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Author Topic: Why is the solar system elliptical?  (Read 9654 times)

another_someone

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Why is the solar system elliptical?
« on: 01/12/2005 21:21:05 »
What has caused the Earth, and I assume the other planets, to have an elliptical orbit orbits around the sun.

As I understand matters, the planets were condensed out of a disk spinning around the early sun.  But, if that disk was created from a spinning sun, I would have expected that disk to be circular, and not elliptical.  So what would have distorted that circle into an ellipse?

The only way I can imagine this happening is some sort of linear drag upon the disk that would have elongated it.  What caused that drag?

The two scenarios I can imagine is either the orbit of the solar system around the galaxy creating some sort of drag, or some early encounter with another star (a close encounter or collision)

daveshorts

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Re: Why is the solar system elliptical?
« Reply #1 on: 02/12/2005 00:44:12 »
I don't know what the total elipticalness of the solar system is, but I think that if you have several circular orbits interactions between the planets will tend to take the orbits away from the perfect circle into ellipses.

another_someone

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Re: Why is the solar system elliptical?
« Reply #2 on: 02/12/2005 02:32:29 »
quote:
Originally posted by daveshorts

I don't know what the total elipticalness of the solar system is, but I think that if you have several circular orbits interactions between the planets will tend to take the orbits away from the perfect circle into ellipses.

I suppose one question one should ask is whether the elongation of the orbits of the various planets are all the same, implying a single event that stretched the orbits, or whether they each have a different degree of elongation of orbit, which would imply a number of small events effecting each planet differently.  The former would imply an external force, the latter would be consistent with internal interactions between the planets causing them to veer from the circular.

In fact, if the orbital distortions are purely due to internal interactions, then I would not only expect the different orbits to have different distortions, but I would expect compensating distortions, so that there would be a nett zero effect (e.g. one planetary orbit distorted in one direction, would be counterbalanced by an opposite distortion by another planet's orbit; taking into account that a small distortion by a massive planet might have to be offset by a very much larger opposing distortion in the orbit of a very light planet).

Is there such a zero sum distortion in the various orbits?
« Last Edit: 02/12/2005 03:26:17 by another_someone »

chris

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Re: Why is the solar system elliptical?
« Reply #3 on: 02/12/2005 12:33:02 »
Dave is right.

The trajectory of an orbiting body (in this case a planet) is a function of all of the gravitational forces acting upon that body.

In other words, all of the planets influence the movements of their neighbours; if you resolve, or balance out, all of these forces they settle into an elliptical orbital path.

Presumably if there were just a single massive body - the sun for example - and one planet, and no other gravitational influences, then I supposed you would expect a semblance of a circular orbit.

Chris

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Soul Surfer

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Re: Why is the solar system elliptical?
« Reply #4 on: 02/12/2005 13:21:22 »
Some of the thinking behind this question sounds a bit like the ancient theory that orbits must be circular because of the "perfection" of the circular shape.

Firstly  the solar system is not "elliptical".  The elliptical orbits of the planets are in no way similar in their ellipticity or aligned in any particular direction.

The normal orbit for one object about another is an ellipse. A circle is just an extreme case of an ellipse where everything happens to be precisely balanced.  Funnily enough even if the orbits of all the planets were set to be perfect circles with their current periods.  They would rapidly become elliptical because of the gravitational effects of the planets on each other.

The more interesting question is why the planets lie quite closely to the same plane and in approximately circular otbits because if you think of a cloud of gas dust anmd lumps contracting under its own gravity the initial orbits of all the particles would be strongly elliptical and falling in from all directions a bit like the orbits of comets.

The answer is that assuming the cloud had a bit of net rotation(angular momentum) about its centre initially forming a disc and then moving towards circular orbits is a way of minimising collisions with other lumps.

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another_someone

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Re: Why is the solar system elliptical?
« Reply #5 on: 02/12/2005 14:49:27 »
quote:
Originally posted by Soul Surfer

Some of the thinking behind this question sounds a bit like the ancient theory that orbits must be circular because of the "perfection" of the circular shape.

There may be analogies to be drawn, but the primary motivation was the notion that if the solar system is a remnant of the early sun, and the sun being a single body, its outer parts would spin in a circular motion.

quote:

Firstly  the solar system is not "elliptical".  The elliptical orbits of the planets are in no way similar in their ellipticity or aligned in any particular direction.

This was part of my auxiliary question.  If the orbits are not similar, then it reduces the likelihood that they were formed by a single external event (albeit, it does not preclude an external event, but it means that some internal rearrangements must have happened whether there was an external event or not).

quote:

The normal orbit for one object about another is an ellipse. A circle is just an extreme case of an ellipse where everything happens to be precisely balanced.

