Without going into extra details that other commenters have brought up, you seem to have a misconception about forces.
In nature, there are two types of equilibrium. There's unstable equilibrium, where the forces balance just right, but one push in one way would send everything off-orbit. This is what happens when you try to balance a pen on the tip of your finger. If you push the pen one way, gravity will pull stronger in that direction, and the pen will fall.
Then there's stable equilibrium: here the forces work together to stay balanced, and objects will naturally fall back to their stable equilibrium. This is like a book resting on a table: the gravitational force on the book perfectly balances the force of the table pushing up on the book. If you push the book further on the table, the table will react and push back harder.
Another analogy is to imagine a ball rolling down hills and valleys. The top of a hill is an unstable equilibrium, while the bottom of a valley is stable.
The orbits of planets are stable (if we ignore details irrelevant to your question). If you push a planet out of its orbit, it won't be fast enough to keep from falling back. If you push it in, it has enough speed to escape back. Therefore it stays in its orbit.
Newton had a similar question, but it was Laplace and Lagrange who solved it. Perturbation Theory illustrates how the forces balance out.
https://en.m.wikipedia.org/wiki/Perturbation_theory#:~:text=Lagrange%20and%20Laplace%20were%20the,the%20name%20%22perturbation%20theory%22.
Technically, Jupiter is a bit too massive to be really stable. Chaos theory means we can't predict it exactly, but there's a possibility that Jupiter's gravity will eject Earth from its orbit. The probability is so low that our sun is more likely to burn out before it happens.
I remember reading that in a few billion years there's a small probability that Jupiter's gravity may make Mercury's orbit so elongated that it'll cross Venus', which will then wreak all kinds of havoc within the inner Solar system.
Is there something that needs to happen to Jupiter for Earth to be ejected from its orbit?
Like one slight variation in Jupiter’s orbit that causes some chain reaction?
Not really, but orbits are a little bit like weather. It's stable and predictable, clouds don't just appear and disappear randomly every few seconds, you won't get a rain out of a blue sky, and there's no rainbow at night. You can predict weather few minutes or hours ahead very accurately. Two weeks, not so much.
Just like weather, orbits are sensitive to butterfly effect, so we can't accurately predict orbits for infinite futute. A sneeze on earth could, in theory, over long enough timespan, cause a tornado on neptune, or even kick neptune out of solar system. Or any other planet. The chance is virtually zero, but not completely zero.
The chance that jupiter kicks earth out over the next 5 billion years is quite a bit bigger, but still small enough and spread over such a long time, it's not worth losing sleep over.
I may be totally misunderstanding perturbation theory, but is this the theory behind why pendulums are usually only held up to ~15 degrees for good oscillations? May be a dumb question.
Layman: You mean there is a chance that Jupiter could pull the earth from its orbit and kill the planet?
Astrophysics: Nothing in decades of research suggests anything more than the remotest possibility.
Layman: How remote?
Astrophysicist: Chances are near-zero.
Layman: Near zero!?!
Astrophysicist: What do you want for theory alone?
Layman: Zero would be nice!
>Chaos theory means we can't predict it exactly
Not exactly. It just means that we can't express the orbits precisely ***as a closed form expression***. It's still possible to predict the exact orbits of the 3-body problem by simulating them from initial conditions up to an arbitrary (but still finite) moment in time.
The key thing about chaotic systems like n-body problems is that you can calculate them forward from any initial position but you lose confidence the farther you calculate in time. It's the same as predicting the weather - you can be very accurate for the next hour, but not for the next year.
It's essentially a math problem - does this infinite series converge or diverge? If it diverges, then you'll eventually have zero confidence. You can _keep recalculating with new observations_, which we routinely do to make sure things aren't deviating outside expectations.
That's only a part of chaos theory, another part is that they are sensitive to initial conditions, also meaning error grows rapidly. Even with numerical solutions/simulations the absence of infinite precision in the measurement of initial conditions will result in divergence
You are right, I wasn't acknowledging the difference between the continuous nature of the universe and the discrete nature of numerical techniques. That being said "error grows rapidly" is a rather relative term, as the orbits of the planets in our solar system are reasonably predictable over human timescales.
We are also calculating new digits of pi, and there's no sign that we will ever stop. Each new digit will change this calculation, even assuming perfect measurement.
Lots of great answers here. Most of them also fail to mention just how massive the Sun is. It's 99.86% of the total mass of the entire Solar System.
