Why are planetary orbits so circular?

In summary, the question of why most planets in our solar system have nearly circular orbits has been extensively explored by theoreticians. While there are various theories and explanations, the most widely accepted one is that the collisions in the early solar system tended to make orbits more circular, leading to the current distribution of nearly circular orbits. Other factors such as the influence of gas giants like Jupiter and the effects of drag have also been considered. Overall, the stability of solutions and the concept of attractors have been used to explain the prevalence of circular orbits in our solar system.
  • #1
oldman
633
5
Over in another forum I've got myself in trouble by suggesting that theoreticians haven't paid enough attention to this question. George Jones has kindlyhttps://www.physicsforums.com/showthread.php?p=2100856#post2100856" that the question seems to have been answered along the lines of "...collisions in the disk tended to make orbits more circular explains why most planets in our solar system have nearly circular orbits", which seems a very reasonable proposition.

I've also come across what looks like a quite exotic answer to this question along totally different lines by the astronomer L. Nottale in a paper entitled SCALE-RELATIVITY, FRACTAL SPACE-TIME AND GRAVITATIONAL STRUCTURES. His work has also been discussed in these forums. I find both the paper and the discussion far above me, though.

Can anyone help to straighten me out with the current consensus on this question? Has it been thoroughly worked through by theoreticians? If so, I'd appreciate a web-accessible simple-minded reference to how this was done.
 
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  • #2
I think the maths is mostly to do with the stablility of solutions …

most solutions after a long time tend to an attractor, which in the case of planetary motion seem to be either circular or hyperbolic …

for the maths background, see http://en.wikipedia.org/wiki/Attractor :smile:
 
  • #3
The eccentricity of mercury's orbit is about 1/5, not all that small. The eccentricity of pluto's orbit is about 1/4 but Pluto is not a planet any more!
 
  • #4
HallsofIvy said:
The eccentricity of mercury's orbit is about 1/5, not all that small. The eccentricity of pluto's orbit is about 1/4 but Pluto is not a planet any more!
Yes indeed.

An eccentricity of 0.2 (Mercury) means however that the major axis differs only by 2% from the minor axis, so even its orbit can still be decribed as "nearly circular". For the other planets "nearly circular" is even more apt! Just a comment. Thanks for your reply.
 
  • #5
tiny-tim said:
I think the maths is mostly to do with the stablility of solutions …

most solutions after a long time tend to an attractor, which in the case of planetary motion seem to be either circular or hyperbolic …

for the maths background, see http://en.wikipedia.org/wiki/Attractor :smile:

Thanks. This suggestion is interesting. But why specifically would a circular orbit be an "attractor" solution for a planet? Has it been established for instance that central-force solutions, when perturbed (randomly?), are chaotic with this kind of attractor? Or is circularity established rather by computer modelling? The Wikipedia reference seems too general to make this obvious. Have you by any chance Astronomy-type references to such matters?
 
  • #6
In a word, drag.

By way of analogy, the orbit of a satellite with in low Earth orbit decays due to atmospheric drag. The decay is not uniform. The satellite's orbit will first become close to circular (the perigee won't change nearly as much as the apogee). Once circularized, game over. The satellite's orbit will become smaller and smaller until it decays into to the thick atmosphere.

Planetesimals were also subject to drag with the surrounding medium. A somewhat massive planetesimal at a given orbital distance (semi-major axis) will orbit slightly faster than a piece of dust with the same semi-major axis. The acceleration of the planet and Sun toward each other is G*(Ms+Mp)/r2. For the piece of dust, the acceleration is just G*(Ms)/r2.

The planetesimals plowed through dust. The difference between the planet's and the dust's velocity (drag is proportional to vrel2) is greatest at perihelion. The impact is seen half an orbit later, so drag tends to reduce apohelion.
 
  • #7
D H said:
In a word, drag.

Planetesimals were also subject to drag with the surrounding medium.

ooh, I don't think so …

the main effect on the early Earth would have been from the gravity of Jupiter …

gravitational orbits tend to be unstable, and a big bully like Jupiter can crate havoc …

any body in the wrong position either gets gently nudged into the right position, or gets turfed out into an elongated orbit …

other planets contribute to a lesser extent …

the "right" positions tend to be at "harmonic" ratios, with nothing in between

for some (but not much :frown:) detail, see http://en.wikipedia.org/wiki/Earth's_orbit and http://en.wikipedia.org/wiki/Stability_of_the_solar_system
The planets' orbits are chaotic over longer timescales, such that the whole Solar System possesses a Lyapunov time in the range of 2–230 million years … in some cases the orbits themselves may change dramatically. Such chaos manifests most strongly as changes in eccentricity, with some planets' orbits becoming significantly more—or less—elliptical.
 
