How stable are the planetary orbits ?

Question

For a circular orbit, the centifugal force perfectly balances the gravitational force.
How stable are these orbits ?

You would think that if the orbit got a little bigger the orbiting body would fly off and if the orbit was a a little smaller, the orbitiing body would spiral into the body it was orbiting.

but neiother of these things happen.

Answer 1

To clarify something you asked...

"You would think that if the orbit got a little bigger the orbiting body would fly off and if the orbit was a a little smaller, the orbitiing body would spiral into the body it was orbiting."

It turns out that classical orbital dynamics disagrees with you here.

I will agree that it's very difficult to maintain a perfectly circular orbit...after all, there's lots of other bodies in the solar system, with their own graviatational pulls, pulling and tugging at your planet's orbit and perturbing it this way and that.

However, I prefer to think about orbits in terms of constant energy trajectories rather than in delicate balances of forces. While it's quite possible (even likely!) to perturb a planet off of a perfect circular path, this won't suddenly cause it to tumble off into space or spiral into the sun. Rather, the planet will follow a shape slightly perturbed from a circle: an elliptical orbit. Since these perturbations aren't taking very much energy from the planet, this new elliptical orbit will be similar in shape to the original circular orbit.

For an extreme example of an elliptical orbit, take Pluto. Its elliptical orbit is eccentric enough that some of it falls inside Neptune's orbit; when Pluto's on this part of its orbit, it's actually closer to the Sun than Neptune is.

Answer 2

Well, to answer just the "question part" (before getting to your additional details), one could make the "freethinker" arguement that since the solar system is 4 billion years old, the planets with stable orbits are the only ones that can still be in orbit today. Planets with too slow of an orbit may have crashed into the sun a few billion years ago. Planets with too much velocity would have the escape velocity to make a great exodus from our solar system... certainly enough time has passed. Sure, things nudge the planets orbits all the time... their own gravitational interaction with each other does this, but, by nature, it evens out. Example. Jupiter orbits the sun faster than Saturn. They are (relatively) neighborly with each other, being just a handful of A.U. apart and a decent combined mass between the two of them. As Jupiter gains on Saturn in their orbits, the force of gravity pulls Jupiter ahead slightly, and slows Saturn down slightly. But the system is conserved once Jupiter passes Saturn, and the pull between the two is now slowing Jupiter slightly, and accelerating Saturn slightly. This is mostly negligible anyway, for most of the planets, but it is always calculatable and is always present. I do not suspect a "negative feedback" is necessary to sustain a stable orbit. Kepler's Laws of planetary motion seem suggest (and have unresounding evidence) that the planets are merely behaving the way that gravity allows them to. For a planet in a stable orbit to get "nudged" into a decaying orbit, the nudge would have to be sustained for quite sometime, or be almost catastrophically large.

Answer 3

A stable system is one that will return to an equilibrium state after it's been perturbed. For example, a ball in the bottom of a bowl, when knocked, will roll around at the bottom of the bowl. On the the other hand, a ball sitting on top of an upside down bowl is easy to knock off. A planet in orbit around the sun is stable in that even when perturbed, will continue to orbit in some new path, most likely a more elliptical path. If "things" like planetoids were to be "tossed at the direction of the sun", the odds favor orbital paths rather than either 1) death spiral into the sun or 2) escape from the solar system, but we'd end up with a messy bunch of highly elliptical orbits at all odd angles, and it would be the presence of such orbits that would actually be the main threat to the stability of any one of them. However, the solar system did form from a flat spinning disk of matter, for reasons of properties of angular momentum of random dust and gas clouds, which is the reason why most planetary orbits ended up being rather circular.

Answer 4

The planetary orbits are very stable, they have been around for billions of years.

Each planet has a certain orbital energy. This might change a tiny tiny bit as it gets hit by meteoroids. The energy determines its orbit. If the energy changes by 1 part in 1 trillion, then the orbit could change by 1 part in 1 trillion.

But even 1 part in 1 trillion would be a lot. The actual number (for meteoroids hitting the Earth) is less than 1 part in 10^18 (that's a 1 with 18 zeros after it) -- probably way less.

So ... planets can't go "flying off" or "spiral into" -- their orbit energy wouldn't allow it.

Answer 5

The orbit is the result of angular momentum verses gravity. The planet or other body wants to go one direction when the force of gravity want is to go another. If the object in orbit speeds up or slows down a little it just goes into a different orbit. That's why we can have planets as far out as Pluto or even further and planets as close to the sun as Mercury. It all has to do with velocity. To obtain enough velocity to break away from the sun's pull would be a tremendous thing for something of that much mass. The amount of energy required would be unbelievable.

Answer 6

If a planet is slowly orbiting a massive Sun, but does not drift into it, then obviously there is a repulsion force as well as the gravitational attraction.

If a planet is continuing in a steady orbit, without any impetus such as the propulsion that we find necessary to keep our probes and satellites from drifting off the path they are set upon, then something is either steadily pulling or pushing the planet to maintain its orbit in the face of all other influences. The influence of the orbiting planets upon each other would otherwise, over time, alter the orbits dramatically.

Why would they not? Each time a given planet lined up with the massive Jupiter, and was perturbed to speed up or slow down due to this influence, unless there were another influence this perturbed planet would remain in motion a bit slower or faster, perpetually. If the planets resume their motion around the Sun after being perturbed by each other, then the impetus setting them in motion is not inherent in the planets as an influence upon each other. A planet slowed by the influence of Jupiter behind its path would not speed up again to resume its steady pace unless this other impetus existed.

This other impetus, has the same basis as the magnetic alignment of the Earth and her Sun. This influence reaches beyond the Solar System, and dictates motion within the Sun not visible but nevertheless present. Just as the core of the Earth revolves at a speed dictated by the thickness of the Earth's liquid core, to chase away from or toward magnetic influences that exist in the Solar System, just so the Sun's core rotates, dragging her children around her like baubles on the ends of her apron strings.

Answer 7

The orbits of the planets are affected by a lot of influences. They are affected by the gravities of other planets as they pass near. They are affected by the gravitational pull from other galaxies, They are affected by their own spin and the distance from the sun. And they are affected by the variences of the sun's own gravity, which in turn is affected by all these things.
Basically, the orbits are fairly balanced and stable, but they are gradually being decayed, and will eventally be pulled into the sun.
However, as this is not likely to happen for several billion years, you don't need to worry.

Answer 8

While the planetary orbits appear stable to us, there are theories about planet formation that seem to indicate that they are not stable over billions of years.
The over 200 exoplanets found all seem to be Jupiter-sized or more all in close orbits around their stars. I won't go into the details of those, but astronomers are re-thinking what we believe about our system.

Venus and Earth are nearly circular orbits, so they are not likely to be de-stabilized much over time. But Mercury and Mars have more eccentric orbits, and over time the influence of the sun and Jupiter could cause one or both to end up spiralling out of the solar system.

Answer 9

Ok, first off, there is no centrifugal force. A planet in orbit around the Sun only experiences one force - the force of gravity between it and the Sun. That's it. It's an unbalanced force, which is why the planet goes around the Sun. If the were another force acting on it that balanced gravity, the planet would move in a straight line at a constant speed, per Newton's First Law.

The orbits of the planets in our solar system are very stable - which is why Earth has been in the same place for 4.5 billion years.

Answer 10

The gravity is constant coming from our sun...we are in NO danger of leaving our orbits. Scientifically speaking...it would take a major event OR the sun exploding/imploding to change the orbit. And when I say a major event, I mean a planet hitting another planet..and that has been run throught the computer and is in NO danger of happening.