I don't see this.  I can understand that if an orbit is created by the capture of a satellite body, the linear motion of the body prior to capture, combined with the angular motion after capture, would lead to an elliptical orbit; but in the absence of any linear motion, I would not expect a lone satellite to be anything but circular in its orbit.

quote:

Funnily enough even if the orbits of all the planets were set to be perfect circles with their current periods.  They would rapidly become elliptical because of the gravitational effects of the planets on each other.

I accept the distinction people are making about multiple satellites creating more complex orbital patterns, but I would argue that any disturbance that one satellite imparts on another must also have a corresponding reaction force, so thus creating a nett zero sum.  It may be that such a nett zero sum is observed in the orbits of the planets – this was also one of the auxiliary questions I asked.

Thinking about this a little more, I realise that in fact the sun and the solar system must be influenced by the interstellar (galactic) wind, and this could easily account for the elliptical aspects of the solar satellite orbits.  The  interstellar wind is sufficient to create a bow wave ahead of the motion of the sun within the galaxy, and so anything behind this shock wave must be influenced by its shape.

DoctorBeaver

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Re: Why is the solar system elliptical?
« Reply #6 on: 02/12/2005 16:58:44 »
If the gravity of planets affects all of the others, would it not be logical to assume that the orbits of Jupiter & Saturn - being the heaviest of the planets & the 2 with the strongest gravitational fields - would be more circular than the others? Is this the case?

another_someone

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Re: Why is the solar system elliptical?
« Reply #7 on: 02/12/2005 22:24:44 »
quote:
Originally posted by DoctorBeaver

If the gravity of planets affects all of the others, would it not be logical to assume that the orbits of Jupiter & Saturn - being the heaviest of the planets & the 2 with the strongest gravitational fields - would be more circular than the others? Is this the case?

Looking at http://en.wikipedia.org/wiki/Planet_%28Table%29, the answer is definitely not so simple.

It is not inevitable that the largest planets would have the most circular orbits, only that if the eccentricities are caused by purely internal interactions, some of the most eccentric orbits should be by the smaller planets (it is possible that some of the smaller planets were simply not affected at all, but those that were influenced should have had a disproportionately greater influence upon their orbits).

Although there is no overall pattern of heavier planets having greater eccentricity than lighter ones, it does seem that where a heavier  and lighter planet are in adjacent orbits, the heavier of the two has the more eccentric orbit.

ukmicky

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Re: Why is the solar system elliptical?
« Reply #8 on: 02/12/2005 22:46:45 »
posted by another-someone What has caused the Earth, and I assume the other planets, to have an elliptical orbit orbits around the sun.

As I understand matters, the planets were condensed out of a disk spinning around the early sun. But, if that disk was created from a spinning sun, I would have expected that disk to be circular, and not elliptical. So what would have distorted that circle into an ellipse?

The only way I can imagine this happening is some sort of linear drag upon the disk that would have elongated it. What caused that drag?

The two scenarios I can imagine is either the orbit of the solar system around the galaxy creating some sort of drag, or some early encounter with another star (a close encounter or collision).

----------------------------------------------------

The disk of material which formed around our sun only influenced the composition of the planets and there direction of travel around the sun.

All planets  orbiting a sun have elliptical orbits, they can't have anything else.

All planets do affect each others orbits as all planets some more than others tug on the sun causing it to wobble, the same wobble which they look for to find planets orbiting distant suns.
In our solar system the planets orbits are only slightly elliptical and are not far off circular. The reason being the large gas giants but mostly jupiter are quite far out and at the distance that they are, they are not large enough and do not produce enough gravity to tug on the sun very much. It’s this tugging or moving of the sun if you like which causes the planets to have noN circular orbits.

Disclaimer I could be wrong ,but thinking about it I feel I’m right or at least could be

Michael
« Last Edit: 03/12/2005 00:40:23 by ukmicky »

Soul Surfer

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Re: Why is the solar system elliptical?
« Reply #9 on: 03/12/2005 00:44:22 »
Quote fom "another someone"

I don't see this. I can understand that if an orbit is created by the capture of a satellite body, the linear motion of the body prior to capture, combined with the angular motion after capture, would lead to an elliptical orbit; but in the absence of any linear motion, I would not expect a lone satellite to be anything but circular in its orbit.

This statement demonstrates a deep and fundamental misunderstanding of the way that gravitational capture and orbits operate.

Firstly it is vital to understand that if we have just two small bodies (ie they don't collide) with some initial arbitrary relative velocity with respect to one another that are attracted by gravity towards each other from a great distance it is impossible for them to go into orbit around each other. The resulting orbit is hyperbolic and they just deflect each other's paths.

The reason for this is that the relative angular momentum of the two bodies will always exceed the escape velocity of the system.

If a closed orbit is to be created (using only gravity) it needs the interposition of a third gravitating body which slows the relative velocities down to below the escape velocity of the gravitating pair.

