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Saturn now has 82 known moons

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#1 ILikePluto

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Posted 07 October 2019 - 11:23 AM

The current count of moons around the planets:

 

Mercury:  0

Venus:  0

Earth:  1

Mars:  2

Jupiter:  79

Saturn:  82

Uranus:  27

Neptune:  14

Pluto:  5

Eris:  1

 

Link


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#2 Astroman007

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Posted 07 October 2019 - 11:26 AM

I like how you included Pluto. smile.gif 

 

Interesting post, thanks.



#3 BillP

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Posted 07 October 2019 - 11:59 AM

I like how you included Pluto. smile.gif

 

Whether it is a "dwarf" planet or a "standard" planet, it is still a planet.  Since his list is including all planet types it needs to be extended to the other dwarf palents besides Eris:

 

Haumea - 2 (Hiʻiaka and Namaka)

Makemake - 1 (provisional at the moment, S/2015 (136472) 1)

Ceres - 0


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#4 ILikePluto

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Posted 07 October 2019 - 12:06 PM

No.  Contrary to what had been thought in 2005, there is a huge gap in diameter between Pluto and Eris on the one hand and all other known trans-Neptunian objects on the other hand.



#5 russell23

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Posted 07 October 2019 - 04:51 PM

Substellar taxonomy will be improved when there is a formal recognition of spherical satellites as a dynamical class of planets called "satellite planets".   The spherical satellites start at a radius of ~200 km.  Mimas is the smallest spherical satellite at 198 km radius.

 

The planets that have satellite planets have the following numbers:

 

Jupiter - 4

Saturn - 7

Uranus - 5

Neptune - 2

 

Pluto - either 1 or Pluto-Charon is a double dwarf planet

Earth - either 1 or Earth-Moon is a  double planet:

 

https://www.scirp.or...x?paperid=81133



#6 llanitedave

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Posted 07 October 2019 - 06:52 PM

Substellar taxonomy will be improved when there is a formal recognition of spherical satellites as a dynamical class of planets called "satellite planets".   The spherical satellites start at a radius of ~200 km.  Mimas is the smallest spherical satellite at 198 km radius.

 

The planets that have satellite planets have the following numbers:

 

Jupiter - 4

Saturn - 7

Uranus - 5

Neptune - 2

 

Pluto - either 1 or Pluto-Charon is a double dwarf planet

Earth - either 1 or Earth-Moon is a  double planet:

 

https://www.scirp.or...x?paperid=81133

I'm still on alert for a double planet system where both planets are nearly the same size.  It can happen to stars, why not planets?



#7 russell23

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Posted 08 October 2019 - 09:25 PM

I'm still on alert for a double planet system where both planets are nearly the same size.  It can happen to stars, why not planets?

Yeah.  That will be interesting.

 

When I posted the link to the paper above to that other forum you directed me to I was told that despite the fact that the Sun has twice the gravitational pull on the Moon than the Earth, the Moon falls into the Earth's Hill Sphere and so the Moon could not be considered a planet.

 

I looked into that a bit more and felt there were a number of problems with arguing the Earth-Moon pair cannot be a double planet because of the Hill Sphere.  The biggest to me was that if we use that argument then there actually can be no such thing as a double planet unless the two bodies are exactly the same mass.  The reason is that in any pair of bodies similar to the Earth-Moon or Pluto-Charon, the less massive body of the pair always falls within the Hill sphere of the larger.   Yet based upon barycenter location the Pluto-Charon system is proposed to be a double planet (double dwarf planet) - even though Charon falls within the Hill sphere of Pluto.  So apparently falling within the Hill sphere does not need to exclude a body from being a "double" rather than a satellite.

 

So either there is no such thing as a double planet, or the Hill sphere radius is not useful in defining what constitutes a double planet.

 

The other thing about the Hill sphere is that, like the mass needed to clear an orbit, the radius of the Hill sphere increases dramatically for a given mass as distance from the Sun increases.   The Hill sphere of the Earth is 1.5 million km.  The Hill sphere of Pluto is 5.76 million km despite its smaller mass. 

