Definition of a planet

The maximum size of a planet is defined in that it is not large enough
to support fusion.
This would put it smaller than a brown dwarf which I think does output
some energy by small scale fusion occurring of deuterium.

I do not think the orbit should effect the definition, e.g. if the
Earth had been knocked into a different orbit by a collision , it
should still be classified as a planet as long as it did remain in
orbit around the sun.

The term planet does not apply just to the solar system so the naming
rules would apply to objects in orbit around other stars.

Personnally I do not consider Pluto a planet, but as it is generally
accepted as a planet this would be the obvious size to use as the
minimum.

Odysseus wrote:

What constitutes an "independent" orbit?

An orbit which is gravitationally dominated by central star of the
planetary system. A body which orbits a larger planet such that the
center of mass of the two-body system resides inside of the larger body
is considered a planetary satellite or a "moon", and not a planet. For
example, Jupiter's moon Ganymede is larger than both Mercury and Pluto
and if it were by itself orbiting the sun, it would probably be referred
to as a planet. However, it orbits Jupiter and therefore is not a
planet.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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lal_truckee wrote:
He's talking strength of materials, you're talking geometry. You are
talking past each other.

I'm not sure about that. I'm not talking solely about geometry; after
all, strength of materials is part of the reason I don't think you can
formulate this definition precisely.

I'd say spherical within a few tenths of percentage points of the
equipotential surface defined by a specific gravity envelope. For earth
we have Everest and the Marianas Trench each of which deviate from the
equipotential surface by less than 1/10 of a percent of the diameter of
the planet. Mons Olympus is taller and Mars is smaller - maybe a 2/10 %
deviation (someone check me!) But that's the idea. I think it's a sound
definition component, coupled with independent orbit around Sol, and
we're onto something.

How is "a few tenths" either precise or non-arbitrary? That's the
problem I have with the definition. Not to say that other definitions
aren't also similarly freighted with problems; they are. And I will
go along with David so far as to say that sphericity is a promising
direction. But he seems to think it's essentially a done deal. Maybe
I've misread him in that regard, but he seems awfully confident about
the definition.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
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"lal_truckee" wrote in message...
.com...

Brian Tung wrote:

I'm unable to grasp why you won't tell me how spherical an object,
naturally formed under the influence of gravity, must be before it will
be considered a planet by this definition. Instead, you give cases
that no thinking human being could possibly be confused about, and
which obviously need no precise definition.

He's talking strength of materials, you're talking geometry. You are
talking past each other. . . .

And yet, aren't the two supposed to go together? If an
object has the strength of materials to form a sphere, then
why can't the scientific precision applied to the strength of
materials also be applied to the spherical shape?

David seems to be implying that a celestial object may be
spherical by some *other* means than its strength of
materials that would, if of sufficient strength, make it more
spherical. So an object that is spherical might not be a
true major planet unless it is determined that it also meets
the strength of materials parameter.

I would like to know if science actually knows of some
other natural means by which a celestial object may happen
to become spherical without also having the material strength
to make it so?

And where does this leave the Moon in this regard? Does
Selene, our Moon, meet Stern's and Levinson's Bulk Density
criteria? (Since we already know that Selene is in its own
orbit around the Sun which is at least as "independent" as the
orbit of Earth.)

happy days and...
starry starry nights!

--
Indelibly yours,
Painius

Brian Tung wrote:

How is "a few tenths" either precise or non-arbitrary? That's the
problem I have with the definition. Not to say that other definitions
aren't also similarly freighted with problems; they are. And I will
go along with David so far as to say that sphericity is a promising
direction. But he seems to think it's essentially a done deal. Maybe
I've misread him in that regard, but he seems awfully confident about
the definition.

I don't see it as a "done deal" by any means, but quite frankly, it is
probably the direction that things need to go. It is certainly less
arbitrary than just setting a minimum diameter for no apparent reason.
Using a density/radius curve for a diameter "cutoff" as Stern and
Levison have done seems a logical way to decide whether an object might
be classified as a planet rather than just an asteroid. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 13th Annual NEBRASKA STAR PARTY *
* July 23-28, 2006, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

Brian Tung wrote:
Anonymous AtWork wrote:
That's an attractive option, but I don't think it will work in practice.
If there's a substantial body out there--say, Earth-sized--there will be
an almost irresistible inclination to call it a planet, whether by that
time it's the ninth planet, tenth, eleventh, whatever. I don't think
that excluding it on the basis of history will work. Some technical
definition will probably be needed.
Why? Because a "*planetary* geologist" is legally barred from studying
a "large circum-solar body"?