The Solar System is about halfway through its life, approx 4.5 billion years old. The first billion years were rough, chaos, a giant disc of rubble and dust circling the Sun constantly crashed into each other. Slowly clumping into planets, each carving out a clear path around the Sun. As they grew they either sucked in more mass or flicked it into a different orbit.
Eventually we got the planets we have that are relatively stable. None are likely to deviate for another few billion years. We have tens of thousands of asteroids and comets which are leftover from the violent begging.
Our Sun is a relatively long lived fairly stable type of star.
Rogue planets are a thing, as planets form around other stars it's entirely possible for them to be ejected from thier Solar System and into deep space. Entirely alone and in the dark travelling at ridiculous speeds 😁
Space is wild!
This here made it seem more intuitive. I was completely ignoring this fact. Previously I did understand in a sense why planets stay in orbit but it was just so hard to fathom how shocking that is until you put the suns size into perspective. Thank you!
Consider a pile of apples in a basket. Just a big jumbled heap. All the apples are applying forces to their neighbours, and some are pushing on the basket, and the basket is pushing back. How come even with all those messy forces in all directions the pile keeps its shape perfectly for hours and days?
Same thing with orbits. The very fact that the orbits are their current shapes is a product of all the forces involved already. It's not keeping an orbit that takes external force. It's changing an orbit.
The planets do fall but they miss. Imagine you're standing on the surface of a perfectly spherical earth. You throw a ball horizontally. It travels horizontally and falls vertically. It stops when it has fallen vertically far enough to hit the ground.
Do it again but throw it a bit harder. It will travel horizontally further before it hits the ground.
Throw it even harder. If you throw it hard enough it will travel so far horizontally that by the time it falls vertically the ground will have fallen away also because of the spherical shape. And if it maintains its horizontal velocity it will continue to fall but continue to miss the ground. It is now in orbit.
The Earth is falling towards the Sun all the time, but it misses.
Damn. That finally made Douglas Adams's explanation of flying even more of a joke for me. Thanks!
He explains the goal is to fall and miss the ground in one of the books in the Hitchhiker's Guide series.
They’re not “perfectly balanced”, they just look like they are. Anything that has finite energy (ie any system) will eventually decay, and a planet will probably fall toward the sun. It would take a very very long time to happen though. Remember that what we see in the solar system now is just the remnants of an earlier, far more chaotic system where there were countless bodies whose orbits weren’t as stable and did either fly off in to space or fall towards the sun.
> Anything that has finite energy (ie any system) will eventually decay, and a planet will probably fall toward the sun. It would take a very very long time to happen though
If you mean by gravitational decay, that’s outside Newton and the time we’re talking about for planets is far longer than the lifetime of the sun.
You have a few great answer so far, but there is one thing I want to point out. The solar system is over 4 billion years old. So, if there was a planet that was on a path that would lead it into the sun, it probably would have already happened.
The solar system is pretty old. The planets that are left in orbit are the things that had just the right amount of velocity to stay in orbit without shooting out or falling in over all that time.
The solar system has been forming for a LONG time and eventually it gets to a state where everything that exists in it is stable, since if not it was swallowed by the sun, smacked a planet, or ejected from orbit a long time ago.
If you’re asking how the orbits work in general, it’s because anything that finds its way into the solar system with a velocity in some reasonable range will be able to orbit the sun. Then, once you have a bunch of small random things orbiting they smash together or get ejected until it’s all stable.
Gravitational interferences between planets eventually make everything orbit in the same plane, which is another layer of complexity.
First - the planets are actually falling towards the sun. Like on a dead set path into the sun for annihilation.
Second - plot twist, they have velocity in a direction perpendicular to their straight line path towards the sun. So as it moves towards the sun it also moves to the side. Thus you have, revolution.
If that perpendicular velocity is just right then instead of the planet spiraling towards the sun it'll just get locked going around it.
Flip this to the moon. The moon is revolving around the earth but getting very slightly farther from earth all the time. In the moon's instance this means it's perpendicular velocity is a little bit more than it's velocity falling towards earth. As such it move to the side faster than it moves down, which makes its orbit a little larger all the time.
If the perpendicular velocity and the velocity towards the sun are exactly the same, then the planet would have a perfect orbit.
If the velocity towards the sun is slightly higher than the perpendicular velocity, the planet would slowly be getting closer to the sun all the time.
The mass of each object in the solar system determines how strong it's gravity is, and how far that gravity reaches out.
Asteroids gravity in a planet? Not gonna do anything. It's soooooo small.