 

There also seems to be some ambiguity as to whether the Hill sphere radius is actually the sphere of influence of a body or whether the practical Hill sphere is perhaps only 1/3 to 1/2 the calculated value.



#8 llanitedave

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Posted 09 October 2019 - 12:08 AM

Yeah.  That will be interesting.

 

When I posted the link to the paper above to that other forum you directed me to I was told that despite the fact that the Sun has twice the gravitational pull on the Moon than the Earth, the Moon falls into the Earth's Hill Sphere and so the Moon could not be considered a planet.

 

I looked into that a bit more and felt there were a number of problems with arguing the Earth-Moon pair cannot be a double planet because of the Hill Sphere.  The biggest to me was that if we use that argument then there actually can be no such thing as a double planet unless the two bodies are exactly the same mass.  The reason is that in any pair of bodies similar to the Earth-Moon or Pluto-Charon, the less massive body of the pair always falls within the Hill sphere of the larger.   Yet based upon barycenter location the Pluto-Charon system is proposed to be a double planet (double dwarf planet) - even though Charon falls within the Hill sphere of Pluto.  So apparently falling within the Hill sphere does not need to exclude a body from being a "double" rather than a satellite.

 

So either there is no such thing as a double planet, or the Hill sphere radius is not useful in defining what constitutes a double planet.

 

The other thing about the Hill sphere is that, like the mass needed to clear an orbit, the radius of the Hill sphere increases dramatically for a given mass as distance from the Sun increases.   The Hill sphere of the Earth is 1.5 million km.  The Hill sphere of Pluto is 5.76 million km despite its smaller mass. 

 

There also seems to be some ambiguity as to whether the Hill sphere radius is actually the sphere of influence of a body or whether the practical Hill sphere is perhaps only 1/3 to 1/2 the calculated value.

That's informative.  Some of those "yabut" arguments do seem rather arbitrary.



#9 DaveC2042

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Posted 09 October 2019 - 12:27 AM

Yeah.  That will be interesting.

 

When I posted the link to the paper above to that other forum you directed me to I was told that despite the fact that the Sun has twice the gravitational pull on the Moon than the Earth, the Moon falls into the Earth's Hill Sphere and so the Moon could not be considered a planet.

 

I looked into that a bit more and felt there were a number of problems with arguing the Earth-Moon pair cannot be a double planet because of the Hill Sphere.  The biggest to me was that if we use that argument then there actually can be no such thing as a double planet unless the two bodies are exactly the same mass.  The reason is that in any pair of bodies similar to the Earth-Moon or Pluto-Charon, the less massive body of the pair always falls within the Hill sphere of the larger.   Yet based upon barycenter location the Pluto-Charon system is proposed to be a double planet (double dwarf planet) - even though Charon falls within the Hill sphere of Pluto.  So apparently falling within the Hill sphere does not need to exclude a body from being a "double" rather than a satellite.

 

So either there is no such thing as a double planet, or the Hill sphere radius is not useful in defining what constitutes a double planet.

 

The other thing about the Hill sphere is that, like the mass needed to clear an orbit, the radius of the Hill sphere increases dramatically for a given mass as distance from the Sun increases.   The Hill sphere of the Earth is 1.5 million km.  The Hill sphere of Pluto is 5.76 million km despite its smaller mass. 

 

There also seems to be some ambiguity as to whether the Hill sphere radius is actually the sphere of influence of a body or whether the practical Hill sphere is perhaps only 1/3 to 1/2 the calculated value.

Isn't the point about Hill Spheres and double planets that they need to be in each others' Hill Spheres?

 

So while the Moon is within the Earth's Hill Sphere, the Earth is not in the Moon's Hill Sphere.  That means the Earth is unambiguously not a satellite of the Moon, so it's not a double planet system.



#10 goodricke1

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Posted 09 October 2019 - 05:11 AM

If Pluto is a planet then every ring particle of Saturn is a moon.



#11 llanitedave

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Posted 09 October 2019 - 10:52 AM

If Pluto is a planet then every ring particle of Saturn is a moon.

