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If gravity is required, what are the planets in the SolarSystem?



 
 
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  #1  
Old March 23rd 04, 08:34 PM
quilty
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Default If gravity is required, what are the planets in the SolarSystem?

http://www.spacedaily.com/news/outerplanets-04b.html

According to an article on SpaceDaily, planetary scientist S. Alan
Stern suggests that the deciding factor in deciding whether or not a
body is a planet should be gravity. If a body is rounded by its own
gravity, then it should be considered a planet.

If this were the rule that decides what is a planet, what would be the
planets in the Solar System? Mr. Stern mentions Pluto, Sedna and Ceres
along with several asteroids and large Kuiper Belt Objects. What would
the planet count be in the Solar System and what would be the total
list of planets?

  #2  
Old March 24th 04, 06:22 AM
Christopher M. Jones
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Default If gravity is required, what are the planets in the SolarSystem?

(quilty) wrote in message om...
http://www.spacedaily.com/news/outerplanets-04b.html

According to an article on SpaceDaily, planetary scientist S. Alan
Stern suggests that the deciding factor in deciding whether or not a
body is a planet should be gravity. If a body is rounded by its own
gravity, then it should be considered a planet.

If this were the rule that decides what is a planet, what would be the
planets in the Solar System? Mr. Stern mentions Pluto, Sedna and Ceres
along with several asteroids and large Kuiper Belt Objects. What would
the planet count be in the Solar System and what would be the total
list of planets?


No body in the Solar System is round. I assume that
you'd have to go by some sort of percentage deviation
from a iso-potential surface.

Anyway, if you make that the criterion (and it's one of
the more sillier, not much better than the arbitrary
"these nine and none others") then we end up with
dozens of planets in our Solar System rather than 9.

Just off the top of my head, we'd add: Earth's Moon,
Io, Europa, Ganymede, Callisto, Titan, Titania, Triton,
and Charon. If we take it all the way down to Ceres'
size / sphericity it'll be a mess. The big question is,
where's the dividing line and WHY?

  #3  
Old March 25th 04, 06:47 PM
quilty
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Default If gravity is required, what are the planets in the SolarSystem?

No body in the Solar System is round. I assume that
you'd have to go by some sort of percentage deviation
from a iso-potential surface.


Perhaps he meant "roundish"?

Just off the top of my head, we'd add: Earth's Moon,
Io, Europa, Ganymede, Callisto, Titan, Titania, Triton,
and Charon. If we take it all the way down to Ceres'
size / sphericity it'll be a mess. The big question is,
where's the dividing line and WHY?


Maybe I was just reading between the lines, but I got the impression
that he was going on the principal that planets orbit a star and moons
orbit a planet which orbits said star. That would leave out all of the
moons that you listed.

Since you seem violently against such criteria, what would you suggest
as a criteria for determining whether an object is a planet?

  #5  
Old April 6th 04, 05:55 AM
Jarvi
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Default If gravity is required, what are the planets in the Solar System?

"quilty" kirjoitti viestissä
om...
No body in the Solar System is round. I assume that
you'd have to go by some sort of percentage deviation
from a iso-potential surface.


Perhaps he meant "roundish"?

Just off the top of my head, we'd add: Earth's Moon,
Io, Europa, Ganymede, Callisto, Titan, Titania, Triton,
and Charon. If we take it all the way down to Ceres'
size / sphericity it'll be a mess. The big question is,
where's the dividing line and WHY?


Maybe I was just reading between the lines, but I got the impression
that he was going on the principal that planets orbit a star and moons
orbit a planet which orbits said star. That would leave out all of the
moons that you listed.

Since you seem violently against such criteria, what would you suggest
as a criteria for determining whether an object is a planet?