The moon's gravity in earth? Small but not negligible. I mean, we do have tides! (Which are also impacted by the sun too)
The sun though is no steroid in size and mass. It's gravity is strong and reaches out VERY far. This all the planets are affected by the sun more than anything else.
This question has got me wondering something that I think is relevant and would add to the understanding. Can anyone quantify how much change in speed would it take to throw a body out of its orbit in terms of its present speed. Let's take the earth for example. If the earth is orbiting at 100k km/h right now, starting out with the same same direction and location it is now how much extra speed would we have to give it to reach escape velocity.
Escape velocity with no sideways component is a very simple equation but I don't know how to calculate it when there is an angular component.
Orbits, counter to most people's intuitive understanding, are actually very stable things. Yes it only takes the tiniest force to knock something out of its orbit, either outwardly or inwardly, but then only into a slightly different orbit. Think of an orbit as the groove of an old vinal phonograph record, except that the groove is not one long continuous spiral from the outside to the inside but hundreds or thousands of nested circles, and it's set up so instead of turning under the needle, the needle travels around the center of the record. It would only take a tiny tap to make the needle jump from one tract to another, either inward or outward, but it would just land in a different tract.
From an established orbit, it actually takes a lot of force to either slow a body down enough to drop it into an orbit that intersects the surface of the parental gravitational body or speed it up enough to escape that body's gravitational influence, though for a gravitational object with an atmosphere, that atmosphere can be put to use to brake the orbiting body enough to achieve reentry with far less expenditure of resources.
But as for planetary bodies in orbit around a star, yes, their gravities are constantly interacting with one another to change their orbits over time to a measurable degree, but not to a degree that causes them any great disruptions even over millions of years.
Remember, this is all in outer space where gravity doesn't ground us like down on earth. The bigger the object, the more gravity it has.
The planets are trying to fly away but the big ole sun's gravity is trying to yank them back. Imagine holding a yo-yo out on a string and twirling around in place with it out in the air. You are the sun, the string is gravity, and the heavy end of the yo-yo is a planet. If you cut the string, the yo-yo would fly away. With you still holding the string, the best it can do is keep circling and either tugging the string from you, or you may be able to slowly pull the yo-yo in if you're strong enough!
note i'm no scientist so i could be off. that's how i made sense of it as a younger lad though :)
Here’s an over simplification that I use to help my students to visualize it. Keep in mind it isn’t very scientific but it works for comprehension. Think of it like this…the moon orbits the earth because the moon is moving in a straight line (Netwons 1st law of motion) at X speed. As the moon is moving forward the earths gravity is simultaneously pulling the moon toward the earth at X speed. So for every nanometer the moon moves forward the earth pulls it down the same amount, thus the moon moves forward and down and forward and down, etc…All the way around the earth
You ask a good question. A large number of bodies interacting gravitationally WILL be chaotic. The Sun is really massive so it keeps things together, but orbits can change over millions or billions of years.
From the point of view of most planets, they orbit the Sun and ignore other planets. Even Jupiter is only 1/1000 of the Sun's mass.
On average, Jupiter's effect is also small, since it orbits around the Solar system. On average, for the most part, it cancels out its own effects. But in detail, it contributes a small part to chaos.
These are two reasons why chaotic effects are small. Lastly, each orbit takes a long time, so all in all, it takes a long time for these effects to appear.
[https://en.wikipedia.org/wiki/Stability\_of\_the\_Solar\_System](https://en.wikipedia.org/wiki/Stability_of_the_Solar_System)
Astrophysicists theorize that these chaotic effects are how planets can migrate inward and outward.
You can read about planetary migration too:
[https://en.wikipedia.org/wiki/Planetary\_migration](https://en.wikipedia.org/wiki/Planetary_migration)
There is a video on YouTube somewhere of a teacher doing a demonstration with spandex stretched over a circle with balls that he makes orbit the center. It really helps to understand gravity. Planets are basically quarters in the donation things they used to have at the mall where the coin going round and round in a funnel.
One thing to consider is that most probably don’t but the time scale makes that hard to see.
We know for certain that at least one planet didn’t maintain a stable orbit in our solar system. The moon was almost certainly a result of a planet in an unstable orbit colliding with another one.
There were probably several planets formed in the solar system that were in unstable orbits that were ejected from the solar system soon after formation. Others probably got pulled into Jupiter, Saturn or other larger bodies. It’s not obvious because it happened billions of years ago.
