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Offline p_noam

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Why do planets have elliptical orbits?
« on: 08/04/2010 15:39:12 »
If the solar system was created from rotating gas that became the sun and planets then the orbits should be round.
if the sun caught in its gravity field objects that became the planet the palnets' orbit should have been in on random plane and different direction rather then on the same plane (almost) and rotating in the same direction.




 

Offline RD

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Why do planets have elliptical orbits?
« Reply #1 on: 08/04/2010 16:00:28 »
The planet's gravitational influence on each other is a cause of eccentricity (elliptical orbits).


Quote
Over thousands of years, the eccentricity of the Earth's orbit varies from nearly 0.0034 to almost 0.058 as a result of gravitational attractions among the planets
http://en.wikipedia.org/wiki/Orbital_eccentricity#Examples
« Last Edit: 08/04/2010 16:36:33 by RD »
 

Offline Soul Surfer

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Why do planets have elliptical orbits?
« Reply #2 on: 08/04/2010 19:17:29 »
I agree with RD's explanation of the ellipticity.  The reason they are all in the same plane is similar to the reason that a lot of galaxies are flat spirals.

When a cloud is contracting under gravity the bits are moving in all sorts of directions but there is always a net axis of rotation.  as the various bits of cloud and dust collide things that run along a surface perpendicular to this axis of rotation are less likely to be disturbed and the cloud gets denser along this plane as more bits in the cloud fall in they either join the plane or tend to get thrown out along the axis of rotation so as the planets condense they are in a plane. 

The flows of some of the material out of the poles is very characteristic of a gravitational collapse process and is a good indicator of stars forming inside of a cloud of dust and gas.  The similar is probably true of the enormous bipolar jets of high energy particles coming out of quasars and active galactic nuclei.
 

Offline LeeE

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Why do planets have elliptical orbits?
« Reply #3 on: 09/04/2010 15:42:08 »
I too agree with RD's explanation of the eccentric orbits of the planets; it is due to the gravitational interaction between the planets.

There seems to be nothing to suggest that the planets were extra-solar objects (where would they have come from?) that were captured by the Sun.
 

Offline Soul Surfer

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Why do planets have elliptical orbits?
« Reply #4 on: 09/04/2010 22:56:00 »
In gravitational interactions it is not possible for one point object to capture another into an orbit with out the help of at least one other gravitating object and the results of this sort of interaction are most likely to produce highly elliptical orbits like comets.  Comets are remote objects captured into close orbits by the sun and some gravitational disturbance.  This fact is not often appreciated by people in their first encounter with the effects of gravity.

The "viscosity" of clouds of gas and particles tends to produce discs and accretions with circular orbits
 

Offline p_noam

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Why do planets have elliptical orbits?
« Reply #5 on: 10/04/2010 22:54:21 »
Thanks
I always thoght the orbit is nice ellipse like you see in pictures
such as : newbielink:http://www.globalwarmingwarriors.com/Cause.htm [nonactive]

but if it is actual 98% round then, OK.
the elliptic orbit was caused as a result of gravitational attractions among the planets
 

Offline graham.d

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Why do planets have elliptical orbits?
« Reply #6 on: 11/04/2010 11:47:17 »
The general description of galactic disk formation given above is probably roughly correct although this is a subject of much development. Nobody has developed a wholly consistent model for galactic disk formation yet and the dynamics of it are far from clear. All models today that are even close to reality take into account the behaviour of dark matter (and little enough is known about that). In these models the dark matter does not form part of the disk (because it does not interact with normal matter) and forms a "halo" around the whole galaxy with its own distribution of angular momentum. Based on this there are models that emulate the typical behaviour of observed galaxies but none, so far, that are able to show the complete formation of such a galaxy - at least none that do so without a degree of inventive manipulation.

There is a good deal of research into this interesting area though and some optimism that a self consistent model will emerge in the near future. A summary with references for those interested is:

http://nedwww.ipac.caltech.edu/level5/Sept09/Burkert/frames.html
 

Offline chris

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Re: Why do planets have elliptical orbits?
« Reply #7 on: 07/10/2015 20:48:36 »
Just bumping this to the top; are we agreed that the correct explanation for the elliptical shape of the planetary orbits in our solar system is owing to gravitational interactions between the orbiting bodies?
 

