How did planets end up orbiting the sun?

[Arjun Artro]

If stars attract everything,and that too with a very high gravitational force, objects like planets would've fallen into the star. But how did they start orbiting the stars?

 

[iNow]

If stars attract everything,and that too with a very high gravitational force, objects like planets would've fallen into the star. But how did they start orbiting the stars?

 

Their orbits are an expression of them falling into the star, just at a time scale that is difficult for us humans with our very short lifespans to notice. Planets are falling into stars and stars are falling into galactic centers and galactic centers are falling into clusters or superclusters. That's what an orbit is.

 

More here: http://spaceplace.nasa.gov/review/podcasts/transcripts/060713_planets_orbit.html

[ACG52]

If stars attract everything,and that too with a very high gravitational force, objects like planets would've fallen into the star. But how did they start orbiting the stars?

 

Well first off, gravity falls off as the square of distance, so it not all that high. When planets formed, they had angular momentum, which expressed itself in their orbits. So they keep going round and round (or eliptical and eliptical).

Edited December 7, 2012 by ACG52

[Spyman]

If stars attract everything,and that too with a very high gravitational force, objects like planets would've fallen into the star. But how did they start orbiting the stars?

Stars and their planets are formed together from large clouds of matter that collapse into a swirling disc. The star forms at the core and the planets by accreating matter in the disc, as such planets are normally formed already in orbit around their star. But during this process it is possible that some protobodies could get ejected from their birth system and then later on get gravitationally captured by a another star or planet they encounter.

 

The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud. This initial cloud was likely several light-years across and probably birthed several stars. As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elements fused by previous generations of stars. As the region that would become the Solar System, known as the pre-solar nebula, collapsed, conservation of angular momentum caused it to rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc. As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU and a hot, dense protostar at the centre. The planets formed by accretion from this disc, in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies. Hundreds of protoplanets may have existed in the early Solar System, but they either merged or were destroyed, leaving the planets, dwarf planets, and leftover minor bodies.

http://en.wikipedia.org/wiki/Solar_System#Formation_and_evolution

[CarbonCopy]

The planets do not fall in due to centrifugal force from left over motion from the gas clouds that created all the of them. This counter acts i the gravitational force and slows down the rate of fall. Its like when you go in a merry-go round you are being pushed outwards, same thing.

[swansont]

The planets do not fall in due to centrifugal force from left over motion from the gas clouds that created all the of them. This counter acts i the gravitational force and slows down the rate of fall. Its like when you go in a merry-go round you are being pushed outwards, same thing.

 

Except you aren't being pushed out. It seems that way if you are in an accelerated reference frame, but in an inertial frame, there is no outward force. Objects moving in ellipses/circles need a force directed inward, which in this case is gravity. Planets are falling in to the sun, but they keep missing owing to their tangential motion.

[CarbonCopy]

 

Except you aren't being pushed out. It seems that way if you are in an accelerated reference frame, but in an inertial frame, there is no outward force. Objects moving in ellipses/circles need a force directed inward, which in this case is gravity. Planets are falling in to the sun, but they keep missing owing to their tangential motion.

 

Ya basically that's the answer.

[Ophiolite]

inow I just gave you a like when I was aiming for the quote function on your post to tell you that you are talking bollocks. Planets are not falling into the sun, even slowly. As the sun is losing mass throught coronal mass ejections and the solar wind planets are in fact, ever so slowly, moving further away from the sun.

 

Perhaps you could join as a sock puppet and give yourself a negative rep to counteract my mistake.

[iNow]

you are talking bollocks. Planets are not falling into the sun, even slowly.

 

 

I stipulate that my post was not as clear as it could have been, but my attempt was to reference the balance between tangential motion and inward curvature as has been shared by others here. Orbits tend to slowly degrade with time, and the question is really no different than asking, "Why don't telecommunications satellites fall into the earth." The answer, of course, is that they do. We just don't tend to notice without access to precise measurements because of the time scales involved.

