Does anyone know why all planets in our solar system except for Venus I think

Discussion in 'Astronomy, Exobiology, & Cosmology' started by Tortise, Mar 24, 2006.

  1. Tortise Registered Senior Member

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    rotate in the same direction? I'm not asking why they rotate around the sun in the same direction and plane, but they all rotate and have "days" in the same direction.
     
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  3. 2inquisitive The Devil is in the details Registered Senior Member

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    Look up retrograde rotation in the solar system. Besides Venus, Uranus and Pluto also have retrograde rotation (rotate in the opposite direction of their orbits around the sun). The consesus is that Venus and Uranus suffered impacts early in the formation of the solar system, reversing their rotation from prograde rotation. I think Pluto is thought to be a captured Kuiper Belt object, hence its ellipital orbit out of the normal plane of the other planets. If the many smaller moons in the solar system are included, more orbit in retrograde orbits than in normal prograde orbits. Most of the these moons are thought to be captured objects, also.

    It is thought the solar system formed from a huge cloud of gas and dust which was disturbed by a Supernova, compacting the cloud and starting it to rotate. As the particles clumped together due to gravity, conservation of angular momentum should have caused all the proto planets to rotate in the same direction, prograde rotation. As I mentioned before, collisions are thought to have altered the rotations of Venus and Uranus. Our own moon is generally accepted to have been formed from the proto-Earth through a huge collision that almost destroyed the Earth, tearing part of the planet off. Some of the pieces formed our moon, and some were attracted back to the proto-Earth. By the way, our moon's orbit is prograde, even though it travels East to West in our skies. The moon orbits the Earth at a slower period than the Earth rotates, hence the seemingly wrong orbital direction for an Earth-based observer.
     
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  5. Tortise Registered Senior Member

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    Thanks.
     
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  7. Lensman Registered Senior Member

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    The planets are believed to have formed by "accretion'. That is, when a "lump" in the early solar system got big enuff, it would start attracting other lumps, which would drift together and sometimes stick, making a bigger lump. Now when lumps from two different orbits drift together, one will be moving faster and the other slower. If they come together slowly, they will most likely hit each other when the faster moving one is on the outside and the slower on the inside. If they stick, this will cause the combined mass to rotate faster in the direction of orbit, just like an ice skater can twirl faster by pulling her arms in. Of course some of the collisions are gonna tend to twirl the total mass the other way, but the average over many, many collisions should cause all the planets to rotate in the direction of their orbit.

    Uranus doesn't really have retrograde rotation. It's just that its axis of rotation has tipped more than 90 degrees. We can be thankful we have a large moon to stabilize earth's axial tilt. For the other planets, axial tilt is chaotic which may occasionally cause it to tilt over extremely far, as Uranus has. Uranus will likely right itself in the "near" future... astronomically speaking.

    I haven't seen any theory that really explains Venus' very odd rotation. I doubt a single impact could be sufficient to reverse a planet's spin like that. An impact that powerful would knock a planet to pieces. One theory is that Venus has a large, denser "lump" on one side and that it is locked into a rotational resonance with Earth. Again I don't see that's a very likely theory. Any heavier "lump" in a planet as large as Venus (or even a lot smaller) should settle to the center. Anyway, the Earth's gravity shouldn't be powerful enuff to affect Venus that strongly.
     
  8. 2inquisitive The Devil is in the details Registered Senior Member

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    I do not understand how you can state that Uranus doesn't really have retrograde rotation. The rotation of the planets is described as prograde or retrograde by assigning an imaginary observational location above and 'north' of the solar system, north as defined by our concept of north on Earth. Your suggestion is that Uranus' rotation is prograde, but tipped more than 90%. When the axis of rotation is 'tipped' more than 90%, the planet appears to rotate in the opposite direction when viewed from the viewing location specified for our solar system. Uranus' axis of rotation is 'tipped' 98%, therefore retrograde. You say Uranus will likely 'right' itself in the future. Do you suggest Uranus' axis of rotation will pass back through the 90% plane to become prograde again? What mechanism would favor the axis shifting in that direction? Why would the axis not eventually become '180%' relative to the orbital plane instead?
     
  9. Gordon Registered Senior Member

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    The simple answer of course really is that no one knows. Everything that has been presented is simply at best a theory or in non scientific jargon a 'guess'.

    There is no evidence to substantiate the collision theories and much evidence against them. In particular as pointed out in another post, the physics simply do not work. You have to believe that on collision sufficient energy was transferred to completely reverse the direction of orbit (a fair amount given the mass of an object like Venus) but that the planet held its solar orbit and remained basically an intact sphere whilst the offending object either went on its way and left the solar system or was completely destroyed (there being no sign of it now!).

    This just does not work!

    Accretion is also a very dodgy theory from a physics point of view. You simply do not get the gravitational forces from swirling gas clouds in space necessary to produce large compressed spherical bodies such as the planets.

    The very valid point made is about how useful it is for Earth to have one moon of the large size that it is (compared to the Earth). As stated this holds the Earth steady and stops it doing a 'Uranus'. If the Earth did roll like some of the other planets, the seasonal variations would probably be so extreme that life as we know it would not exist - so be very thankful indeed for that large dead lump of rock up there!
     
  10. c7ityi_ Registered Senior Member

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    uranus and it's moons are like a little independent solar system.
     
  11. Xylene Valued Senior Member

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    Could be that Venus was flipped end over end some time in the past, and rotates retrograde because its south pole is north and vice versa

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    Just a passing thought
     
  12. Xylene Valued Senior Member

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    So's Jupiter, for that matter--and all the major gas giants, I guess--minor stars with their own systems of micro-planets.
     
  13. Lensman Registered Senior Member

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    I agree the astronomy books say that, but it's geometric nonsense. If you insist the rotation is retrograde, then you can't simultaneously claim it's tipped more than 90°. If you insist its rotation is retrograde, then the tilt is only 82°. If you admit it's tilted 98°, then by definition its rotation is prograde. One or the other-- you can't have both, any more than you can have a double negative in math.

    Of course.

    Same thing that tipped it "too far" over in the first place. Random perturbations coupled with chaotic motion.

    A. Conservation of energy and gyroscopic motion. Just like a gyroscope, the farther it tips over the faster it will right itself.

    B. Because none of the other gas giants have, and they all must have chaotically tipping axes.
     
    Last edited: Apr 26, 2006
  14. Lensman Registered Senior Member

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    Why do you say that? The math seems to work, computer models do result in planets forming, and now we've even seen the accretion disc around another star, with planets forming in it. How much more evidence can there possibly be?
     
  15. Pete It's not rocket surgery Registered Senior Member

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    That's very interesting.
    Do you have a reference where I can read more about it?
     
  16. D H Some other guy Valued Senior Member

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    What nonsense! Venus, Uranus, and Neptune have retrograde rotation by the commonly accepted definition of the word <i>retrograde</i>. You are confusing the rotation rate with retrograde.

    Sure you can. -(-1) is well defined; it is just 1.

    Mathematically, the rotation is retrograde if the inner product of the planet's orbital and rotational angular momentum vectors is negative.


    Most think collisions with massive bodies tipped the planets over. I don't know what you don't think the gas giants don't have, but they are rotating and do have angular momentum due to that rotation. That angular momentum must be conserved unless the planet is acted upon by some external torque (such as a planet smacking into it).
     

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