Offline alancalverd

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Re: Why do planets have elliptical orbits?
« Reply #8 on: 07/10/2015 21:37:52 »
A circle is a special case of an ellipse. Nothing to do with the perturbations of other planets: all orbits are to a first approximation elliptical. It's a long story (and I've long since forgotten the trick of solving the gravitational equations) but summarised in Kepler's observational Laws and analysed by Newton and others:

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Newton is credited with understanding that the second law is not special to the inverse square law of gravitation, being a consequence just of the radial nature of that law; while the other laws do depend on the inverse square form of the attraction. Carl Runge and Wilhelm Lenz much later identified a symmetry principle in the phase space of planetary motion (the orthogonal group O(4) acting) which accounts for the first and third laws in the case of Newtonian gravitation, as conservation of angular momentum does via rotational symmetry for the second law.
 

Offline jeffreyH

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Re: Why do planets have elliptical orbits?
« Reply #9 on: 07/10/2015 22:52:39 »
This may be instructive for those interested.

https://en.wikipedia.org/wiki/Indefinite_orthogonal_group
 

Offline Thebox

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Re: Why do planets have elliptical orbits?
« Reply #10 on: 08/10/2015 00:08:17 »
Just bumping this to the top; are we agreed that the correct explanation for the elliptical shape of the planetary orbits in our solar system is owing to gravitational interactions between the orbiting bodies?


Well no I don't agree but I have been told to hold off on my opinionated comments in other threads.
 

Offline jeffreyH

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Re: Why do planets have elliptical orbits?
« Reply #11 on: 08/10/2015 02:01:32 »
A perfect circular orbit is in a state of equilibrium where the kinetic energy of the orbital velocity exactly matches the radial force of gravity. This is also because the orbit can be thought of as being exactly perpendicular to the field at all points along its path. This is unusual if we think of the gravitational field as quantised. An elliptical orbit is only perpendicular to the gravitational field at aphelion and perihelion. The energies are still in a balanced state in such an orbit. The gravitational field takes away as much kinetic energy as it gives back. With many bodied systems this is more complex. When very dense masses orbit each other any accretion of matter from one to the other will ultimately destabilise the system. Which will change the angular momentum over time.
 

Offline chris

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Re: Why do planets have elliptical orbits?
« Reply #12 on: 08/10/2015 08:26:07 »
Sorry people, but I'm of reasonable intelligence and I don't understand a fraction of what has been written above.

I've really like to get a clear explanation for this phenomenon here, because I come across a lot of substandard answers to this question across the web, so it would be good to set the record straight.

Can we work together to do this?
 

Offline MolonLabe

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Re: Why do planets have elliptical orbits?
« Reply #13 on: 08/10/2015 10:22:28 »
I'll have a go from the point of view of Newtonian mechanics. The planet experiences an attractive force towards the sun. From Newton's laws we can deduce the law of conservation of angular momentum. Because the force on the planet is always towards one point (the centre of the sun) the angular momentum of the planet around this point is constant. Interestingly, the nature of this force (inverse r^2) is irrelevant. But this central force has the result that the orbit of the planet is a conic section, and the particular conic section is determined by its angular velocity. So it could be a hyperbola, a parabola, an ellipse, a circle, a point, or a straight line. Obviously, the ellipse is the only one we see for planets, although some comets have hyperbolic orbits. The maths to show that the orbit must be a conic section is a bit tricky, but I remember doing this in 'A'-level Further Maths
 
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Offline evan_au

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Re: Why do planets have elliptical orbits?
« Reply #14 on: 08/10/2015 11:37:40 »
Quote from: p_noam
If the solar system was created from rotating gas that became the sun and planets then the orbits should be round.
During the formation of a dense protoplanetary disk, friction would tend to make the material assume circular orbits in a disk. The disk will have the average orientation of the original material. Any dust in different, random orbits will lose energy rapidly, in frequent collisions, while dust in circular orbits will retain their energy over longer periods.

When considered on a timescale of years to centuries, the planets must follow one of the "conic sections" (circle, ellipse, parabola or hyperbola), because these are the only shapes where:
(kinetic energy of the planets speed) + (potential energy in the Sun's gravitational well) = (a constant at all points in their path)
So Conservation of Energy demands that they follow one of these paths.

Considered over thousands to millions of years, orbits are perturbed (or disturbed) by the orbits of the neighboring planets, resulting in the orbits becoming more or less elliptical, and changing where they are pointing in space. (Mercury has an additional slow drift due to the effects of general relativity close to the Sun).

Considered over billions of years, these perturbations build up; some planets will change places, some will be thrown out of the Solar System, or crash into the Sun. On these long timescales, the orbits of planets must be described as chaotic, rather than elliptical. (In the past, there was also an intense bombardment from large comets and asteroids, which would have also perturbed the orbits.)