 

I would be curious to learn more about your correction, though. I would think that the loss of solar mass you reference might make planets fall inward more slowly, but surely not move away as you've suggested.

Edited December 8, 2012 by iNow

[D H]

I would think that the loss of solar mass you reference might make planets fall inward more slowly, but surely not move away as you've suggested.

 

You can't just think in physics. You have to do the math. So, do the math. Assume a body is orbiting circularly and the Sun loses a tiny bit of mass. I'll use the Sun's standard gravitational parameter [imath]\mu\equiv GM_{\text{sun}}[/imath] instead of mass; it makes the math easier and it's what is observable. Initially, the relation between velocity, the Sun's mass, and orbital radius is given by

 

[math]v^2 = \frac \mu a = \frac \mu r[/math]

 

Now suppose the Sun gains or loses a bit of mass: [imath]\mu \to \mu + \Delta \mu[/imath]. Note that the increment here is positive; losing mass means [imath]\Delta \mu < 0[/imath].

 

Per the vis-visa equation,

 

[math]v^2 = \frac \mu r = (\mu + \Delta \mu) \left(\frac 2 r - \frac 1 {a'}\right)[/math]

 

where [imath]a'[/imath] is the semi-major axis length after the mass loss. Denoting this as [imath]a'=a+\Delta a[/imath] with [imath]a=r[/imath] yields

 

[math]\frac \mu a = (\mu + \Delta \mu) \left(\frac 2 a - \frac 1 {a+\Delta a}\right)[/math]

 

Grind this through and you'll get

 

[math]\frac{\Delta a}{a} = -\,\frac{\mu}{\Delta \mu}[/math]

 

In other words, a loss of mass ([imath]\Delta \mu < 0[/imath]) causes the orbit to expand.

 

There are other effects besides mass loss from solar wind on a planet's orbit. There's also mass loss from radiation. This currently dominates over mass loss from the solar wind. When the Sun was young it was the other way around. However, mass loss is mass loss. It doesn't matter how the Sun is losing mass. Mass loss causes orbits to expand. Effects other than addition to mass loss include

• Radiation pressure (radial component)
  The radial component of radiation pressure exerts an anti-sunward force on objects. Gravitation is of course inward; both are inverse square law forces. Define [imath]\beta[/imath] as the ratio of the outward force from radiation pressure to the inward gravitational force. [imath]\beta>1[/imath] for extremely small particles. Radiation pressure ejects very small particles from the solar system. [imath]\beta[/imath] deceases as particle size increases assuming constant density. Eventually increasing particle size will make the inward gravitational force dominate over the outward radiation pressure force. At this stage, the radial component of radiation pressure becomes a no-op. The anti-sunward component of solar radiation pressure merely makes the Sun appear less massive.
   
  
• Radiation pressure (tangential component)
  Thanks to the finite speed of light, sunlight hits an orbiting body with a non-zero tangential component directed against the body's velocity vector. This creates a drag on the body called Poynting-Robertson drag. This causes small particles (but not so small as to be ejected by the radial component of radiation pressure) to spiral inward toward the Sun. Because radiation pressure increases with the square of radius while mass increases with the cube, this Poynting-Robertson drag becomes smaller and smaller as body size increases. It is incredibly small for planet-sized bodies.
   
  
• Solar wind drag
  The solar wind, like radiation pressure, has both radial and tangential components. The effects of each component are very similar to those of radiation pressure.
   
  
• Yarkovsky effect
  The Sun warms the sunward facing side of an orbiting body. This results in an anisotropic distribution of the thermal radiation from the orbiting body. If the body rotates, the outgoing radiation is more intense on the dusk side of the planet than the dawn side. This results in an acceleration in the dawn direction. The Yarkovsky effect mades a body with a prograde rotation spiral outward from the Sun; a body with a retrograde rotation with spiral inward.
   
