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Date: 26 Aug 2006 13:04:56
From: John Freck
Subject: What is the term for a nearly spherical celestial body?
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What is the proper term for a celestial body that is nearly spherical
body? Is there any term, a word or noun phrase, that describes and
takes altogether our moon, Pluto, the spherical moons of Jupiter, and
Saturn, Ceres, and other dwarf planents?
John
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Date: 26 Aug 2006 19:54:40
From: canopus56
Subject: Re: What is the term for a nearly spherical celestial body?
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John Freck wrote:
> What is the proper term for a celestial body that is nearly spherical
> body? Is there any term, a word or noun phrase, that describes and
> takes altogether our moon, Pluto, the spherical moons of Jupiter, and
> Saturn, Ceres, and other dwarf planents?
The language used in IAU Resolution 5A is: "A 'planet' is a celestial
body that . . . (b) has sufficient mass for its self-gravity to
overcome rigid body forces so that it assumes a hydrostatic equilibrium
(nearly round) shape. . . ."
As JMB notes in this thread, "nearly round" or "nearly spherical" means
"hydrostatic equilibrium." The Stern paper cited in another thread in
this newsgroup -
Stern, S.A., & Levison, H.F. 2002.
Regarding the criteria for planethood and
proposed planetary classification schemes.
Status: To appear in IAU Proceedings 2000.
http://www.boulder.swri.edu/~hal/planet_def.html
http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf
- discusses hydrostatic equilibrium for planets, KBOs, TNO's and
asteriod belt members. Whether a planet, drawf planet or pluton is at
hydrostatic equilibrium depends on its density.
Stern's Figure 1 is instructive and easy to follow.
For a rocky planet like the Earth with a density of 5.7gm/cm^3,
hydrostatic equilibrium can be achieved by objects as small as 100km in
radius (200km dia.). The Earth of course is many times this minimum
size.
For a lower density body like Titan at about 2.2gm/cm^3, hydrostatic
equilibrium, the minimum body radius is larger - over 300km (600km
dia.) Titan is much larger than this minimum.
Note that asteriod Vesta falls below this minimum requirement.
I haven't looked them up, but there are undoubtedly many Jupiter and
Saturn moons that are larger than this minimum diameter.
The result is that spheriosity alone is not a sufficient criteria to
distinguish between the many populations of known objects in the solar
system. Some of those populations include -
Planets
Asteriod Belt objects
Trans-Neptunian objects
Kuiper Belt objects
Oort Cloud objects
Moons of Jupiter, Saturn and Earth
That is why IAU Resolution 5A contains the other two criteria: "(1) A
'planet' is a celestial body that (a) is in orbit around the Sun, . . .
and (c) has cleared the neighbourhood around its orbit."
Subcriteria (a) (orbit) excludes the moons of Jupiter, Saturn and Earth
as planets.
Subcriteria (b) (spheriosity) excludes a plethora of objects, like
asteriods, Centaurs, short and long period comets
Subcriteria (c) (% of mass in oribtal shell) excludes everything else
- leaving only "planets".
- Canopus56
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Date: 27 Aug 2006 02:29:14
From: Wally Wonderful
Subject: Re: What is the term for a nearly spherical celestial body?
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canopus56 wrote:
> John Freck wrote:
> > What is the proper term for a celestial body that is nearly spherical
> > body? Is there any term, a word or noun phrase, that describes and
> > takes altogether our moon, Pluto, the spherical moons of Jupiter, and
> > Saturn, Ceres, and other dwarf planents?
>
> The language used in IAU Resolution 5A is: "A 'planet' is a celestial
> body that . . . (b) has sufficient mass for its self-gravity to
> overcome rigid body forces so that it assumes a hydrostatic equilibrium
> (nearly round) shape. . . ."
>
> As JMB notes in this thread, "nearly round" or "nearly spherical" means
> "hydrostatic equilibrium." The Stern paper cited in another thread in
> this newsgroup -
>
> Stern, S.A., & Levison, H.F. 2002.
> Regarding the criteria for planethood and
> proposed planetary classification schemes.
> Status: To appear in IAU Proceedings 2000.
> http://www.boulder.swri.edu/~hal/planet_def.html
> http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf
>
> - discusses hydrostatic equilibrium for planets, KBOs, TNO's and
> asteriod belt members. Whether a planet, drawf planet or pluton is at
> hydrostatic equilibrium depends on its density.
>
> Stern's Figure 1 is instructive and easy to follow.