Quote
Why do planets have elliptical orbits?
There are 3 paths that a body can take around a massive object like the Sun:
  • Elliptical (a circle is a special case of an ellipse). This path repeats.
  • Parabolic: If the object just exactly reaches escape velocity. It never comes back.
  • Hyperbolic: If the object exceeds escape velocity. It never comes back.

Since by definition, planets "hang around" for a long time, then they must follow an elliptical (or circular) orbit, by a process of elimination.

Quote
if the sun caught in its gravity field objects that became the planet the planets' orbit should have been in on random plane and different direction
Some examples of this:
  • Objects forming far from the Sun were in a very low-density environment, and did not collide with a high-density protoplanetary disk, so they kept their original random motions. Comets (and, we expect, other objects from the Kuiper Belt and Oort Cloud) are in this category, and tend to have random orbits.
  • The outer moons of Jupiter also have more random orbits, and are thought to be captured asteroids, rather than objects that formed around Jupiter.
   
 

Offline Thebox

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Re: Why do planets have elliptical orbits?
« Reply #15 on: 08/10/2015 12:46:13 »
Sorry people, but I'm of reasonable intelligence and I don't understand a fraction of what has been written above.

I've really like to get a clear explanation for this phenomenon here, because I come across a lot of substandard answers to this question across the web, so it would be good to set the record straight.

Can we work together to do this?
Hello Chris, I will answer this according to present knowledge with a simplistic answer.


Planets have an elliptical orbit because the gravity force entropy of the entire Universe is randomly displaced rather than an isotropic force of the space itself,it is an isotropic force of the random placed masses.


I will also leave an answer in new theories section for you of my opinion the real reason.
« Last Edit: 08/10/2015 12:49:14 by Thebox »
 

Offline PmbPhy

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Re: Why do planets have elliptical orbits?
« Reply #16 on: 08/10/2015 15:58:09 »
Quote from: p_noam
If the solar system was created from rotating gas that became the sun and planets then the orbits should be round.
if the sun caught in its gravity field objects that became the planet the palnets' orbit should have been in on random plane and different direction rather then on the same plane (almost) and rotating in the same direction.
The orbital angular momentum of any system is a vector which points in a direction which does not change. After a long time the gases form into planets and one or more stars. The force on a planet due to the Sun (the name that we give our particular star) is given by the force of gravity which is F = GMm/r2n where M is the mass of the Sun and m is the mass of the plant. The energy of the planets vary with energy.

If the total energy E pf the planet orbiting a star is related to the shape of the orbit then

E < 0 then the orbit is an ellipse

E = 0 then the orbit is a circle

E > 0 then the orbit is a hyperbola

So you see the total energy has to be exactly equal to E for the orbit to be exactly a circle.

If the orbit is a hyperbola then the planet, or what have you, will have escaped from the solar system a very long time ago.
 

Offline MolonLabe

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Re: Why do planets have elliptical orbits?
« Reply #17 on: 08/10/2015 17:36:42 »

If the total energy E pf the planet orbiting a star is related to the shape of the orbit then

E < 0 then the orbit is an ellipse

E = 0 then the orbit is a circle

E > 0 then the orbit is a hyperbola

So you see the total energy has to be exactly equal to E for the orbit to be exactly a circle.

If the orbit is a hyperbola then the planet, or what have you, will have escaped from the solar system a very long time ago.

All true, but nobody yet has shown that the orbit has to be a conic section (which the conservation of angular momentum demands). The above list omits the trivial cases of zero angular momentum, that the orbit is a straight line (into the sun - crash) and an even more obscure one, the point at the centre of the sun.
 

Offline PmbPhy

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Re: Why do planets have elliptical orbits?
« Reply #18 on: 08/10/2015 18:08:15 »
Quote from: MolonLabe
All true, but nobody yet has shown that the orbit has to be a conic section (which the conservation of angular momentum demands).
Circles, ellipses and hyperbolas are all conic sections

Quote from: MolonLabe
The above list omits the trivial cases of zero angular momentum, that the orbit is a straight line (into the sun - crash) and an even more obscure one, the point at the centre of the sun.
I omitted it because it's not part of the subject. The subject is Why do planets have elliptical orbits? A straight line into the Sun is not an orbit.
 

Offline MolonLabe

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Re: Why do planets have elliptical orbits?
« Reply #19 on: 08/10/2015 18:46:20 »
Quote from: MolonLabe
All true, but nobody yet has shown that the orbit has to be a conic section (which the conservation of angular momentum demands).
Circles, ellipses and hyperbolas are all conic sections

Yes, I said so in my previous post which you obviously missed.