The ellipticity of the orbit is just determined by the relative angular momenum of the two bodies between the maximum defined by the escape velocity and the absolute minimum defined by a circular orbit.  In any capture or change of orbit process it is extreemely unlikely that the resultant angular momentum of any object will be the absolute mininmum for its given orbit.  That is there is very little reason why any orbit should be EXACTLY circular

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another_someone

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Re: Why is the solar system elliptical?
« Reply #10 on: 03/12/2005 07:20:42 »
quote:
Originally posted by Soul Surfer

Firstly it is vital to understand that if we have just two small bodies (ie they don't collide) with some initial arbitrary relative velocity with respect to one another that are attracted by gravity towards each other from a great distance it is impossible for them to go into orbit around each other. The resulting orbit is hyperbolic and they just deflect each other's paths.

The reason for this is that the relative angular momentum of the two bodies will always exceed the escape velocity of the system.

If a closed orbit is to be created (using only gravity) it needs the interposition of a third gravitating body which slows the relative velocities down to below the escape velocity of the gravitating pair.

Accepted - I had not thought through properly what I was saying – mea culpa (the proviso is when the escape velocity approaches the speed of light – but that is a different topic).

another_someone

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Re: Why is the solar system elliptical?
« Reply #11 on: 03/12/2005 09:02:58 »
quote:

The disk of material which formed around our sun only influenced the composition of the planets and there direction of travel around the sun.

But if the influenced the direction of travel, then it must follow that they also must have created in initial parameters for their motion, for what else could have created those initial parameters?  That these parameters may later have been modified by other factors is then a matter of idenifying those other factors.

quote:

All planets  orbiting a sun have elliptical orbits, they can't have anything else.

All planets do affect each others orbits as all planets some more than others tug on the sun causing it to wobble, the same wobble which they look for to find planets orbiting distant suns.
In our solar system the planets orbits are only slightly elliptical and are not far off circular. The reason being the large gas giants but mostly jupiter are quite far out and at the distance that they are, they are not large enough and do not produce enough gravity to tug on the sun very much. It’s this tugging or moving of the sun if you like which causes the planets to have noN circular orbits.

Disclaimer I could be wrong ,but thinking about it I feel I’m right or at least could be

OK, when I originally posed the question, I had not actually done any background search on the amount of eccentricity of the orbits of the planets, and having now done that, and found the eccentricities both to be very slight, and highly varied, and with the relative correlation between neighbouring massive and light planets (the peculiarity to this pattern is the disparity between the eccentricity of the Earth and its neighbour Venus), I agree that this would be more consistent with internal gravitational interactions between the planets than with an external drag upon the orbits of the planets.

I cannot see your argument about the eccentricity of the orbits being mediated by the sun (maybe with the exception of the orbit of Mercury, which is relatively highly eccentric).  I understand what you say about the sun wobbling under planetary influence, but that alone need not be an eccentric wobble, and if the sun were to mediate the eccentricities of the planets, then one would expect those planets that were lighter, and closer to the sun, to have more eccentric orbits.  In fact, Uranus, which is both heavier and further out than the Earth, has a more eccentric orbit than the Earth; while Venus, which is of comparable mass to the Earth (only fractionally lighter), but closer to the sun (being the second closest planet), is the least eccentric of all the orbits.

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Re: Why is the solar system elliptical?
« Reply #12 on: 03/12/2005 11:13:37 »
quote:
Although there is no overall pattern of heavier planets having greater eccentricity than lighter ones, it does seem that where a heavier and lighter planet are in adjacent orbits, the heavier of the two has the more eccentric orbit.

The HEAVIER of the 2 has the more eccentric orbit? That would imply that a small planet has a greater influence over a large planet than does a large planet over a small 1. That doesn't sound right to me.

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another_someone

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Re: Why is the solar system elliptical?
« Reply #13 on: 03/12/2005 11:31:43 »
quote:
Originally posted by DoctorBeaver

quote:
Although there is no overall pattern of heavier planets having greater eccentricity than lighter ones, it does seem that where a heavier and lighter planet are in adjacent orbits, the heavier of the two has the more eccentric orbit.

The HEAVIER of the 2 has the more eccentric orbit? That would imply that a small planet has a greater influence over a large planet than does a large planet over a small 1. That doesn't sound right to me.

"There is no such thing on earth as an uninteresting subject; the only thing that can exist is an uninterested person."
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Apologies – brain and fingers disconnected (I think it might be said my brain has simply been disconnected over the last 36 hours).