>
> For a rocky planet like the Earth with a density of 5.7gm/cm^3,
> hydrostatic equilibrium can be achieved by objects as small as 100km in
> radius (200km dia.). The Earth of course is many times this minimum
> size.
>
> For a lower density body like Titan at about 2.2gm/cm^3, hydrostatic
> equilibrium, the minimum body radius is larger - over 300km (600km
> dia.) Titan is much larger than this minimum.
>
> Note that asteriod Vesta falls below this minimum requirement.
>
> I haven't looked them up, but there are undoubtedly many Jupiter and
> Saturn moons that are larger than this minimum diameter.
>
> The result is that spheriosity alone is not a sufficient criteria to
> distinguish between the many populations of known objects in the solar
> system. Some of those populations include -
>
> Planets
> Asteriod Belt objects
> Trans-Neptunian objects
> Kuiper Belt objects
> Oort Cloud objects
> Moons of Jupiter, Saturn and Earth
>
> That is why IAU Resolution 5A contains the other two criteria: "(1) A
> 'planet' is a celestial body that (a) is in orbit around the Sun, . . .
> and (c) has cleared the neighbourhood around its orbit."
>
> Subcriteria (a) (orbit) excludes the moons of Jupiter, Saturn and Earth
> as planets.
>
> Subcriteria (b) (spheriosity) excludes a plethora of objects, like
> asteriods, Centaurs, short and long period comets
>
> Subcriteria (c) (% of mass in oribtal shell) excludes everything else
>
> - leaving only "planets".
>
> - Canopus56
so... if Mercury had a piece of it knocked off it would no longer be a
planet?
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Date: 27 Aug 2006 02:38:59
From: Sjouke Burry
Subject: Re: What is the term for a nearly spherical celestial body?
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John Freck wrote:
> What is the proper term for a celestial body that is nearly spherical
> body? Is there any term, a word or noun phrase, that describes and
> takes altogether our moon, Pluto, the spherical moons of Jupiter, and
> Saturn, Ceres, and other dwarf planents?
>
> John
>
Planet family??
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Date: 26 Aug 2006 23:46:49
From: Rick Evans
Subject: Re: What is the term for a nearly spherical celestial body?
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"John Freck" <v1313w@yahoo.com > wrote in message
news:1156622696.336308.247490@p79g2000cwp.googlegroups.com...
> What is the proper term for a celestial body that is nearly spherical
> body? Is there any term, a word or noun phrase, that describes and
> takes altogether our moon, Pluto, the spherical moons of Jupiter, and
> Saturn, Ceres, and other dwarf planents?
Spheroid.
--
Hilton Evans
----------------------------------------------------------------
Lat +42° 11' 07"
Lon -71° 04' 35"
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http://www.chempensoftware.com
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http://mysite.verizon.net/hiltonevans33050/astroimaging/astroimaging.htm
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Date: 26 Aug 2006 22:39:03
From: Jean-Marc Becker
Subject: Re: What is the term for a nearly spherical celestial body?
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"John Freck" <v1313w@yahoo.com > a écrit dans le message de news:
1156622696.336308.247490@p79g2000cwp.googlegroups.com...
> What is the proper term for a celestial body that is nearly spherical
> body? Is there any term, a word or noun phrase, that describes and
> takes altogether our moon, Pluto, the spherical moons of Jupiter, and
> Saturn, Ceres, and other dwarf planents?
>
>
Hydrostatic equilibrium.
JMB
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Date: 26 Aug 2006 13:45:32
From: SkySea
Subject: Re: What is the term for a nearly spherical celestial body?
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>"John Freck" <v1313w@yahoo.com> wrote:
> What is the proper term for a celestial body that is nearly spherical
> body? Is there any term, a word or noun phrase, that describes and
> takes altogether our moon, Pluto, the spherical moons of Jupiter, and
> Saturn, Ceres, and other dwarf planents?
>"Jean-Marc Becker" <no.adress@nowhere.com> wrote:
>Hydrostatic equilibrium.
There's also "oblate spheroid" for those planets like Earth that are a
bit flattened by rotation.
=============
- Dale Gombert (SkySea at aol.com)
122.38W, 47.58N, W. Seattle, WA
http://flavorj.com/~skysea
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Date: 27 Aug 2006 17:14:15
From: Paul Schlyter
Subject: Re: What is the term for a nearly spherical celestial body?
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In article <1156622696.336308.247490@p79g2000cwp.googlegroups.com >,
John Freck <v1313w@yahoo.com > wrote:
> What is the proper term for a celestial body that is nearly spherical
> body? Is there any term, a word or noun phrase, that describes and
> takes altogether our moon, Pluto, the spherical moons of Jupiter, and
> Saturn, Ceres, and other dwarf planents?
