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AlienRatDog
professor emeritus
Reged: 07/15/05
Posts: 587
Loc: Detroit
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OK Everyone just a question for you guys (and gals). What do you think the solar system should consist of??
I think 10 planets (include XENA!!) but the minimum should be plutos mass. The object should orbit the star independently. If there is a binary planet, the more massive one should be the plant, less massive the satellite.
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Abe -- the poor PhD student
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Vixen Portamount
Nagler 31mm, 22mm, 12mm
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 6878
Loc: Beatrice, Nebraska
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Quote:
OK Everyone just a question for you guys (and gals). What do you think the solar system should consist of??
I think 10 planets (include XENA!!) but the minimum should be plutos mass. The object should orbit the star independently. If there is a binary planet, the more massive one should be the plant, less massive the satellite.
The solar system should consist of the sun, at least 11 planets (Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto/Charon (binary planet), and Xena for starters), planetary satellites, asteroids, comets, smaller Kuiper Belt objects (KBO's), interplanetary dust and gas, and the solar wind. That should just about cover it. Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
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Greg K.
Reged: 12/11/03
Posts: 10123
Loc: Clifton Park, NY
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The solar system is gonna consist of a bunch of stuff no matter what we call it.
Personally I think they should have just picked a 2000km diameter cut of (making Pluto and anything larger qualify), but it's really arbitrary. The IAU's definition is as good as any and better than most.
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 11081
Loc: Los Angeles
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I favor a strict definition:
1.large enough for gravity to pull the object into a round shape
2.small enough to not have nuclear fusion taking place at its core
3.in orbit around the sun, not another body. The orbit is concave toward the sun at all times.
4.if in orbit with another body, the barycenter of its co-orbit must not be within the body of the object orbited.
Further, I think the definition should be further defined:
1.Rocky planets
2.Gas planets
3.Ice planets.
Using the above definitions, Group 1 would include:
Mercury, Venus, Earth, Luna, Mars, Ceres, Pallas, Vesta, Hygiea (the exact shape of the last 3 is not well known--Hubble has imaged Ceres)
Group 2 would include Jupiter, Saturn, Uranus, Neptune.
Group 3 would contain Pluto, Charon, Sedna, Quaoar, Santa, Xena, and others (this list will grow). As Saturn's moons have shown us, an icy body can form a round shape with a smaller diameter than a rocky body.
As you know, Luna's orbit is everywhere concave to the sun. It is in solar orbit, but highly perturbed by the Earth. If the sun disappeared, Luna would fly away. It is too massive, too far away, and moving too fast to be gravitationally in orbit around the Earth. Even though the barycenter of the apparent orbit lies within the Earth, the Earth and Luna should be considered a double planet.
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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photonovore
Moonatic
Reged: 12/24/04
Posts: 2488
Loc: tacoma wa
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"It is too massive, too far away, and moving too fast to be gravitationally in orbit around the Earth. Even though the barycenter of the apparent orbit lies within the Earth, the Earth and Luna should be considered a double planet."
I think you expressed the reasons many planetary scientists already consider this system a defacto binary planetary system. 
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Mardi
4" achromat, ETX-70.
Whitepeak Lunar Observatory Website
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 10898
Loc: Amargosa Valley, NV, USA
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Standard gravitational parameter for earth (GM) = SGP = 398,600km^3 s^-2
r = 384403 km (avg earth-moon dist.)
T = ((2*pi)/sqrt(SGP))*r^1.5 -- Circular orbit period equation.
T = 2371873 sec, or 27.45 days, with an average velocity of 1.018 km/sec.
This is what the moon's orbital period should be if it was just orbiting the earth, with no solar influence.
The moon's actual sidereal period is 27.32 days, according to Wikipedia. This gives an average velocity of 1.023 km/sec -- only a little bit faster. This is well below escape velocity from that distance, which is about 1.44 km/sec.
So, although the moon is moving slightly faster than Earth's gravity alone would dictate, it's not enough to allow the moon to escape without the sun's influence. If the sun disappeared, it would simply move to a slightly higher orbit.
I really hope we never have to do that experiment! 
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"S.O.E." (Sauron's Other Eye) 16" Royce conical mirror: A permanent work in progress.
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photonovore
Moonatic
Reged: 12/24/04
Posts: 2488
Loc: tacoma wa
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Well, whaddya know!  A good lesson not to trust intuition when it comes to orbital physics! Thanks Dave, (for the first new lunar tidbit I learned today!!)
Upon reflection I remember that the reason many planetary scientists consider the moon/earth system a defacto double planet is because of the unusually high relative mass relationship compared to satellites elsewhere in the solar system. again.
So, how much hold does Pluto have on Charon, absent the sun's influence, I wonder?
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Mardi
4" achromat, ETX-70.
Whitepeak Lunar Observatory Website
Edited by photonovore (08/20/06 02:29 PM)
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 10898
Loc: Amargosa Valley, NV, USA
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The sun's gravitational influence on orbiting bodies is tidal in nature: what's important is the difference in the distance of the objects from the sun between one point of the orbit and another. At Pluto/Charon's distance from the sun, coupled with the high inclination of their mutual orbit, I'd doubt that there's any noticeable solar differential between the two bodies.