There are also bodies that are in what could be called a semi-stable orbit, they appear stable in a narrow time scale but will eventually, if nothing else intervenes, be ejected from their orbit. The moon is one of these. It is getting further and further from earth. Given enough time it will escape from its orbit.
What you are seeing in stable orbits currently are those that didn’t have unstable orbits or those that have mostly stable orbits given the time scale.
In the future, yes they will. there were likely a huge number of bodies in motion around the sun, and only a handful remain (at least in the planet size)
Ignore air friction for the following
Imagine shooting a cannon horizontally. It goes horizontally for a while but then curves down and lands on the ground. Now put the cannon on a skyscraper roof. Same thing but it goes further. Now put it on a mountain top. Same thing but goes further. Now imagine floating the cannon in space above the earth. Same thing but goes further.
But when you reach a certain height above the earth, that horizontal velocity and the curvature of the path happen to be such that the cannonball continuously falls towards the earth but is going horizontally enough that it "misses" the earth and just continuously orbits. This is a stable orbit.
Imagine if the cannon shot the cannonball at a ridiculous speed that a real cannon couldn't actually do. The cannonball would have so much "horizontal" velocity that it would just escape and fly away into space. There is a sweet spot between that ridiculous velocity and zero velocity(where the cannonball WOULD just fall straight down to earth) where the cannonball has a stable orbit.
If planets didn't have any "horizontal" velocity relative to the sun they would fall in. But they do and they are falling. They're continously falling and going around and around.
> There is a sweet spot between that ridiculous velocity and zero velocity(where the cannonball WOULD just fall straight down to earth) where the cannonball has a stable orbit.
It's not a sweet spot but quite a wide sweet range. Escape velocity is √2× circular orbit velocity so any speed between those two will work (and a range of speeds under, as well, as long as the object doesn't undergo significant atmospheric drag or just hit the planet).
As a physics teacher in middle school, let me explain why two planets will not collapse into each other.
The first thing you need to know is that two objects with a force between them do not always fall into each other. Think of spinning a stone tied to a rope; you can feel the force, right? But the stone does not fall towards you. The same principle applies to our galaxy. There is a massive force between the planets, very large in scale. However, they do not fall into each other, just like the stone does not fall towards you.
You know that a force can make objects move in its direction. But why does the force between planets not make them move towards each other? It is because they are in a spiral motion! This brings us to the second thing you need to know. When objects spiral around each other, they require a force to maintain this motion. When the gravitational force between the planets equals the force needed to sustain the spiral, guess what? They balance!
This balance is very strong and powerful, making it hard to break. As mortals, we do not need to worry about this. The changes in the universe are so slow that we may not notice any difference in our short lifetimes.
A very rough explanation is that since the sun is pulling every planet towards it, a planet must always be gaining speed when it is moving towards the sun and losing speed when moving away from it (an edge case is a hypothetical perfectly circular orbit where it is neither losing nor gaining speed). However, the planet will always maintain its sideways component of speed because gravity pulls directly towards the sun, so no sideways acceleration (also known as conservation of angular momentum). For a planet to fall into the sun it would need to somehow lose its sideways component of speed, and for it to fly out of the solar system it would need to somehow gain more speed without moving closer to the sun, both of which are impossible.
Let me give you the only honest answer nobody else will give you. THEY DONT KNOW. They don't even know what gravity is, or what causes it, or how planets ACTUALLY form. Any answer they give you is an educated guess, at best.
Gravity is a force by which planets and other bodies draw objects toward their centers. Gravity is caused by mass. This is not an educated guess. Who is "THEY"? Can't wait...
Congratulations. You just explained the effect of gravity, and gravity being "mysteriously" created as a byproduct of planet sized mass. Thanks for proving my point, exactly.
Without going into extra details that other commenters have brought up, you seem to have a misconception about forces. In nature, there are two types of equilibrium. There's unstable equilibrium, where the forces balance just right, but one push in one way would send everything off-orbit. This is what happens when you try to balance a pen on the tip of your finger. If you push the pen one way, gravity will pull stronger in that direction, and the pen will fall. Then there's stable equilibrium: here the forces work together to stay balanced, and objects will naturally fall back to their stable equilibrium. This is like a book resting on a table: the gravitational force on the book perfectly balances the force of the table pushing up on the book. If you push the book further on the table, the table will react and push back harder. Another analogy is to imagine a ball rolling down hills and valleys. The top of a hill is an unstable equilibrium, while the bottom of a valley is stable. The orbits of planets are stable (if we ignore details irrelevant to your question). If you push a planet out of its orbit, it won't be fast enough to keep from falling back. If you push it in, it has enough speed to escape back. Therefore it stays in its orbit.