And what about globular clusters, planetary nebulae, or type E0
elliptical galaxies?
There are lots of nearly spherical celestial objects - and there is no
name or phrase which catches them all, except "nearly spherical
celestial objects"....
--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/
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Date: 27 Aug 2006 17:26:02
From: John Freck
Subject: Re: What is the term for a nearly spherical celestial body?
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Llanzlan Klazmon wrote:
> "John Freck" <v1313w@yahoo.com> wrote in news:1156622696.336308.247490
> @p79g2000cwp.googlegroups.com:
>> What is the proper term for a celestial body that is nearly spherical
>> body? Is there any term, a word or noun phrase, that describes and
>> takes altogether our moon, Pluto, the spherical moons of Jupiter, and
>> Saturn, Ceres, and other dwarf planents?
John
> Such a term would include all stars so wouldn't be much use in terms of
> classifying things.
How is the new usage useful? Is pluto a dwarf planet? I think that you
didn't catch on to the possiblity of subcategories of planets.
Stars are stars even if they orbit larger stars. Stars are not planets,
but stars can lose fussion adn become planets.
We can have all of these being perfectly good planet names, all with
good definitions, and easily understood. Current the drawf planet
Pluto is--not--a planet. Then why is 'planet' in its category dub?
Why isn't dwarf planet a type of planet?
Extinct star planets
Brown dwarf planets
Gas giant planets
Large rocky planets
Planetary moons
Minor planets.
Rogue planet (a planet traversing interstellar space)
Major planets (the first 5)
If Neptune orbited Jupiter, then would Neptune be a planet or a moon or
a planetary gas gaint moon? Can there be double planets?
A planet is a spherical body that is not a star. There can be many
possible adjectives and nouns used ot modify the main noun. For
example, what does 'planet' in 'dwarf planet' mean? How can a 'dwarf
planet' not be a 'planet'? And who said astronomers were good
technical writers anyway, or even good educators?
John Freck
> Klazmon.
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Date: 27 Aug 2006 16:16:54
From: canopus56
Subject: Re: What is the term for a nearly spherical celestial body?
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John Freck wrote:
> Paul Schlyter wrote:
> > In article <1156622696.336308.247490@p79g2000cwp.googlegroups.com>,
> > John Freck <v1313w@yahoo.com> wrote:
<snip >
> And, of course, we can use
> adjectives to create definable terms that are perfectly whole in
> themselves. We can use terms like these:
> Major planets, minor planets, dwarf planets, planetary moons, ect.
It would be interesting to see how well the definition holds up against
the what little knowledge we have on the 180 or so extra-solar planets.
I believe Brian Tung did this for one extrasolar planet in another
thread. - Canopus56
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Date: 28 Aug 2006 10:13:10
From: Llanzlan Klazmon
Subject: Re: What is the term for a nearly spherical celestial body?
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"John Freck" <v1313w@yahoo.com > wrote in news:1156622696.336308.247490
@p79g2000cwp.googlegroups.com:
> What is the proper term for a celestial body that is nearly spherical
> body? Is there any term, a word or noun phrase, that describes and
> takes altogether our moon, Pluto, the spherical moons of Jupiter, and
> Saturn, Ceres, and other dwarf planents?
>
> John
>
Such a term would include all stars so wouldn't be much use in terms of
classifying things.
Klazmon.
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Date: 27 Aug 2006 11:16:16
From: John Freck
Subject: Re: What is the term for a nearly spherical celestial body?
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Paul Schlyter wrote:
> In article <1156622696.336308.247490@p79g2000cwp.googlegroups.com>,
> John Freck <v1313w@yahoo.com> wrote:
>
> > What is the proper term for a celestial body that is nearly spherical
> > body? Is there any term, a word or noun phrase, that describes and
> > takes altogether our moon, Pluto, the spherical moons of Jupiter, and
> > Saturn, Ceres, and other dwarf planents?
>
> And what about globular clusters, planetary nebulae, or type E0
> elliptical galaxies?
>
> There are lots of nearly spherical celestial objects - and there is no
> name or phrase which catches them all, except "nearly spherical
> celestial objects"....
The word planets is just a word, of course. And, of course, we can use
adjectives to create definable terms that are perfectly whole in
themselves. We can use terms like these:
Major planets, minor planets, dwarf planets, planetary moons, ect.
John Freck
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