# The Naked Scientists Forum

### Author Topic: Why the Planets of a Solar System all orbit their Star at the same Velocity.  (Read 782 times)

#### RTCPhysics

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##### Why the Planets of a Solar System all orbit their Star at the same Velocity.
« on: 22/05/2016 11:38:01 »
A solar system is reliant for its existence upon the balancing of two opposing forces. The first force creates an attraction between bodies of matter and the second force is the ‘kinetic energy’ of the body of matter, whose magnitude determines whether the individual planets will lock into orbit, spiral inwards towards the star or break free from the system. The limit that determines how fast a planet can orbit within a gravitational field, is called its ‘escape velocity’.

The observation that all the planets of a solar system orbit their star at the same velocity, implies that each planet must have the same ‘escape velocity’.  So the question that needs to be answered is “How does this come about?”

For the purposes of this explanation, Newton’s concept of gravity has been adopted for the attracting force between bodies of matter, rather than Einstein’s ‘General Theory of Relativity’. Both are known to work equally well, when calculating the trajectories of space probes to reach other planets, but the use of ‘gravity’ to describe the attracting force between two bodies of matter is an easier concept to adopt for this explanation.

Newton coined the term ‘gravity’ and quantified it by his iconic formula that: the gravitational force between two bodies of matter is proportional to the product (not sum) of their masses and inversely proportional to the square of their distance apart. Newton's formula is based upon the concept that every ‘unit of matter’ in one body of matter, whether an atom or a molecule, an electron or a quark, attracts every other ‘unit of matter’ within another body of matter and the force between the two bodies arises from the accumulation of these unit forces. Hence m1 x m2 rather than m1 + m2.

So the magnitude of the gravitational attraction operating between two masses of the same size is exactly the same as the attracting force between two masses which have the same total mass, but have been split into a larger mass and a smaller mass. This latter situation models the situation that we have within our own solar system.

But if the ‘distance’ between these two bodies of matter is doubled, then the joint mass of the two bodies needs to be quadrupled in order to maintain the same level of gravitational attraction. And if the distance is trebled then the joint mass needs to be increased nine fold. So distance has a more attenuating effect upon gravitational attraction than variations in the amount of mass and this has played an important role in the development of our solar system from its original existence as a swirling mass of ‘gas plasma’.

The counter balancing force to ‘gravity’ for a body of matter in orbit around another, is the ‘kinetic energy’ of the orbiting body, which is referred to as its ‘centrifugal force’.  The reality of the centrifugal force can be experienced by anyone at any time, simply by swinging a weight on a rope around themselves and feeling the pulling sensation that it induces upon the arms and shoulders by the circulating weight.  Without the centrifugal force that each orbiting body of matter experiences, then all the planets within the solar system would simply spiral inwards towards the sun.

So, although the star is the ‘primary gravitational force’ that keeps the planets in their orbits, it is the ‘secondary gravitational forces’ between the planets, that holds the key to why the planets all orbit their star at the same velocity.

If Mercury was on its own in orbit around the sun, then it would orbit at its ‘escape velocity’. Slower and it would spiral into the Sun. Faster and it would exit its orbit of the Sun like a ‘sling shot’.  However, by introducing a neighbouring planet, such as Venus into an outer orbit, the presence of another planet has a significant impact upon the orbital velocity of Mercury.

As both planets are orbiting in the same direction and in the same plane, they are attracted by gravity towards each other, with the force being at a maximum when they are at their nearest point of approach. The gravitational force acts to bring the orbits of the two planets nearer together, with the planet of larger mass having the greatest effect. So Mercury would be moved into an orbit further away from the Sun and Venus is moved into an orbit nearer to the Sun. This process repeats itself upon every pass that Mercury makes by Venus.

This coming together of the two planets as they interact at their closest point, is counterbalanced by the centrifugal force generated by this orbital change. Venus speeds up, Mercury slows down, both experiencing different escape velocities to those which they would have had without each other’s presence. Venus is experiencing a greater inwards attraction through the joint gravitational effects of the Sun and Mercury, so it speeds up to a new faster ‘escape velocity’, whereas Mercury is experiencing a smaller gravitational attraction, as Venus counteracts the Sun’s gravitational influence. So Venus speeds up and Mercury slows down and equilibrium is achieved when both planets are orbiting at the same ‘escape velocity’.