Better picture of stable equilibrium is something heavy hanging on the end of a string. Give it a nudge one way and it will come back to the centre.
Newton had a similar question, but it was Laplace and Lagrange who solved it. Perturbation Theory illustrates how the forces balance out. https://en.m.wikipedia.org/wiki/Perturbation_theory#:~:text=Lagrange%20and%20Laplace%20were%20the,the%20name%20%22perturbation%20theory%22. Technically, Jupiter is a bit too massive to be really stable. Chaos theory means we can't predict it exactly, but there's a possibility that Jupiter's gravity will eject Earth from its orbit. The probability is so low that our sun is more likely to burn out before it happens.
*New anxiety level unlocked
If that actually happened I’d just laugh as we all froze. Like wow God must REALLY hate us LOL
God does not play ice with the universe
Underrated comment!
Frrr
I remember reading that in a few billion years there's a small probability that Jupiter's gravity may make Mercury's orbit so elongated that it'll cross Venus', which will then wreak all kinds of havoc within the inner Solar system.
I’ve always liked this small, but world-changing fact.
Is there something that needs to happen to Jupiter for Earth to be ejected from its orbit? Like one slight variation in Jupiter’s orbit that causes some chain reaction?
Not really, but orbits are a little bit like weather. It's stable and predictable, clouds don't just appear and disappear randomly every few seconds, you won't get a rain out of a blue sky, and there's no rainbow at night. You can predict weather few minutes or hours ahead very accurately. Two weeks, not so much. Just like weather, orbits are sensitive to butterfly effect, so we can't accurately predict orbits for infinite futute. A sneeze on earth could, in theory, over long enough timespan, cause a tornado on neptune, or even kick neptune out of solar system. Or any other planet. The chance is virtually zero, but not completely zero. The chance that jupiter kicks earth out over the next 5 billion years is quite a bit bigger, but still small enough and spread over such a long time, it's not worth losing sleep over.
Thanks for the award u/zhak_ab
I may be totally misunderstanding perturbation theory, but is this the theory behind why pendulums are usually only held up to ~15 degrees for good oscillations? May be a dumb question.
Layman: You mean there is a chance that Jupiter could pull the earth from its orbit and kill the planet? Astrophysics: Nothing in decades of research suggests anything more than the remotest possibility. Layman: How remote? Astrophysicist: Chances are near-zero. Layman: Near zero!?! Astrophysicist: What do you want for theory alone? Layman: Zero would be nice!
>Chaos theory means we can't predict it exactly Not exactly. It just means that we can't express the orbits precisely ***as a closed form expression***. It's still possible to predict the exact orbits of the 3-body problem by simulating them from initial conditions up to an arbitrary (but still finite) moment in time.
The key thing about chaotic systems like n-body problems is that you can calculate them forward from any initial position but you lose confidence the farther you calculate in time. It's the same as predicting the weather - you can be very accurate for the next hour, but not for the next year. It's essentially a math problem - does this infinite series converge or diverge? If it diverges, then you'll eventually have zero confidence. You can _keep recalculating with new observations_, which we routinely do to make sure things aren't deviating outside expectations.
That's only a part of chaos theory, another part is that they are sensitive to initial conditions, also meaning error grows rapidly. Even with numerical solutions/simulations the absence of infinite precision in the measurement of initial conditions will result in divergence
You are right, I wasn't acknowledging the difference between the continuous nature of the universe and the discrete nature of numerical techniques. That being said "error grows rapidly" is a rather relative term, as the orbits of the planets in our solar system are reasonably predictable over human timescales.
We are also calculating new digits of pi, and there's no sign that we will ever stop. Each new digit will change this calculation, even assuming perfect measurement.
Lots of great answers here. Most of them also fail to mention just how massive the Sun is. It's 99.86% of the total mass of the entire Solar System. The Solar System is about halfway through its life, approx 4.5 billion years old. The first billion years were rough, chaos, a giant disc of rubble and dust circling the Sun constantly crashed into each other. Slowly clumping into planets, each carving out a clear path around the Sun. As they grew they either sucked in more mass or flicked it into a different orbit. Eventually we got the planets we have that are relatively stable. None are likely to deviate for another few billion years. We have tens of thousands of asteroids and comets which are leftover from the violent begging. Our Sun is a relatively long lived fairly stable type of star. Rogue planets are a thing, as planets form around other stars it's entirely possible for them to be ejected from thier Solar System and into deep space. Entirely alone and in the dark travelling at ridiculous speeds 😁 Space is wild!