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  • #8
tiny-tim said:
ooh, I don't think so …
That remains the dominant view, so much so that the discovery that many exoplanets have highly eccentric orbits was quite the shock.

the main effect on the early Earth would have been from the gravity of Jupiter …
That Jupiter was a major player, no doubt. That it directly caused orbits to be circular, big doubt. The suspected role played by gas giants during planetary formation was the same as now: The tend to make a terrestrial planetesimals' orbits more, not less, eccentric. During planetary formation, that would have exposed a perturbed planetesimal to a thicker supply of planetary medium. Drag would have circularized the orbit.

Think about it this way: Seven of the eight planets in our solar system have orbits with extremely small eccentricities. If the solar system is as chaotic as your post suggests, that is a truly amazing coincidence. If, on the other hand, the primary effect of the chaotic nature is to merely make the planetary positions unpredictable over any extended period of time, then it is not such an amazing coincidence.

The time scale over which the orbits in our solar remain predictable in terms of state is relative short -- a few millions of years. The time scale over which orbital shape (semi-major axis, eccentricity, inclination) remains stable is very long -- much longer than the life span of the solar system for most planets. Mercury is the big exception. The time scale over which Mercury's orbit maintains its basic shape is a few billion years.

In short, the solar system started out with planets in nearly circular orbits. Their orbits remain nearly circular because, except for Mercury (and maybe Mars), the time scale to complete chaos is much greater than the total lifespan of the Sun.
 
  • #9
D H said:
That Jupiter was a major player, no doubt. That it directly caused orbits to be circular, big doubt. The suspected role played by gas giants during planetary formation was the same as now: The tend to make a terrestrial planetesimals' orbits more, not less, eccentric.

I agree Jupiter generally tends to make orbits more eccentric …

but this gets rid of anything that's in the wrong position, or in the right position but with the wrong behaviour …

leaving only well-brought up planets like Earth …

so perhaps I should have said: what causes the present planets to have nearly circular orbits? the effect of (mostly) Jupiter turfing out anything that didn't :wink:
 
  • #10
tiny-tim said:
I agree Jupiter generally tends to make orbits more eccentric …

but this gets rid of anything that's in the wrong position, or in the right position but with the wrong behaviour …

leaving only well-brought up planets like Earth …

so perhaps I should have said: what causes the present planets to have nearly circular orbits? the effect of (mostly) Jupiter turfing out anything that didn't :wink:

In this model would the gravitational ejection from the solar system happen pre or post planetesimals?
 
  • #11
So far we have: inelastic collisions with dust, 'evaporative cooling'--interactions with smaller bodies, and planetary interactions.

Planetary interactions alone have time reverse symmetry don't they?, unless there's some stochastic thing among many planets I don't see. If so, as Jupiter would tend to stabilize the Earth from an excentric orbit to a circular orbit, it would also destabilize an Earth in a circular orbit to an excentric orbit.
 
  • #12
D H said:
In a word, drag...The planetesimals plowed through dust. The difference between the planet's and the dust's velocity ... is greatest at perihelion. The impact is seen half an orbit later, so drag tends to reduce apohelion.

Thanks for this clear answer to what turned out to be a naive question. The further discussion by yourself, Phrak, Tiny Tim and Nabeshin is illuminating too.
 
  • #13
Two explanations have been suggested on this forum: (i) drag, and (ii) the influence of other planets, especially Jupiter. The influence of Jupiter does not explain why Jupiter itself has a near circular orbit, and, more importantly, why its Galilean satellites have very nearly circular orbits, or why the rings of Saturn are nearly circular, or why even the Moon's orbit has low eccentricity. It seems that almost everything in a planet-forming nebula, or a satellite-forming one, tends to move together in a circle, despite some persistent small-scale chaos. What goes (locally) too fast is slowed down, and what goes too slow is pushed along. Of course, it is a complicated process, and it is not due entirely to two-body collisions with dust grains or boulders. But in one word that's mostly what it is: drag.
 