What is effectively happening is that the two planets are adjusting the distance between themselves, such that the compounded gravitational pull that each experiences from the Sun and the other planet is at exactly the same amount. By experiencing the same gravitational force, the two planets will orbit at exactly the same ‘escape velocity’.

This process is repeated for each of the planets that exist in our solar system. During its formation, the planets constantly adjusted their distances from one another, reaching equilibrium when the gravitational pull of the inner planets upon them was matched by the gravitational pull of the outer planets upon them and all the planets settled into the same ‘escape velocity’. Measure one planet’s orbital velocity and you know the rest, which of course, includes our earth.

For the outermost planet, Pluto, (assuming that another planet does not exist in the outermost region of our solar space), its ‘escape velocity’ is determined by the total sum of all the other planets inward gravitational pull upon it, including the Sun. From Pluto’s perspective, the Sun is just another distant planet pulling it inwards.

An analogy for this 'planetary equilibrium situation' is a race upon an athletics track, where the participants are all running at the same speed, but confined to their circular lanes. The runners in the inner lanes will draw ahead of the outer lane runners and eventually will lap them, simply because the inner runners are covering a shorter distance upon each lap.

If any of the planets experienced a traumatic event which changed its orbital speed, shifted its orbital plane or expelled it out of the solar system, then all the planets would start the process of adjusting the distances between themselves and settle down into a pattern, which once again gave them all the same ‘escape velocity’, albeit at a different level from that which they have now.

So the gravitational pull upon an individual located on the earth is not just a function of the earth’s gravity, but is a compounded effect from all the planets including the Sun. In which case we should strictly say, that we live in a ‘Planetary system’, not a ‘Solar system’.

« Last Edit: 22/05/2016 11:48:18 by RTCPhysics »

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#### Thebox

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##### Re: Why the Planets of a Solar System all orbit their Star at the same Velocity.
« Reply #1 on: 22/05/2016 11:47:59 »
A solar system is reliant for its existence upon the balancing of two opposing forces. The first force creates an attraction between bodies of matter and the second force is the ‘kinetic energy’ of the body of matter, whose magnitude determines whether the individual planets will lock into orbit, spiral inwards towards the star or break free from the system. The limit that determines how fast a planet can orbit within a gravitational field, is called its ‘escape velocity’.

The observation that all the planets of a solar system orbit their star at the same velocity, implies that each planet must have the same ‘escape velocity’.  So the question that needs to be answered is “How does this come about?”

For the purposes of this explanation, Newton’s concept of gravity has been adopted for the attracting force between bodies of matter, rather than Einstein’s ‘General Theory of Relativity’. Both are known to work equally well, when calculating the trajectories of space probes to reach other planets, but the use of ‘gravity’ to describe the attracting force between two bodies of matter is an easier concept to adopt for this explanation.

Newton coined the term ‘gravity’ and quantified it by his iconic formula that: the gravitational force between two bodies of matter is proportional to the product (not sum) of their masses and inversely proportional to the square of their distance apart. Newton's formula is based upon the concept that every ‘unit of matter’ in one body of matter, whether an atom or a molecule, an electron or a quark, attracts every other ‘unit of matter’ within another body of matter and the force between the two bodies arises from the accumulation of these unit forces. Hence m1 x m2 rather than m1 + m2.

So the magnitude of the gravitational attraction operating between two masses of the same size is exactly the same as the attracting force between two masses which have the same total mass, but have been split into a larger mass and a smaller mass. This latter situation models the situation that we have within our own solar system.

But if the ‘distance’ between these two bodies of matter is doubled, then the joint mass of the two bodies needs to be quadrupled in order to maintain the same level of gravitational attraction. And if the distance is trebled then the joint mass needs to be increased nine fold. So distance has a more attenuating effect upon gravitational attraction than variations in the amount of mass and this has played an important role in the development of our solar system from its original existence as a swirling mass of ‘gas plasma’.