Quote from: MolonLabe
The above list omits the trivial cases of zero angular momentum, that the orbit is a straight line (into the sun - crash) and an even more obscure one, the point at the centre of the sun.
I omitted it because it's not part of the subject. The subject is Why do planets have elliptical orbits? A straight line into the Sun is not an orbit.
[/quote]

You mention the hyperbola. That is no more of an orbit than a straight line is. And you still have not answered the question Why do planets have elliptical orbits?. You have explained why it is the preferred conic section, but not why it is a conic section.
 

Offline PmbPhy

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Re: Why do planets have elliptical orbits?
« Reply #20 on: 08/10/2015 20:19:39 »
Quote from: MolonLabe
You mention the hyperbola. That is no more of an orbit than a straight line is.
It's considered an orbit in physics parlance. Please learn the terminology at:
https://en.wikipedia.org/wiki/Orbit
It's referred to as an open trajectory and is a conic section. A straight line isn't.

Quote from: MolonLabe
And you still have not answered the question Why do planets have elliptical orbits?. You have explained why it is the preferred conic section, but not why it is a conic section.
I already told you why. And your comment about "preferred conic section" is meaningless. I've correctly explained everything that the OP wanted to know. I have no desire to talk about irrelevant, and in this case erroneous, semantics. I really don't see why you're making a big deal out of this. In any case the problem is your ignorance in the language of the physics of gravitation and Keppler's laws and the orbits of planets/asteroids/comets etc.

 

Offline evan_au

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Re: Why do planets have elliptical orbits?
« Reply #21 on: 08/10/2015 21:02:14 »
Quote from: PmbPhy
A straight line into the Sun is not an orbit.
A straight line into the Sun from a point within the Solar System is an extreme ("degenerate") ellipse, where the minor axis=0.
If it is a high-speed dive into the Sun (faster than escape velocity), a straight line is a degenerate hyperbola.
But in both cases it is swallowed by the Sun, so it is, at best, a temporary resident of the solar system.

A circle is the other extreme of an ellipse, where the minor axis=major axis.
« Last Edit: 08/10/2015 21:16:26 by evan_au »
 

Offline MolonLabe

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Re: Why do planets have elliptical orbits?
« Reply #22 on: 08/10/2015 21:07:21 »
Quote from: MolonLabe
You mention the hyperbola. That is no more of an orbit than a straight line is.
It's considered an orbit in physics parlance. Please learn the terminology at:
https://en.wikipedia.org/wiki/Orbit [nofollow]
It's referred to as an open trajectory and is a conic section. A straight line isn't.

Quote from: MolonLabe
And you still have not answered the question Why do planets have elliptical orbits?. You have explained why it is the preferred conic section, but not why it is a conic section.
I already told you why. And your comment about "preferred conic section" is meaningless. I've correctly explained everything that the OP wanted to know. I have no desire to talk about irrelevant, and in this case erroneous, semantics. I really don't see why you're making a big deal out of this. In any case the problem is your ignorance in the language of the physics of gravitation and Keppler's laws and the orbits of planets/asteroids/comets etc.


No, you have not explained why a planet's orbit is an ellipse, as opposed to (say) x^4 + y^4 = 1 or some other closed loop equation. It is, but show me the mathematical derivation.

Your suggestion that I am ignorant about the physics of gravitation is laughable. I am perfectly familiar with Newton's Laws, and although Kepler is irrelevant, at least I can spell his name.

You might want to check up on conic sections, by the way - you might even find a couple of straight lines through the origin.
 

Offline jeffreyH

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Re: Why do planets have elliptical orbits?
« Reply #23 on: 09/10/2015 05:29:12 »
You can find the mathematical derivation here.

https://en.wikipedia.org/wiki/Kepler_orbit#Johannes_Kepler
 

Offline jeffreyH

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Re: Why do planets have elliptical orbits?
« Reply #24 on: 09/10/2015 05:39:26 »
Conic sections were considered by Thomas Harriot who corresponded with Kepler via one of Keplers associates. In the Harriot papers there is evidence that Kepler passed on work he had done on the orbits since Harriot showed the parabolic nature of the orbits of comets. Harriot also observed the moon and sunspots through a telescope and produce illustrations of both. I didn't read this on wikipedia. I researched it personally. I have a microfilm of the Harriot papers which I transferred to CD and passed to other researchers. I know Pete and also know that he is knowledgeable in physics. Why would you even require him to provide such a derivation? If you think you know more then why not just post it? We're here to help each other. It isn't a competition. You don't get a prize.
 

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Re: Why do planets have elliptical orbits?
« Reply #24 on: 09/10/2015 05:39:26 »

 

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