  • #14
The explanation that the planetary scientists I know seem to like is the anthropic one. If the planets didn't have circular orbits, we wouldn't been here to see them.

One reason this this turning into a very popular explanation is that so far in our studies of exoplanets, it appears that a solar system with nice round orbits seems to be uncommon.

Also the paper that you reference is pure-crankdom. The trouble with any sort of fractal theory is that if you have some gravity/fractal/QM effect that explains the motion of planets, then you should be able to create some easy-to-do experiment to show that this happens in the lab. For example, if there is some fractal/QM/gravity effect then surely GPS satellites ought to behave differently in some orbits and not others.
 
  • #15
oldman said:
Thanks for this clear answer to what turned out to be a naive question.

It's not a naive question. It's is very deep and complex one. And as far as I can tell, the answer right now is "we don't really know."

There is an old scientist joke. Observer goes up to a theorist, and says "I've discovered something wonderful." Theorist responds "Great! I've got a theory for that." Next day, observer looks disappointed. "It looks like I pointed my telescope backwards, and everything I told you was wrong, and the facts are actually the complete opposite." Theorist says "No problem. I've got a theory for that too!"
 
  • #16
twofish-quant said:
Also the paper that you reference is pure-crankdom.
Nice catch! This is an old thread, but ... cranky link deleted.
 
  • #17
D H said:
Nice catch! This is an old thread, but ... cranky link deleted.

One problem with cranky papers is that the truth may be much weirder than the cranks. It's not out of the question at this point that we may be one of the few solar systems that have regular stable circular orbits.

The other thing is that for things like the Galliean satellites. The orbits are synchronized so there is a physical reason for their orbits being where they are. The other solar system objects are not "locked" but one wonders if they once where.

Also, the dividing line between productive theorist and total crank is less clear than it seems. Frank Tipler who popularized the anthropic principle is both totally brilliant and totally nuts. (If he turns out to be right about the omega point, I'll have drinks with him at the Restaurant at the End of the Universe and apologize for that statement.)

The same could be said for Roger Penrose.
 
  • #18
Are planetary orbits analogous to the orbits of electrons? And if so, do their orbits infer anything towards answering this question?
 
  • #19
narrator said:
Are planetary orbits analogous to the orbits of electrons? And if so, do their orbits infer anything towards answering this question?
No and no. The Rutherford and Bohr models of the atom tried (and ultimately failed) to have electrons orbiting the nucleus in a matter analogous to how planets orbit their central star. Those are old, inaccurate models. Orbitals don't look anything like planetary orbits.
 
  • #20
Since the sun and solar system are traveling along through space, would it be incorrect to say that the planets are following a spiral or corkscrew path, instead of a circular or elliptical path?

Respectfully submitted,
Steve
 
  • #21
narrator said:
Are planetary orbits analogous to the orbits of electrons? And if so, do their orbits infer anything towards answering this question?
Adding to my previous answer, the issue is not how planets orbit their central star. The science behind planetary orbits is essentially a solved problem. The issue is why planets are in the orbits that they are in.

Scientists thought they had answers to this question. However, when astronomers started finding planetary systems beyond our solar system what they found did not match theory. They instead found hot jupiters, planets with highly eccentric orbits, and even planets with retrograde orbits. So a bit of a back to the drawing board move may be needed.
 
  • #22
Dotini said:
Since the sun and solar system are traveling along through space, would it be incorrect to say that the planets are following a spiral or corkscrew path, instead of a circular or elliptical path?
No. You are implying that there is some preferred frame of reference.
 
  • #23
D H said:
No. You are implying that there is some preferred frame of reference.

With respect to the Universe's centre of mass?
 
  • #24
As far as I recall an undergrad essay I did on Solar System formation [decades ago], the hypothesis is focussed around the accretion disc that forms, from which the Solar System emerges. Imagine once everything settles down, like rings around Saturn, and eventually the planets form as those materials 'condense' from that matter. Clearly, if everything had roundy-round motion in the plane, the planets that are made from that stuff will go roundy-round. Clearly it is not essential, as there are many eccentric objects in the Solar System. So one might presume that the circular motion of the accretion disc is an origin, against which there are divergences according to the 'accidents' that each planet has had with other 'condensed' objects not traveling around in circles.