The counter balancing force to ‘gravity’ for a body of matter in orbit around another, is the ‘kinetic energy’ of the orbiting body, which is referred to as its ‘centrifugal force’.  The reality of the centrifugal force can be experienced by anyone at any time, simply by swinging a weight on a rope around themselves and feeling the pulling sensation that it induces upon the arms and shoulders by the circulating weight.  Without the centrifugal force that each orbiting body of matter experiences, then all the planets within the solar system would simply spiral inwards towards the sun.

So, although the star is the ‘primary gravitational force’ that keeps the planets in their orbits, it is the ‘secondary gravitational forces’ between the planets, that holds the key to why the planets all orbit their star at the same velocity.

If Mercury was on its own in orbit around the sun, then it would orbit at its ‘escape velocity’. Slower and it would spiral into the Sun. Faster and it would exit its orbit of the Sun like a ‘sling shot’.  However, by introducing a neighbouring planet, such as Venus into an outer orbit, the presence of another planet has a significant impact upon the orbital velocity of Mercury.

As both planets are orbiting in the same direction and in the same plane, they are attracted by gravity towards each other, with the force being at a maximum when they are at their nearest point of approach. The gravitational force acts to bring the orbits of the two planets nearer together, with the planet of larger mass having the greatest effect. So Mercury would be moved into an orbit further away from the Sun and Venus is moved into an orbit nearer to the Sun. This process repeats itself upon every pass that Mercury makes by Venus.

This coming together of the two planets as they interact at their closest point, is counterbalanced by the centrifugal force generated by this orbital change. Venus speeds up, Mercury slows down, both experiencing different escape velocities to those which they would have had without each other’s presence. Venus is experiencing a greater inwards attraction through the joint gravitational effects of the Sun and Mercury, so it speeds up a new faster ‘escape velocity’, whereas Mercury is experiencing a smaller gravitational attraction, as Venus counteracts the Sun’s gravitational influence. So Venus speeds up and Mercury slows down and equilibrium is achieved when both planets are orbiting at the same ‘escape velocity’.

What is effectively happening is that the two planets are adjusting the distance between themselves, such that the compounded gravitational pull that each experiences from the Sun and the other planet is at exactly the same amount. By experiencing the same gravitational force, the two planets will orbit at exactly the same ‘escape velocity’.

This process is repeated for each of the planets that exist in our solar system. During its formation, the planets constantly adjusted their distances from one another, reaching equilibrium when the gravitational pull of the inner planets upon them was matched by the gravitational pull of the outer planets upon them and all the planets settled into the same ‘escape velocity’. Measure one planet’s orbital velocity and you know the rest, which of course, includes our earth.

For the outermost planet, Pluto, (assuming that another planet does not exist in the outermost region of our solar space), its ‘escape velocity’ is determined by the total sum of all the other planets inward gravitational pull upon it, including the Sun. From Pluto’s perspective, the Sun is just another distant planet pulling it inwards.

An analogy for this 'planetary equilibrium situation' is a race upon an athletics track, where the participants are all running at the same speed, but confined to their circular lanes. The runners in the inner lanes will draw ahead of the outer lane runners and eventually will lap them, simply because the inner runners are covering a shorter distance upon each lap.

If any of the planets experienced a traumatic event which changed its orbital speed, shifted its orbital plane or expelled it out of the solar system, then all the planets would start the process of adjusting the distances between themselves and settle down into a pattern, which once again gave them all the same ‘escape velocity’, albeit at a different level from that which they have now.

So the gravitational pull upon an individual located on the earth is not just a function of the earth’s gravity, but is a compounded effect from all the planets including the Sun. In which case we should strictly say, that we live in a ‘Planetary system’, not a ‘Solar system’.

Thank you for sharing your thoughts, it was a very interesting read, I have used the athletic track in examples myself in the past when explaining ''plasma'' problems. I will read it again later with a fresher head before I comment on any points, but I do like the ''sound'' of this.
I also believe thermodynamics play a part and also a sort of ''electrodynamics buoyancy''.

I will get back to you ''later''.