?! I'm not sure how you get that from what I wrote.

My point is, people--not just scientists--like to classify.

You said it would be "needed". Now you just talk about how people "like
to" classify. Which is it--a need or a want?

David I'll throw this into this post. Here on Earth the Moon and the
Sun moves the liquid water. On Ganymede it creates the volcanoes and
helps along with createring cracks its icy surface. Here you see an
object 665,000 miles away heateed up by the great gravitational force of
its massive parent. I read that Io Europa,and Ganymede were formed
around Jupiter while the planet itself was in the process of forming. I
find that needs a lot of thinking about. Bert PS One couold come up
with a theory when the sun and moon line up it could help volcano
activity here on Earth.(spring volano season) go figure

David Knisely wrote:
I don't see it as a "done deal" by any means, but quite frankly, it is
probably the direction that things need to go. It is certainly less
arbitrary than just setting a minimum diameter for no apparent reason.

I think we agreed on that the last time.

Using a density/radius curve for a diameter "cutoff" as Stern and
Levison have done seems a logical way to decide whether an object might
be classified as a planet rather than just an asteroid. Clear skies to you.

Well, I agree it's more logical than just setting a minimum diameter.
But so far, I don't think you've contradicted my initial statement,
which was merely that I was unsatisfied with this definition because it
couldn't be made (or at least hasn't yet) both precise and non-
arbitrary. (Yes, I realize those aren't absolute terms, but there's
still a significant component of each as it stands.)

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

Anonymous AtWork wrote:
You said it would be "needed". Now you just talk about how people "like
to" classify. Which is it--a need or a want?

Why so rude? Not a good way to repay someone who answered your Jupiter
question, after all.

You ask your question as though there were a sharp dividing line,
especially when it comes to non-physical needs. If you want to avoid
having this same discussion each time a new significantly-sized body
is discovered, then yes, you'll *need* to have such a definition. If
you don't mind it that much, then you may only want to have one.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

On 13 Jun 2006 08:34:36 -0700, "Double-A" wrote:


steve wrote:
With this set to be finalised fairly soon in September 2006 I think it
is essential that a few sensible parameters are added.

1) A body in orbit around a star.

2) Size must be larger than Pluto ( or another agreed specified size
and mass) to give a minimum size UNLESS extra-terrestial life is found
on the planet when a much smaller size would be allowed.

3) If the size of a moon is similar (to a stated percentage e.g. 80%)
to the larger body in its group it to can be called a planet if it
meets (1) and (2)
Thus it is possibe to have binary planets.

4) It would probably be necessary to specify a maximum distance from
the star in which the planet is in orbit to dis-allow wandering rocks.


Should there be a maximum size for a planet?

What about brown dwarfs in orbit around a star?

Brown dwarfs give light, but they do not have fusion reactions
occurring as is necessary in the usual definition of a star.

What about the continuum of objects between the size of Jupiter and
brown dwarfs?

Where do we draw the line?

And on the low end, why shouldn't Ceres be the minimum planet size?

It has almost half the diameter of Pluto.

It is spherical.

It contains 25% of all the mass in the asteroid belt.

It fills a gap in the planet spacing predicted by Bode's Law.

It probably has more water (ice) than all the fresh water on Earth.

And it orbits the Sun and has a 9 hour day.


http://www.astrobio.net/news/modules...cle&sid= 1711

Ceres for 5th planet!

Dobule-A


do away with "planet" as some kind of superior designation. all
current "planets" keep their common designation. everything else gets
a catalog number and keeps it.

planets are a romantic designation. the stuff we are discovering now,
in volume and number, exceeds any ability to stay romantic about it.

maybe, someday, when people are living on these bodies they can come
up with their own vernacular ID name. meanwhile, its pointless trying
to categorize objects that may have designations that reside in
several classes. its all about the gray, not black and white.