A bit like asking why pancakes are circular... because the pan they are made from was circular.
 
  • #25
cmb said:
With respect to the Universe's centre of mass?

The universe does not have a centre of mass. The universe does not have a centre. More accurately, everywhere is the centre.

In a nutshell:

No matter where you go, there you are - writ cosmic.
 
  • #26
I find it curious that the two planets in our solar system that have eccentric orbits are the very two that are not bounded on both sides by other planets. It certainly seem to lend some credence to a "sheperding" hypothesis.
 
  • #27
DaveC426913 said:
I find it curious that the two planets in our solar system that have eccentric orbits are the very two that are not bounded on both sides by other planets. It certainly seem to lend some credence to a "sheperding" hypothesis.
What two planets are you talking about? Only one of the eight planets, Mercury, has anything close to an eccentric orbit. Moreover, a big part of Mercury's eccentricity results from Jupiter.
 
  • #28
DaveC426913 said:
The universe does not have a centre of mass.

This was partly a cheeky quip... But only partly. The universe must have a centre of mass, unless the mass is infinite. It's not possible for a finite amount of mass not to have a centre of mass.
 
  • #29
cmb said:
It's not possible for a finite amount of mass not to have a centre of mass.
You are implicitly assuming that space is cartesian here. It isn't.
 
  • #30
D H said:
You are implicitly assuming that space is cartesian here. It isn't.

Not sure I am. The centre of mass of the universe is a point that is equally the age of the universe away from all points in our 3D space, is it not?
 
  • #31
cmb said:
Not sure I am. The centre of mass of the universe is a point that is equally the age of the universe away from all points in our 3D space, is it not?
By that definition, every point in the universe is the center of the universe. Another way to say it: The universe has no center.

All of the discussion started by post #20 is incredibly off-topic. Start a new thread, or better yet, find an existing thread that is already devoted to this topic.
 
  • #32
D H said:
What two planets are you talking about? Only one of the eight planets, Mercury, has anything close to an eccentric orbit. Moreover, a big part of Mercury's eccentricity results from Jupiter.
Pluto's is eccentric enough to pass within Uranus'. And it's tilted.
 
  • #33
cmb said:
Not sure I am. The centre of mass of the universe is a point that is equally the age of the universe away from all points in our 3D space, is it not?
As D_H points out, so was my point: all points in the universe meet that criteria.
 
  • #34
DaveC426913 said:
Pluto's is eccentric enough to pass within Uranus'. And it's tilted.
I was being nitpicky. Pluto is no longer considered to be a planet.
 
  • #35
D H said:
I was being nitpicky. Pluto is no longer considered to be a planet.
Is too. Dwarf planets are a subset of planets. :biggrin:
 

1. Why do planets orbit in a circular path around the sun?

Planets orbit in a circular path around the sun because of the force of gravity. Gravity pulls objects towards each other, and in the case of planets, the force of gravity between the sun and the planet causes the planet to continuously fall towards the sun while also moving forward, resulting in a circular orbit.

2. Are all planetary orbits perfectly circular?

No, not all planetary orbits are perfectly circular. While the force of gravity causes planets to orbit in a circular path, other factors such as the gravitational pull of other planets and the shape of the planet's orbit can cause slight variations in the circularity of the orbit. Some planets, such as Mercury, have more elliptical orbits.

3. How do scientists measure the circularity of a planet's orbit?

Scientists use a measurement called eccentricity to determine the circularity of a planet's orbit. Eccentricity is a number between 0 and 1, with 0 representing a perfectly circular orbit and 1 representing a completely elliptical orbit. The closer the eccentricity is to 0, the more circular the orbit is.

4. Can the circularity of a planet's orbit change over time?

Yes, the circularity of a planet's orbit can change over time. This can happen due to various factors such as the gravitational pull of other planets, collisions with other objects, and the planet's own rotation. However, these changes are usually very gradual and do not significantly affect the overall circularity of the orbit.

5. Are there any benefits to having a circular orbit?

Yes, having a circular orbit has several benefits. A circular orbit allows a planet to maintain a consistent distance from the sun, which is important for maintaining a stable climate and atmosphere. It also allows for more predictable and regular seasons, which can be beneficial for life on the planet. Additionally, a circular orbit reduces the chances of collisions with other objects in space.

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