#### chiralSPO

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##### Re: Why the Planets of a Solar System all orbit their Star at the same Velocity.
« Reply #2 on: 22/05/2016 13:24:27 »
A solar system is reliant for its existence upon the balancing of two opposing forces. The first force creates an attraction between bodies of matter and the second force is the ‘kinetic energy’ of the body of matter, whose magnitude determines whether the individual planets will lock into orbit, spiral inwards towards the star or break free from the system. The limit that determines how fast a planet can orbit within a gravitational field, is called its ‘escape velocity’.

The observation that all the planets of a solar system orbit their star at the same velocity, implies that each planet must have the same ‘escape velocity’.  So the question that needs to be answered is “How does this come about?”

For the purposes of this explanation, Newton’s concept of gravity has been adopted for the attracting force between bodies of matter, rather than Einstein’s ‘General Theory of Relativity’. Both are known to work equally well, when calculating the trajectories of space probes to reach other planets, but the use of ‘gravity’ to describe the attracting force between two bodies of matter is an easier concept to adopt for this explanation.

Newton coined the term ‘gravity’ and quantified it by his iconic formula that: the gravitational force between two bodies of matter is proportional to the product (not sum) of their masses and inversely proportional to the square of their distance apart. Newton's formula is based upon the concept that every ‘unit of matter’ in one body of matter, whether an atom or a molecule, an electron or a quark, attracts every other ‘unit of matter’ within another body of matter and the force between the two bodies arises from the accumulation of these unit forces. Hence m1 x m2 rather than m1 + m2.

So the magnitude of the gravitational attraction operating between two masses of the same size is exactly the same as the attracting force between two masses which have the same total mass, but have been split into a larger mass and a smaller mass. This latter situation models the situation that we have within our own solar system.

But if the ‘distance’ between these two bodies of matter is doubled, then the joint mass of the two bodies needs to be quadrupled in order to maintain the same level of gravitational attraction. And if the distance is trebled then the joint mass needs to be increased nine fold. So distance has a more attenuating effect upon gravitational attraction than variations in the amount of mass and this has played an important role in the development of our solar system from its original existence as a swirling mass of ‘gas plasma’.

The counter balancing force to ‘gravity’ for a body of matter in orbit around another, is the ‘kinetic energy’ of the orbiting body, which is referred to as its ‘centrifugal force’.  The reality of the centrifugal force can be experienced by anyone at any time, simply by swinging a weight on a rope around themselves and feeling the pulling sensation that it induces upon the arms and shoulders by the circulating weight.  Without the centrifugal force that each orbiting body of matter experiences, then all the planets within the solar system would simply spiral inwards towards the sun.

So, although the star is the ‘primary gravitational force’ that keeps the planets in their orbits, it is the ‘secondary gravitational forces’ between the planets, that holds the key to why the planets all orbit their star at the same velocity.

If Mercury was on its own in orbit around the sun, then it would orbit at its ‘escape velocity’. Slower and it would spiral into the Sun. Faster and it would exit its orbit of the Sun like a ‘sling shot’.  However, by introducing a neighbouring planet, such as Venus into an outer orbit, the presence of another planet has a significant impact upon the orbital velocity of Mercury.

As both planets are orbiting in the same direction and in the same plane, they are attracted by gravity towards each other, with the force being at a maximum when they are at their nearest point of approach. The gravitational force acts to bring the orbits of the two planets nearer together, with the planet of larger mass having the greatest effect. So Mercury would be moved into an orbit further away from the Sun and Venus is moved into an orbit nearer to the Sun. This process repeats itself upon every pass that Mercury makes by Venus.

This coming together of the two planets as they interact at their closest point, is counterbalanced by the centrifugal force generated by this orbital change. Venus speeds up, Mercury slows down, both experiencing different escape velocities to those which they would have had without each other’s presence. Venus is experiencing a greater inwards attraction through the joint gravitational effects of the Sun and Mercury, so it speeds up to a new faster ‘escape velocity’, whereas Mercury is experiencing a smaller gravitational attraction, as Venus counteracts the Sun’s gravitational influence. So Venus speeds up and Mercury slows down and equilibrium is achieved when both planets are orbiting at the same ‘escape velocity’.