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"S.O.E." (Sauron's Other Eye) 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Next Project: The "Eye of Sauron" Observatory!
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 10898
Loc: Amargosa Valley, NV, USA
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By the way, the calculations I gave were for a circular orbit. Since the moon's orbit is slightly elliptical, the actual results should be somewhat different -- but not by much. The main effect of the sun's influence is to cause the alignment of the moon's orbital ellipse to precess over time -- thus giving us the eclipse cycles.
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"S.O.E." (Sauron's Other Eye) 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Next Project: The "Eye of Sauron" Observatory!
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matt
Vendor (Scopemania)
Reged: 07/28/03
Posts: 10037
Loc: Chaville, France
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I think that with the barycenter stuff, or no-satellite rules, astronomers mistake "nature" and "function".
If a twin of Earth orbited Jupiter, wouldn't you want to call it a planet? I think that the term "planet" does not necessarily refer to the object's environment but to the object itself. Case in point: look at free-floating "planets" such as those found in the Orion nebula. How do you know if they formed around a protostellar disk or alone?
my rules:
if it's round and smaller than a brown dwarf, it's a planet. There. Period. That's the nature of the object. If it's not round, it's an planetoid.
Then you have functions, and some objects can have many. The 'old" Nine planets can be the "classical" planets, the Big Nine, whatever. I would point out that when Pluto was discovered, its planetary status was not questioned. Ceres is both a planet and an asteroid. Sedna is both a planet and a KBO (Or KBPlanet if you wish). Ganymede is a planet which happens to be a satellite of Jupiter.
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Matt
CI700 mount with various scopes on top.
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 11081
Loc: Los Angeles
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Dave,
C=pi x D
D=480,000 miles on the Moon's orbit
C=1,507,963 miles in 27.32 days
= 2300mph (rounded off) =38.3mi/min = .64mi/sec=1.03 km/sec
Escape Velocity = 1.44km/sec?
Then the Moon is gravitationally a satellite of the Earth.
But, a geosynchronous satellite orbits at 5760mph at 22,000 mi altitude, with a sidereal period of 23hr56min.
At 10.9X the distance of a geosynchronous orbit, how could a stable orbital speed be 2300/5760, or 40% as fast? I understand your formulae, but don't get the solution that leads to having 1.44k/sec (3214mph) as an escape velocity. I would logically think a stable orbital speed at 240,000 mi would be a lot slower than 2300mph.
Also, if it's in orbit around the Earth, why doesn't it have any part of its orbit convex to the sun, as do all other satellites?
Where did I go wrong? Where's the error? I'd appreciate learning something, here, as dense as that solid noggin of mine obviously is.
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 10898
Loc: Amargosa Valley, NV, USA
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You're right that the moon's orbit is everywhere concave to the sun, just like Earth. And you're right that the sun's gravitational pull on the moon is greater than the earth's pull on it. But what's significant is the fact that the earth and moon are a system that orbit the sun together. So even though the sun is pulling harder on the moon than the earth is, it is also pulling on the earth with the same average acceleration. So the dynamics of the moon with respect to the earth depends almost exclusively on the earth's gravity.
It's really just a space-based example of Galileo's (apocryphal) experiment with the two weights at the Leaning Tower of Pisa. Even though the weights were of different masses, they fell at the same rate. The earth and moon fall around the sun in essentially the same orbit. Their deviations from that orbit take the form of a mutual orbit of their own around each other.
Other planets with moons are farther from the sun than Earth is. Their orbits are wider and the curve less pronounced. They are also more massive (except for Mars, but its moons are pretty close in) and their moons orbit faster. It's just easier for them to arrange bodies at orbital distances that have a convex motion component.
As far as escape velocity, the rule is that escape velocity for any body is the square root of 2 times the velocity for a circular orbit at a particular distance. So escape velocity for a geosynchronous satellite is 8146 mph.
The change in speed with distance is related to the formula I gave above: the key is *r^1.5
Speed doesn't drop off linearly, and it doesn't drop off as the square of the orbital distance (even though the gravitational pull itself does); it decreases at an exponential rate, and the exponent is 3/2.
That's not a particularly intuitive value, and explains why we get results we wouldn't normally expect.
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"S.O.E." (Sauron's Other Eye) 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Next Project: The "Eye of Sauron" Observatory!
Edited by llanitedave (08/21/06 12:37 AM)
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Anonymous
Unregistered
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I have enjoyed your discussion on formulas for planets. Personally, having followed the humble Ceres for several nights now I would leave it off the list on the grounds that it makes little impact on the world at large or even those with a telecope.
Eddy
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photonovore
Moonatic
Reged: 12/24/04
Posts: 2488
Loc: tacoma wa
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I found out that even Pluto's satellite Charon is bound to Pluto not the Sun and it's barycenter and mass relationship to the parent body are far weaker than the Moon's. ref. : "Charon feels a stronger gravitational acceleration from Pluto than from the Sun (as do all other satellites)." ---Steve Kortenkamp at the Planetary Science Institute
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Mardi
4" achromat, ETX-70.
Whitepeak Lunar Observatory Website
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