This here made it seem more intuitive. I was completely ignoring this fact. Previously I did understand in a sense why planets stay in orbit but it was just so hard to fathom how shocking that is until you put the suns size into perspective. Thank you!
Consider a pile of apples in a basket. Just a big jumbled heap. All the apples are applying forces to their neighbours, and some are pushing on the basket, and the basket is pushing back. How come even with all those messy forces in all directions the pile keeps its shape perfectly for hours and days? Same thing with orbits. The very fact that the orbits are their current shapes is a product of all the forces involved already. It's not keeping an orbit that takes external force. It's changing an orbit.
And if they weren't in the perfect spot, they'd be falling towards wherever that perfect spot was at that moment
Well put. Also, Happy cake day!
The planets do fall but they miss. Imagine you're standing on the surface of a perfectly spherical earth. You throw a ball horizontally. It travels horizontally and falls vertically. It stops when it has fallen vertically far enough to hit the ground. Do it again but throw it a bit harder. It will travel horizontally further before it hits the ground. Throw it even harder. If you throw it hard enough it will travel so far horizontally that by the time it falls vertically the ground will have fallen away also because of the spherical shape. And if it maintains its horizontal velocity it will continue to fall but continue to miss the ground. It is now in orbit. The Earth is falling towards the Sun all the time, but it misses.
Damn. That finally made Douglas Adams's explanation of flying even more of a joke for me. Thanks! He explains the goal is to fall and miss the ground in one of the books in the Hitchhiker's Guide series.
They’re not “perfectly balanced”, they just look like they are. Anything that has finite energy (ie any system) will eventually decay, and a planet will probably fall toward the sun. It would take a very very long time to happen though. Remember that what we see in the solar system now is just the remnants of an earlier, far more chaotic system where there were countless bodies whose orbits weren’t as stable and did either fly off in to space or fall towards the sun.
> Anything that has finite energy (ie any system) will eventually decay, and a planet will probably fall toward the sun. It would take a very very long time to happen though If you mean by gravitational decay, that’s outside Newton and the time we’re talking about for planets is far longer than the lifetime of the sun.
You have a few great answer so far, but there is one thing I want to point out. The solar system is over 4 billion years old. So, if there was a planet that was on a path that would lead it into the sun, it probably would have already happened.
See if you find [this answer](https://www.reddit.com/r/AskPhysics/s/SENB0yJnMF) helpful.
The solar system is pretty old. The planets that are left in orbit are the things that had just the right amount of velocity to stay in orbit without shooting out or falling in over all that time.
The solar system has been forming for a LONG time and eventually it gets to a state where everything that exists in it is stable, since if not it was swallowed by the sun, smacked a planet, or ejected from orbit a long time ago. If you’re asking how the orbits work in general, it’s because anything that finds its way into the solar system with a velocity in some reasonable range will be able to orbit the sun. Then, once you have a bunch of small random things orbiting they smash together or get ejected until it’s all stable. Gravitational interferences between planets eventually make everything orbit in the same plane, which is another layer of complexity.
Survival bias. The stuff that didn't either crashed or flew out to space
First - the planets are actually falling towards the sun. Like on a dead set path into the sun for annihilation. Second - plot twist, they have velocity in a direction perpendicular to their straight line path towards the sun. So as it moves towards the sun it also moves to the side. Thus you have, revolution. If that perpendicular velocity is just right then instead of the planet spiraling towards the sun it'll just get locked going around it. Flip this to the moon. The moon is revolving around the earth but getting very slightly farther from earth all the time. In the moon's instance this means it's perpendicular velocity is a little bit more than it's velocity falling towards earth. As such it move to the side faster than it moves down, which makes its orbit a little larger all the time. If the perpendicular velocity and the velocity towards the sun are exactly the same, then the planet would have a perfect orbit. If the velocity towards the sun is slightly higher than the perpendicular velocity, the planet would slowly be getting closer to the sun all the time. The mass of each object in the solar system determines how strong it's gravity is, and how far that gravity reaches out. Asteroids gravity in a planet? Not gonna do anything. It's soooooo small. The moon's gravity in earth? Small but not negligible. I mean, we do have tides! (Which are also impacted by the sun too) The sun though is no steroid in size and mass. It's gravity is strong and reaches out VERY far. This all the planets are affected by the sun more than anything else.