What is effectively happening is that the two planets are adjusting the distance between themselves, such that the compounded gravitational pull that each experiences from the Sun and the other planet is at exactly the same amount. By experiencing the same gravitational force, the two planets will orbit at exactly the same ‘escape velocity’.

This process is repeated for each of the planets that exist in our solar system. During its formation, the planets constantly adjusted their distances from one another, reaching equilibrium when the gravitational pull of the inner planets upon them was matched by the gravitational pull of the outer planets upon them and all the planets settled into the same ‘escape velocity’. Measure one planet’s orbital velocity and you know the rest, which of course, includes our earth.

For the outermost planet, Pluto, (assuming that another planet does not exist in the outermost region of our solar space), its ‘escape velocity’ is determined by the total sum of all the other planets inward gravitational pull upon it, including the Sun. From Pluto’s perspective, the Sun is just another distant planet pulling it inwards.

An analogy for this 'planetary equilibrium situation' is a race upon an athletics track, where the participants are all running at the same speed, but confined to their circular lanes. The runners in the inner lanes will draw ahead of the outer lane runners and eventually will lap them, simply because the inner runners are covering a shorter distance upon each lap.

If any of the planets experienced a traumatic event which changed its orbital speed, shifted its orbital plane or expelled it out of the solar system, then all the planets would start the process of adjusting the distances between themselves and settle down into a pattern, which once again gave them all the same ‘escape velocity’, albeit at a different level from that which they have now.

So the gravitational pull upon an individual located on the earth is not just a function of the earth’s gravity, but is a compounded effect from all the planets including the Sun. In which case we should strictly say, that we live in a ‘Planetary system’, not a ‘Solar system’.

Yes the planets interact with each other gravitationally. But the effect is very small compared to each planet's interaction with the central star.

The rest of this is completely misguided as the planets in our solar system are all orbiting with different velocities!
Mercury reaches speeds up to 56 km/s while Venus travels at a more modest 35 km/s. Earth is even slower at 30 km/s, and the speeds decrease as you go farther and farther from the star (Saturn is going at a mere 10 km/s).

#### jeffreyH

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##### Re: Why the Planets of a Solar System all orbit their Star at the same Velocity.
« Reply #3 on: 22/05/2016 13:43:24 »
In the case of circular orbits for a full explanation see:

https://en.m.wikipedia.org/wiki/Circular_orbit

This also deals with the relativistic case.

#### RTCPhysics

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##### Re: Why the Planets of a Solar System all orbit their Star at the same Velocity.
« Reply #4 on: 25/05/2016 11:49:50 »
Quote from ChiralSPO

"Yes the planets interact with each other gravitationally. But the effect is very small compared to each planet's interaction with the central star.

The rest of this is completely misguided as the planets in our solar system are all orbiting with different velocities!

Mercury reaches speeds up to 56 km/s while Venus travels at a more modest 35 km/s. Earth is even slower at 30 km/s, and the speeds decrease as you go farther and farther from the star (Saturn is going at a mere 10 km/s)."

1.   All the Planets must circulate around the central star at their ‘escape velocities’, otherwise they will spiral in towards each other under the gravitational influence of the central star or alternatively, exit the solar system.

2.   I stated at the start of my article, that the Planets had been ‘observed’ to orbit around the Sun at the same velocities and that this was inexplicable, as under the sole influence of the Sun’s gravitational field, they should all travel at different speeds from fast to slow, reflecting the rapid fall off with distance in the magnitude of the sun’s gravitational field, mitigated only by the varying masses of the planets.

3.   However, if Newton’s formula is to be believed, then the gravitational attraction between two planets approaches infinity as the distance between them tends to zero. So by adjusting the distance between the orbits of two neighbouring planets, the gravitational force between them can reduce or negate the inwards pull of the central star upon them, whatever the star’s mass.

4.   The ‘escape velocities’ published by Nasa.gov do not agree with the ‘orbiting velocities’ that you have quoted. (Mercury: 4.3Km/sec, Venus: 10.4Km/sec, Earth: 11.2Km/sec, Saturn: 35.5Km/sec, Pluto: 1.3Km/sec).