It is the lighter with the greater eccentricity (the exception seems to be that Venus is less eccentric than the Earth, although it is fractionally lighter; but then the Earth is also influenced by Mars on its other side, which, while lighter than even Venus, is still twice as heavy as Mercury, which is on the other side of Venus).
« Last Edit: 03/12/2005 11:33:08 by another_someone »

Soul Surfer

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Re: Why is the solar system elliptical?
« Reply #14 on: 03/12/2005 13:54:50 »
THe eccentricity of planetary orbits probably has more to do with detailed events during their accretion from the orriginal panetessimals than the environment that the whole solar syastem was passing through The location of the smaller planets has a great deal to do with the orbits of the largest planets notably jupiter.

One point to note some of these replies talk about the planetary dsic being drawn from the sun in some way.  This may have been in old theories of planetary formation the current thinking is that the planets were formed awith the sun and were an important sink for the excess of angular momentum that existed in the solar nebula because without them the sun would have rotated too fast and become unstable.

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DoctorBeaver

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Re: Why is the solar system elliptical?
« Reply #15 on: 03/12/2005 17:17:41 »
another_someone - thank gawd for that! My poor little brain was going into turbo mode trying to figure out why the heavier planet would be affected more [xx(]

ukmicky

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Re: Why is the solar system elliptical?
« Reply #16 on: 04/12/2005 05:30:24 »
At the moment the earth orbit is very circular however every every 100,000 years and every 413,000 years it goes through a cycle where it becomes more eccentric.
Reading about it some believe the cycle is to long to be caused from gravitational forces within the solar system and must be caused through external influences like dark matter or even companion stars ?.
There does seems to be much speculation in the scientific community as to the influences and reasons regarding planetary orbits but it seems to me that most of it is guesswork.

Michael

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Re: Why is the solar system elliptical?
« Reply #17 on: 04/12/2005 10:36:30 »
quote:
At the moment the earth orbit is very circular however every every 100,000 years and every 413,000 years it goes through a cycle where it becomes more eccentric.

How did they discover that? We haven't been studying the Earth's orbit for that long!

another_someone

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Re: Why is the solar system elliptical?
« Reply #18 on: 04/12/2005 12:10:56 »
quote:
Originally posted by ukmicky

At the moment the earth orbit is very circular however every every 100,000 years and every 413,000 years it goes through a cycle where it becomes more eccentric.
Reading about it some believe the cycle is to long to be caused from gravitational forces within the solar system and must be caused through external influences like dark matter or even companion stars ?.
There does seems to be much speculation in the scientific community as to the influences and reasons regarding planetary orbits but it seems to me that most of it is guesswork.

Michael

http://en.wikipedia.org/wiki/Nemesis_%28star%29
quote:

Nemesis is the name given to a hypothetical red dwarf star or brown dwarf, orbiting the Sun at a distance of about 50,000 to 100,000 AU, somewhat beyond the Oort cloud. Richard A. Muller suggests that the most likely object is a red dwarf with magnitude between 7 and 12 [1]. Such a bright object would undoubtedly already be in existing star catalogs, but its true nature would only be detectable by measuring its parallax; due to orbiting the Sun it would have a very low proper motion and would escape detection by proper motion surveys that have found stars like the 9th magnitude Barnard's star.
Nemesis' existence was proposed by Muller in 1984 (Nature, vol 308, pp 715-717, 1984) to explain an apparent 26-million year cycle in the occurrence of mass extinctions on Earth as noted by Raup and Sepkoski.
According to the theory, Nemesis periodically (roughly every 26 million years), passes through a denser region of the Oort cloud, disrupting the orbits of comets, and sending millions of comets into the inner solar system and potential collision with the Earth. It was initially nicknamed the "death star" (after the fictional Star Wars weapon), although this nickname has fallen out of usage.
No direct evidence for Nemesis has been found, however, and the existence of a periodicity in the Earth's series of mass extinctions is disputed.
It is also worth noting that Matese and Whitman have suggested that the cycle might be caused by the solar system oscillating across the galactic plane. These oscillations may lead to gravitational disturbances in the Oort cloud with the same proposed consequences as the orbit of "Nemesis". However, the period of oscillation is not well-constrained observationally, and may differ from the needed 26 million years by as much as 40%.
If Nemesis exists, it may be detected by the planned Pan-STARRS astronomical survey, or similar future projects.

ekc

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Re: Why is the solar system elliptical?
« Reply #19 on: 06/12/2005 19:02:37 »
quote:
Originally posted by ukmicky

At the moment the earth orbit is very circular however every every 100,000 years and every 413,000 years it goes through a cycle where it becomes more eccentric.
Reading about it some believe the cycle is to long to be caused from gravitational forces within the solar system and must be caused through external influences like dark matter or even companion stars ?.

That said, there is a systematic variation in the Earth's orbital eccentricity caused by local solar system interactions, and it is thought to have a period of approximately 100,000 years.  Google "Milankovitch cycles" for more information.

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Re: Why is the solar system elliptical?
« Reply #19 on: 06/12/2005 19:02:37 »