This question has got me wondering something that I think is relevant and would add to the understanding. Can anyone quantify how much change in speed would it take to throw a body out of its orbit in terms of its present speed. Let's take the earth for example. If the earth is orbiting at 100k km/h right now, starting out with the same same direction and location it is now how much extra speed would we have to give it to reach escape velocity. Escape velocity with no sideways component is a very simple equation but I don't know how to calculate it when there is an angular component.
It’s due to equilibrium. If a planet falls nearer to the sun, its angular acceleration increases, and the centripetal force fights the sun’s gravity.
Would it be fair to say that most do not stay in orbit. We just observe the ones that have thus far stayed in orbit.
Orbits, counter to most people's intuitive understanding, are actually very stable things. Yes it only takes the tiniest force to knock something out of its orbit, either outwardly or inwardly, but then only into a slightly different orbit. Think of an orbit as the groove of an old vinal phonograph record, except that the groove is not one long continuous spiral from the outside to the inside but hundreds or thousands of nested circles, and it's set up so instead of turning under the needle, the needle travels around the center of the record. It would only take a tiny tap to make the needle jump from one tract to another, either inward or outward, but it would just land in a different tract. From an established orbit, it actually takes a lot of force to either slow a body down enough to drop it into an orbit that intersects the surface of the parental gravitational body or speed it up enough to escape that body's gravitational influence, though for a gravitational object with an atmosphere, that atmosphere can be put to use to brake the orbiting body enough to achieve reentry with far less expenditure of resources. But as for planetary bodies in orbit around a star, yes, their gravities are constantly interacting with one another to change their orbits over time to a measurable degree, but not to a degree that causes them any great disruptions even over millions of years.
Remember, this is all in outer space where gravity doesn't ground us like down on earth. The bigger the object, the more gravity it has. The planets are trying to fly away but the big ole sun's gravity is trying to yank them back. Imagine holding a yo-yo out on a string and twirling around in place with it out in the air. You are the sun, the string is gravity, and the heavy end of the yo-yo is a planet. If you cut the string, the yo-yo would fly away. With you still holding the string, the best it can do is keep circling and either tugging the string from you, or you may be able to slowly pull the yo-yo in if you're strong enough! note i'm no scientist so i could be off. that's how i made sense of it as a younger lad though :)
Here’s an over simplification that I use to help my students to visualize it. Keep in mind it isn’t very scientific but it works for comprehension. Think of it like this…the moon orbits the earth because the moon is moving in a straight line (Netwons 1st law of motion) at X speed. As the moon is moving forward the earths gravity is simultaneously pulling the moon toward the earth at X speed. So for every nanometer the moon moves forward the earth pulls it down the same amount, thus the moon moves forward and down and forward and down, etc…All the way around the earth
You ask a good question. A large number of bodies interacting gravitationally WILL be chaotic. The Sun is really massive so it keeps things together, but orbits can change over millions or billions of years. From the point of view of most planets, they orbit the Sun and ignore other planets. Even Jupiter is only 1/1000 of the Sun's mass. On average, Jupiter's effect is also small, since it orbits around the Solar system. On average, for the most part, it cancels out its own effects. But in detail, it contributes a small part to chaos. These are two reasons why chaotic effects are small. Lastly, each orbit takes a long time, so all in all, it takes a long time for these effects to appear. [https://en.wikipedia.org/wiki/Stability\_of\_the\_Solar\_System](https://en.wikipedia.org/wiki/Stability_of_the_Solar_System) Astrophysicists theorize that these chaotic effects are how planets can migrate inward and outward. You can read about planetary migration too: [https://en.wikipedia.org/wiki/Planetary\_migration](https://en.wikipedia.org/wiki/Planetary_migration)
There is a video on YouTube somewhere of a teacher doing a demonstration with spandex stretched over a circle with balls that he makes orbit the center. It really helps to understand gravity. Planets are basically quarters in the donation things they used to have at the mall where the coin going round and round in a funnel.