If ‘escape velocities’ and ‘orbiting velocities’ are different entities, then there must be another force, other than gravity and centrifugal force, that acts upon the planets. Otherwise the logic in the article still seems valid.

#### chiralSPO

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##### Re: Why the Planets of a Solar System all orbit their Star at the same Velocity.
« Reply #5 on: 25/05/2016 13:37:44 »

4.   The ‘escape velocities’ published by Nasa.gov do not agree with the ‘orbiting velocities’ that you have quoted. (Mercury: 4.3Km/sec, Venus: 10.4Km/sec, Earth: 11.2Km/sec, Saturn: 35.5Km/sec, Pluto: 1.3Km/sec).

These escape velocities ↑ are how fast one has to go to escape the gravitational well of the planet, from the surface of the planet. These numbers have nothing to do with the Sun or the orbital motion of the planets, and are determined purely by the mass and size of the planets themselves.

#### hamdani yusuf

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##### Re: Why the Planets of a Solar System all orbit their Star at the same Velocity.
« Reply #6 on: 31/05/2016 17:53:51 »
3.   However, if Newton’s formula is to be believed, then the gravitational attraction between two planets approaches infinity as the distance between them tends to zero. So by adjusting the distance between the orbits of two neighbouring planets, the gravitational force between them can reduce or negate the inwards pull of the central star upon them, whatever the star’s mass.
https://en.wikipedia.org/wiki/Lagrangian_point

#### impyre

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##### Re: Why the Planets of a Solar System all orbit their Star at the same Velocity.
« Reply #7 on: 01/06/2016 14:25:21 »
The escape velocity for a system depends primarily on the parent object (though all bodies contribute to the total system mass). The escape velocity is a representation of the total energy needed to establish a trajectory with an eccentricity >= 1, which defines a path that doesn't return (non-periodic). This is a property of the system as a whole, and thus is shared among its members. Even sub-systems will have their own internal escape velocity (such as escaping earth) which if reached will take the propelled object into the larger parent system (sun-centered orbit in this case) where the influence due to the old parent object (earth) becomes negligible and a new object (sun) becomes the most influential. Depending on the altitude of the orbit, varying starting kinetic and potential energies can result in very different amounts of acceleration required for escape. For an object orbiting very far away, even small accelerations can be enough to accrue the total energy required for escape.
Also, centrifugal force is not a thing. Centripetal force is a thing, and in this context the centripetal force at work is due to gravity. "Centrifugal" force is simply a poor way of describing an object's tendency to continue moving in the same direction unless acted on by an outside force. A better word for this is inertia. Inertia is what provides the counterbalance to the centripetal force provided by gravity.
Gravitational force cannot be negated in the way you describe. It's easier to think of gravity acting on groups of bodies as acting on that group's barycenter. You must consider that the force of gravity created by the star affects not one planet, but both. It pulls both in the same direction, toward the star. For two neighboring planets orbiting at different altitudes, this means that the higher planet will have both the star and the lower planet pulling it down, and the lower planet will feel the higher planet pulling it up and the star pulling it down. If you sum all the forces due to gravity it will still be a net force pulling toward the center of mass of the system as a whole. One way planets often find stable arrangements in this situation is by simply orbiting each other, becoming tidally locked. This has the effect that they get to "take turns" taking the greater load of being the higher planet. Another method is with harmonic periods with other bodies in the system.
Another poster mentioned LaGrangian points, but I feel that it's important to point out that most LaGrangian points are less like "wells" where stability can be achieved easily because gravity is "cancelled" and more like "humps" where the object could really fall either way. This is especially true of the points that lie on the line established between the orbiting body and the parent (such as you were describing). The only points where stability can really be achieved are on the L4 and L5, which are ahead of and behind the orbiting body in it's orbit.