  
• Gravitational radiation
  Per general relativity, an accelerating object will radiate energy in the form of gravity waves. This makes very massive objects orbiting very close to one another spiral inward. (1993 Nobel Prize in Physics). In our solar system, this is essentially a non-effect. The effect on the Earth, for example, is to cause it to spiral inward at about 10^-15 meters per day.

 

For planetary sized objects, all of the above are essentially non-effects, completely dominated by the spiraling out due to solar mass loss.

Edited December 9, 2012 by D H

[Ophiolite]

Thank you for reminding me of the Yarkovsky effect and explaining with greater clarity than I've previously seen. And the Poynting-Robertson drag. It keeps cropping up in the papers I read on planetary formation, but is never explained - it quite rightly being assumed anyone reading a research paper on such a topic will know what it is. I never had got around to looking into it more deeply. (More interested in the chemistry of meterorites than their orbital preferences.)

[iNow]

Excellent. Thank you, both. I need to update the way I think about this topic.

Edited December 9, 2012 by iNow

[imatfaal]

Thanks for post #2 iNow - I agreed with it at the time and would have answer in the same terms; and after your dressing down by Ophiolite and the, frankly superb, detailed post by DH I feel I have really learned something!

[StringJunky]

Thanks for post #2 iNow - I agreed with it at the time and would have answer in the same terms; and after your dressing down by Ophiolite and the, frankly superb, detailed post by DH I feel I have really learned something!

 

I agree. Even though DH goes over my head quite often, because of my inadequate maths knowledge, I appreciate the clarity and detail of his posts and I always learn something from them. The effort and desire to impart what he knows in as clear and unambiguous a manner as possible is commendable..

[Roger_XR]

I have often heard of the "angular momentum" from the creation of our solar system - and indeed the creation of the other systems and the galaxy itself. It would seem that the alignment of the planets follows the plane that the Sun rotates on with little above or below that plane.

The first question would be: What would determine the initial "angular momentum" direction that the solar system began to rotate about.

Next: It would seem perhaps that the galaxy itself formed in a similar fashion (and long before) with a large Dark Hole mass at its center. The difference being that our galaxy's center has stars and dust and other matter above and below the rotational plane of the main body of the galaxy. Would it seem reasonable to think that the Dark Hole at the center of the Galaxy is rotating in the same direction as the main body of the Galaxy much as the planets in our solar system follow the Sun?

[Civat]

I really did not knoe about any of this before.. Thanks for this

[ox1111]

I have often heard of the "angular momentum" from the creation of our solar system - and indeed the creation of the other systems and the galaxy itself. It would seem that the alignment of the planets follows the plane that the Sun rotates on with little above or below that plane.

The first question would be: What would determine the initial "angular momentum" direction that the solar system began to rotate about.

Next: It would seem perhaps that the galaxy itself formed in a similar fashion (and long before) with a large Dark Hole mass at its center. The difference being that our galaxy's center has stars and dust and other matter above and below the rotational plane of the main body of the galaxy. Would it seem reasonable to think that the Dark Hole at the center of the Galaxy is rotating in the same direction as the main body of the Galaxy much as the planets in our solar system follow the Sun?

 

At the very beginning of gravitational collapse a rotational direction is chosen or formed. Once the rotation is chosen or formed all the rest of collapsing matter will follow suit. Much like a vortex in a sink drain, although the direction is usually chosen by gravity on earth depending on which hemisphere you live on, if you use your hand at the beginning to start the water swirling in the opposite direction the vortex will happily go in the direction opposite to gravity and Earth's rotation. It truly is a butterfly effect

 

I truly love that you use dark hole Center instead of blackhole that most people would've wrote. While black holes are theoretical scientists speak of them as fact and I believe this to be a mistake. Not that blackhole don't exist which they may not just that they may not be what they seem. To answer your question I would agree that whatever is at the center of our galaxy would most likely be rotating the same direction as the galaxy itself although mathematically it is possible for it to go in the opposite direction.