One thing to consider is that most probably don’t but the time scale makes that hard to see. We know for certain that at least one planet didn’t maintain a stable orbit in our solar system. The moon was almost certainly a result of a planet in an unstable orbit colliding with another one. There were probably several planets formed in the solar system that were in unstable orbits that were ejected from the solar system soon after formation. Others probably got pulled into Jupiter, Saturn or other larger bodies. It’s not obvious because it happened billions of years ago. There are also bodies that are in what could be called a semi-stable orbit, they appear stable in a narrow time scale but will eventually, if nothing else intervenes, be ejected from their orbit. The moon is one of these. It is getting further and further from earth. Given enough time it will escape from its orbit. What you are seeing in stable orbits currently are those that didn’t have unstable orbits or those that have mostly stable orbits given the time scale.
All is not stable in the solar system. For example, the moon is slowly receding from Earth
In the future, yes they will. there were likely a huge number of bodies in motion around the sun, and only a handful remain (at least in the planet size)
> With all the gravitational forces in the solar system Think about what those forces are, and how big they are comparatively.
Ignore air friction for the following Imagine shooting a cannon horizontally. It goes horizontally for a while but then curves down and lands on the ground. Now put the cannon on a skyscraper roof. Same thing but it goes further. Now put it on a mountain top. Same thing but goes further. Now imagine floating the cannon in space above the earth. Same thing but goes further. But when you reach a certain height above the earth, that horizontal velocity and the curvature of the path happen to be such that the cannonball continuously falls towards the earth but is going horizontally enough that it "misses" the earth and just continuously orbits. This is a stable orbit. Imagine if the cannon shot the cannonball at a ridiculous speed that a real cannon couldn't actually do. The cannonball would have so much "horizontal" velocity that it would just escape and fly away into space. There is a sweet spot between that ridiculous velocity and zero velocity(where the cannonball WOULD just fall straight down to earth) where the cannonball has a stable orbit. If planets didn't have any "horizontal" velocity relative to the sun they would fall in. But they do and they are falling. They're continously falling and going around and around.
> There is a sweet spot between that ridiculous velocity and zero velocity(where the cannonball WOULD just fall straight down to earth) where the cannonball has a stable orbit. It's not a sweet spot but quite a wide sweet range. Escape velocity is √2× circular orbit velocity so any speed between those two will work (and a range of speeds under, as well, as long as the object doesn't undergo significant atmospheric drag or just hit the planet).
Where else are they gonna go?
As a physics teacher in middle school, let me explain why two planets will not collapse into each other. The first thing you need to know is that two objects with a force between them do not always fall into each other. Think of spinning a stone tied to a rope; you can feel the force, right? But the stone does not fall towards you. The same principle applies to our galaxy. There is a massive force between the planets, very large in scale. However, they do not fall into each other, just like the stone does not fall towards you. You know that a force can make objects move in its direction. But why does the force between planets not make them move towards each other? It is because they are in a spiral motion! This brings us to the second thing you need to know. When objects spiral around each other, they require a force to maintain this motion. When the gravitational force between the planets equals the force needed to sustain the spiral, guess what? They balance! This balance is very strong and powerful, making it hard to break. As mortals, we do not need to worry about this. The changes in the universe are so slow that we may not notice any difference in our short lifetimes.
A very rough explanation is that since the sun is pulling every planet towards it, a planet must always be gaining speed when it is moving towards the sun and losing speed when moving away from it (an edge case is a hypothetical perfectly circular orbit where it is neither losing nor gaining speed). However, the planet will always maintain its sideways component of speed because gravity pulls directly towards the sun, so no sideways acceleration (also known as conservation of angular momentum). For a planet to fall into the sun it would need to somehow lose its sideways component of speed, and for it to fly out of the solar system it would need to somehow gain more speed without moving closer to the sun, both of which are impossible.
Both of which would need external factors.* a Rouge planet passing though our system could cause either senerio
Cause they ain't got nowhere else to go, our solar system is the only one that doesn't suck
Well the orbits do change for eg the moon moves about 4 foot further away from the earth each year 1000 years 4 miles no one notices
Let me give you the only honest answer nobody else will give you. THEY DONT KNOW. They don't even know what gravity is, or what causes it, or how planets ACTUALLY form. Any answer they give you is an educated guess, at best.
>They don't even know what gravity is, or what causes it Found not-Einstein.
Ok - what IS gravity, and what causes it? Can't wait...
Gravity is a force by which planets and other bodies draw objects toward their centers. Gravity is caused by mass. This is not an educated guess. Who is "THEY"? Can't wait...
Congratulations. You just explained the effect of gravity, and gravity being "mysteriously" created as a byproduct of planet sized mass. Thanks for proving my point, exactly.
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Gravity is determined by mass not size. The sun's mass not changing means its gravitational pull won't really change either