Gravity is proportial to mass. If all objects are put in a same list in the
order of their mass we can see that there are some clear limits of different
groups:
1.Sun is a star, it is a thousand times as massive as the biggest planet
Jupiter.
2. The giant gas planets are clearly one group.
3.The terrestial planets could be considered another group as the smallest
gas planet Uranus is 14.6 times more massive than Earth. However both these
groups are clearly planets. Both groups have roughly as massive metal and
stone cores.
4. The moons, asteroids, comets and kuiper belt objects(KBO) are clearly
smaller in size. The smallest planet Mercury is 4,5 times more massive than
biggest of them, our Moon.
5. Pluto is 18.4 times lighter than the smallest planet Mercury, so it is
not a planet by weight. It is clearly a Kuiper belt object, the biggest, but
only slightly bigger than the next 100 KBOs known today and belongs to that
group. There are many moons bigger than Pluto. Pluto is called a planet only
because it was found years before other KBOs

  #6  
Old April 6th 04, 10:14 PM
Joe Strout
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Default If gravity is required, what are the planets in the Solar System?

In article ,
"Jarvi" wrote:

4. The moons, asteroids, comets and kuiper belt objects(KBO) are clearly
smaller in size. The smallest planet Mercury is 4,5 times more massive than
biggest of them, our Moon.


Our Moon is not the biggest of these, by mass or by radius. Sorting by
radius:

Name Orbits Dist(km) Rad(km) Mass(kg)
---- ------ -------- ------- --------
Mars Sun 227940 3398 6.42e23
Ganymede Jupiter 1070 2631 1.48e23
Titan Saturn 1222 2575 1.35e23
Mercury Sun 57910 2439 3.30e23
Callisto Jupiter 1883 2400 1.08e23
Io Jupiter 422 1815 8.93e22
Moon Earth 384 1738 7.35e22
Europa Jupiter 671 1569 4.80e22

.....and sorting by mass, we have:

Name Orbits Dist(km) Rad(km) Mass(kg)
---- ------ -------- ------- --------
Mars Sun 227940 3398 6.42e23
Mercury Sun 57910 2439 3.30e23
Ganymede Jupiter 1070 2631 1.48e23
Titan Saturn 1222 2575 1.35e23
Callisto Jupiter 1883 2400 1.08e23
Io Jupiter 422 1815 8.93e22
Moon Earth 384 1738 7.35e22
Europa Jupiter 671 1569 4.80e22

So, not to pick a nit, but our Moon is smaller than Ganymede, Titan,
Callisto, and Io no matter how you look at them.

Also, Mercury is smaller than Ganymede and Titan by radius, and only
about a factor of 2 larger than Ganymede by mass -- about the same as
the difference between Mercury and Mars. So I don't see a clear
distinction between planets and moons here.

,------------------------------------------------------------------.
| Joseph J. Strout Check out the Mac Web Directory: |
| http://www.macwebdir.com |
`------------------------------------------------------------------'

  #7  
Old April 7th 04, 11:09 AM
Jarvi
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Default If gravity is... Joe Strout is right

Joe Strout is right below. Thank you Joe, for correcting my error. I had
forgotten the biggest moons: Ganymede, Titan callisto and Io.

The right conclusion is: All planets, moons, asteroids, Kuiper belt objects,
comets (and other Oort cloud objects) orbiting our Sun form by mass a
continuous serie where are no major gaps in relative mass between two
following objects smaller than Earth untill microscopic dust particles.

However There is major gap in mass 43:1 between the smallest terrestial
planet Mars and Pluto. So we should not call Pluto a planet, if we do not
call all Kuiper belt objects planets as they all form another continous
series by mass with no major gaps by mass.

Antti Järvi





"Joe Strout" kirjoitti viestissä
...
In article ,
"Jarvi" wrote:

4. The moons, asteroids, comets and kuiper belt objects(KBO) are clearly
smaller in size. The smallest planet Mercury is 4,5 times more massive

than
biggest of them, our Moon.


Our Moon is not the biggest of these, by mass or by radius. Sorting by
radius:

Name Orbits Dist(km) Rad(km) Mass(kg)
---- ------ -------- ------- --------
Mars Sun 227940 3398 6.42e23
Ganymede Jupiter 1070 2631 1.48e23
Titan Saturn 1222 2575 1.35e23
Mercury Sun 57910 2439 3.30e23
Callisto Jupiter 1883 2400 1.08e23
Io Jupiter 422 1815 8.93e22
Moon Earth 384 1738 7.35e22
Europa Jupiter 671 1569 4.80e22

....and sorting by mass, we have:

Name Orbits Dist(km) Rad(km) Mass(kg)
---- ------ -------- ------- --------
Mars Sun 227940 3398 6.42e23
Mercury Sun 57910 2439 3.30e23
Ganymede Jupiter 1070 2631 1.48e23
Titan Saturn 1222 2575 1.35e23
Callisto Jupiter 1883 2400 1.08e23
Io Jupiter 422 1815 8.93e22
Moon Earth 384 1738 7.35e22
Europa Jupiter 671 1569 4.80e22

So, not to pick a nit, but our Moon is smaller than Ganymede, Titan,
Callisto, and Io no matter how you look at them.