And what's wrong with that?

 

 

Actually, I don't see the logic of the comparison.



#12 russell23

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Posted 09 October 2019 - 04:41 PM

Isn't the point about Hill Spheres and double planets that they need to be in each others' Hill Spheres?

 

So while the Moon is within the Earth's Hill Sphere, the Earth is not in the Moon's Hill Sphere.  That means the Earth is unambiguously not a satellite of the Moon, so it's not a double planet system.

That is exactly what I looked into a while back.   I don't think it is actually that simple.  If you look at numbers using one of the online hill sphere calculators you start to see some problems. 

 

So the Hill sphere of the Earth is 1.5 million km and the Moon's orbit is 385,000 km.  The Moon is well inside the Earth's Hill sphere radius (HSR). 

 

The Moon's HSR for its orbit relative to the Earth is 58,200 km.  So there is no overlap.  But then we can ask the question ... What mass would the Moon have to have in order for its HSR to overlap the Earth?  What if the Moon was half the Earth's mass?  Nope then the Hill sphere is 200,000 km.

 

0.75 Earth masses -->  229,000

1.0 Earth mass --> 252,000

3.5 Earth masses -->  there we go.  That gives a Hill sphere radius of 385,000 km.

 

So what does that mean?   The Moon needs to have a mass 3.5 times the Earth to have a HSR = to its orbital semi-major axis? 

 

But the problem here is that if the Earth-Moon system is a "double planet" then the Moon's HSR should be calculated relative to its primary - the Sun.  In fact it is known that the Sun exerts twice the gravitational force on the Moon that the Earth does.   If we calculate the HSR for the Moon's orbit relative to the Sun it comes out to 340,000 km.  That is not quite an overlap with its distance from the Earth, but since the distance from the Earth is increasing it turns out in the past the Moon was close enough that the Earth was within the Moon's HSR. 

 

Another example:  Pluto-Charon

 

The Pluto-Charon system has been proposed as a double dwarf planet because the orbital barycenter lies outside the physical radius of Pluto.   But Pluto does not fall within the HSR of Charon if we treat Pluto as the primary.  So Pluto does not fall within the HSR of Charon and yet astronomers have proposed it is a double system.   So apparently the HSR is not important for a double system?  

 

If the Sun is the primary then it turns out Pluto does fall within the HSR of Charon.  But at the orbital distance of Pluto-Charon the HSR is extremely large - nearly 6 million km for Pluto and nearly 3 million km for Charon.

 

I didn't check all spherical "satellite planets" (yet) but relative to the Sun all four Galilean Moons, Titan, Triton, and even the smallest spherical moons (Mimas, Enceladus) have a HSR that overlaps their planet if the Sun is treated as the primary.

 

There are different ways we might consider this:

 

1.  The Hill sphere is not useful in determining what constitutes a double planet.

 

2.  The Hill sphere demonstrates that "double planet" is a meaningless concept.

 

3.  The Hill sphere is not useful in determining what constitutes a double planet, but it is useful in checking that a double planet is in fact a pair of bodies that will remain joined in their orbital dance around the primary star.   After all - we know that a pair that is being considered for a "double" must at least have the smaller inside the HSR of the larger. 

 

Now some people have objected to the proposal that the Earth-Moon system is a double planet on the grounds that it is only because the Earth-Moon is close to the Sun that the Sun has a stronger gravitational force on the Moon than the Earth.   In other words - the Earth-Moon system is just a matter of circumstances.  That is true, but that argument then can also be used against the IAU definitions.    After all, if Pluto was in the Earth's orbit it could clear it.  If Mercury was in Pluto's orbit it could not clear it - just circumstances. 

 

So either we accept dynamical definitions despite the situational variation in circumstances that will occur.  Or we don't.  If we do, then the Earth-Moon system is a double planet because:

 

1.  The Moon is massive enough to be spherical.

2.  The Moon is massive enough to clear the orbital zone.

3.  The Sun - not the Earth -  is the orbital primary based upon strength of gravitational force.

 

That checks all three boxes of the IAU definition for planet.


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