate in the infrared and emit more energy than they re-radiate from the sun.

Cheers

Well, this is actually what I meant

, emitting more than received.
I was not sure about other than Jupiter though, but I should have guessed..
So, lets say that the excess energy, radiated by those bodies need to be generated by nuclear fusion processes, to consider them stars (or companion stars). This way we can still have all our planets remain planets

astroron (Ron) · #10 08-06-2007, 10:20 AM

What do you call a body that is either gas or rock or a combination of both that is in interstellar space and not in orbit around any star?

With the latest ruling from the IAU, the Greek defination goes out the window.

bojan · #11 08-06-2007, 10:36 AM

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Quote:

Originally Posted by astroron

What do you call a body that is either gas or rock or a combination of both that is in interstellar space and not in orbit around any star?

Obviously, this would not be a planet, according to classical definition.
Should we call it "Free Planet", perhaps? Or just use descriptive name "Planet-like gas or rocky object"

I am sure we will find them sooner or later... and it will turn out they were formed near stars as planets, but were ejected at some point in time long ago in a planet system formation turmoil.... Maybe some of them could be formed on its own, but this is very unlikely, for the gravitational collapse of dust cloud to start in a first place it really needs to have some minimal critical mass to start with (or continue, if initiated by, say, shock wave from nearby supernova explosion or strong stellar wind from nearby hot stars - provided it does not evaporate due to UV radiation before it manages to reach sufficient density and collapse).
And I am sure future IAU or equivalent will have their own opinion about it....

ballaratdragons (Ken) · #12 08-06-2007, 04:51 PM

and what if there are giant Mercury/Venus/Earth size Asteroids located in the Oort Cloud. Are they Planets, Minor Planets, Comets, or 'Large Asteroids'? Labelling objects can be a tricky and argumentative business.

Maybe we could just call them 'Floaties' and be done with it

[1ponders] (Paul) · #13 08-06-2007, 05:35 PM

Let's not start this argument again. Remember what happened to Pluto last time.

There is no simple answer to this. Planet literally means "wanderer" and related to those objects in the sky that didn't stay still agains the background of fixed stars. The sun and moon were planets once, and then Ceres for about 2 years back in the early 1800s.

Even though we call them planets, the IAU doesn't recognise bodies orbiting other stars as planets. I don't know what they recognise them as. They probably dont know.

Only objects (that fit into the IAUs recent "planetary" definition) that orbit out sun can be classes planets.

If you will find an interesting read on this topic by Gibor Basri from the Uni of California. http://astro.berkeley.edu/~basri/def...et/Mercury.htm

Outbackmanyep · #14 13-06-2007, 11:21 PM

Quote:

Originally Posted by bojan

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Obviously, this would not be a planet, according to classical definition.
Should we call it "Free Planet", perhaps? Or just use descriptive name "Planet-like gas or rocky object"

How about "Parent Star Challenged" ?
Or "Runaway Planet" ??

Or that terrible word "Anomaly"

I hate that word....