Also, Mercury is smaller than Ganymede and Titan by radius, and only
about a factor of 2 larger than Ganymede by mass -- about the same as
the difference between Mercury and Mars. So I don't see a clear
distinction between planets and moons here.

,------------------------------------------------------------------.
| Joseph J. Strout Check out the Mac Web Directory: |
| http://www.macwebdir.com |
`------------------------------------------------------------------'


  #8  
Old April 7th 04, 08:08 PM
Sander Vesik
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Default If gravity is required, what are the planets in the Solar System?

Jarvi wrote:

3.The terrestial planets could be considered another group as the smallest
gas planet Uranus is 14.6 times more massive than Earth. However both these
groups are clearly planets. Both groups have roughly as massive metal and
stone cores.
4. The moons, asteroids, comets and kuiper belt objects(KBO) are clearly
smaller in size. The smallest planet Mercury is 4,5 times more massive than
biggest of them, our Moon.
5. Pluto is 18.4 times lighter than the smallest planet Mercury, so it is
not a planet by weight. It is clearly a Kuiper belt object, the biggest, but
only slightly bigger than the next 100 KBOs known today and belongs to that
group. There are many moons bigger than Pluto. Pluto is called a planet only
because it was found years before other KBOs


This is not quite that straightworward - there are moons in this system that
fit very well inbetween the mass of mars and mercury.

--
Sander

+++ Out of cheese error +++

  #9  
Old April 8th 04, 07:13 AM
dave schneider
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Default If gravity is... Joe Strout is right

"Jarvi" wrote:
Joe Strout is right below. Thank you Joe, for correcting my error. I had
forgotten the biggest moons: Ganymede, Titan callisto and Io.

The right conclusion is: All planets, moons, asteroids, Kuiper belt objects,
comets (and other Oort cloud objects) orbiting our Sun form by mass a
continuous serie where are no major gaps in relative mass between two
following objects smaller than Earth untill microscopic dust particles.


Well, the moons can all be distinguished by what they orbit around.
Pluto does not seem to orbit around a[nother] planet, and as you point
out, drops off scale compared to [other] planets.

Continuing to characterize the size of KBOs will no doubt add to the
precision of our classification.

However There is major gap in mass 43:1 between the smallest terrestial
planet Mars and Pluto. So we should not call Pluto a planet, if we do not
call all Kuiper belt objects planets as they all form another continous
series by mass with no major gaps by mass.

Antti Järvi


  #10  
Old April 21st 04, 09:34 PM
Rodney Kelp
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Default If gravity is required, what are the planets in the Solar System?

I thought it had to do with orbit. If it orbits a sun, it's a planet. If it
orbits a planet it's a moon. Large elliptical orbits by tiny objects don't
count as planets. Comets and asteroids can't have moons because of
insufficient mass, therefore can't be planets.
Another line of reasoning might be that what we call it is what it is.
You could have a planet floating around in deep space that has escaped it's
sun or the sun burnt out. A lost planet? A rogue moon?


"quilty" wrote in message
m...
http://www.spacedaily.com/news/outerplanets-04b.html

According to an article on SpaceDaily, planetary scientist S. Alan
Stern suggests that the deciding factor in deciding whether or not a
body is a planet should be gravity. If a body is rounded by its own
gravity, then it should be considered a planet.

If this were the rule that decides what is a planet, what would be the
planets in the Solar System? Mr. Stern mentions Pluto, Sedna and Ceres
along with several asteroids and large Kuiper Belt Objects. What would
the planet count be in the Solar System and what would be the total
list of planets?



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