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#### RTCPhysics

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##### Re: Why the Planets of a Solar System all orbit their Star at the same Velocity.
« Reply #8 on: 02/06/2016 17:04:56 »

4.   The ‘escape velocities’ published by Nasa.gov do not agree with the ‘orbiting velocities’ that you have quoted. (Mercury: 4.3Km/sec, Venus: 10.4Km/sec, Earth: 11.2Km/sec, Saturn: 35.5Km/sec, Pluto: 1.3Km/sec).

These escape velocities ↑ are how fast one has to go to escape the gravitational well of the planet, from the surface of the planet. These numbers have nothing to do with the Sun or the orbital motion of the planets, and are determined purely by the mass and size of the planets themselves.

Thanks for your reply. Complete 'error' on my part by reading the wrong line of the NASA Planetary Fact Sheet.

If these figures for planetary orbital velocities are correct and I now have no reason to doubt them, I'm wondering why I gained and retained this clear recollection of an Astrophysicist, taking part in a TV documentary upon the subject of Dark Matter and Dark Energy, explaining that all the planets in the solar system had been found to orbit the sun at the same velocity, rather than falling off in velocity, according to their size and distance from the sun. Did anyone else view the documentary and can clarify the point being made?

The object of my article was to explain how a 'common orbital velocity', could happen during the formation of the solar system and the explanation relied upon the twin concepts of each planet having an 'orbital escape velocity' where the gravitational pull is exactly matched by the centrifugal force and the impact upon the planets' orbits and orbital velocities, that results from their interacting 'gravitational forces' as they moved into the proximity of each other in their separate orbits.

If, as now seems likely, I completely misunderstood the point that the Astrophysicist was making as he laid out his nine stones upon the bonnet of his car to explain this recent (my assumption) scientific finding, then it removes any rationale for me to have written this article. But thanks for your replies and advice. It has been appreciated.

#### chiralSPO

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##### Re: Why the Planets of a Solar System all orbit their Star at the same Velocity.
« Reply #9 on: 02/06/2016 18:19:02 »

Thanks for your reply. Complete 'error' on my part by reading the wrong line of the NASA Planetary Fact Sheet.

If these figures for planetary orbital velocities are correct and I now have no reason to doubt them, I'm wondering why I gained and retained this clear recollection of an Astrophysicist, taking part in a TV documentary upon the subject of Dark Matter and Dark Energy, explaining that all the planets in the solar system had been found to orbit the sun at the same velocity, rather than falling off in velocity, according to their size and distance from the sun. Did anyone else view the documentary and can clarify the point being made?

The object of my article was to explain how a 'common orbital velocity', could happen during the formation of the solar system and the explanation relied upon the twin concepts of each planet having an 'orbital escape velocity' where the gravitational pull is exactly matched by the centrifugal force and the impact upon the planets' orbits and orbital velocities, that results from their interacting 'gravitational forces' as they moved into the proximity of each other in their separate orbits.

If, as now seems likely, I completely misunderstood the point that the Astrophysicist was making as he laid out his nine stones upon the bonnet of his car to explain this recent (my assumption) scientific finding, then it removes any rationale for me to have written this article. But thanks for your replies and advice. It has been appreciated.

Glad we could help. And thank you for acknowledging a misunderstanding, it is very refreshing, and allows the discussion to continue in a way that is most useful and interesting for all involved! :-D (and I doubt anyone would hold a misunderstanding against you--we are all here to learn)

I suspect the astrophysicist may have been explaining that the rotation curves of some galaxies, and clusters of galaxies appear not to follow the trend observed for the planets. We would expect orbiting bodies near to the center of mass to move more quickly than those orbiting farther from the center (as is observed in our solar system), but for these galaxies the more distant stars are moving too quickly. This is the impetus for the initial proposal of the existence of dark matter.

See more here: https://en.wikipedia.org/wiki/Galaxy_rotation_curve

#### jeffreyH

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##### Re: Why the Planets of a Solar System all orbit their Star at the same Velocity.
« Reply #10 on: 02/06/2016 18:55:14 »
I second what chiralSPO said. It is important to realise that everyone can be wrong. It shouldn't be an issue for someone to admit that they are wrong. I am, often!

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##### Re: Why the Planets of a Solar System all orbit their Star at the same Velocity.
« Reply #10 on: 02/06/2016 18:55:14 »