Why do the Planets in our Solar System Orbit the Sun Counter-Clockwise?

Question: The planets in our solar system are orbiting the Sun counter clockwise, why? Do the laws of physics dictate that all planet orbit their respective stars counter clockwise or is it possible to have a solar system where the planets are in a clockwise motion around their star?  – David

Answer: Most of the objects in our solar system, including the Sun, planets, and asteroids, all rotate counter-clockwise.  This is due to the initial conditions in the cloud of gas and dust from which our solar system formed.  As this gas and dust cloud began to collapse it also began to rotate.  That rotation just happened to be in a counter-clockwise direction.  There is nothing special about a counter-clockwise rotation, though.  We could easily have found ourselves living in a solar system which was rotating clockwise about our Sun, if that was the initial state of rotation of the gas and dust cloud from which our solar system formed.  Note, though, that there are two oddballs in our solar system that do not rotate in the same way as the rest of the planets.  Uranus rotates about an axis that is nearly parallel with its orbital plane (i.e. on its side), while Venus rotates about its axis in a clockwise direction.  These oddities are thought to be caused by events, such as collisions, which occurred during the formation of the solar system.

Jeff Mangum

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2 Responses to Why do the Planets in our Solar System Orbit the Sun Counter-Clockwise?

  1. Þorsteinn Sigurðsson says:

    Hi,
    I was just wandering whether the orbital direction of our planets and Sun could be dictated by our orbital direction around the galaxy, just as the rotation around our Sun seems to dictate the rotation of the planets.
    I have not managed to find any sources indicating this relationship between our solar system and our galaxy.

    Secondly,
    Are you positive that orbital direction of our planets is the same as their rotation, counter clockwise? I’ve been scratching my head over this all night and find Internet sources to be confusing to say the least. They don’t agree on either the orbital direction of our planets or their rotation.
    The reason I ask is threefold:
    1. Because of angular momentum, matter in lower orbit around the Sun moves faster than matter in higher orbit. As matter condenses into planetary objects friction between the object and incoming matter would cause it to rotate in the opposite direction of its orbital direction.
    2. All planetary objects have been hammered by incoming asteroids, comets, and even other planets for billions of years. Majority of those impacts have been from objects in higher orbit around the Sun moving too slowly to maintain orbit. As they tend to hit planets on the outer side as they move in their orbit they should cause them to rotate, again in the opposite direction of their orbital direction.
    3. All planetary objects have a diameter and that diameter causes an orbital difference between the side that is closer to the Sun and the side that is further away from the Sun. The side that is further away from the Sun has a slightly longer orbit then the other. Condensed matter forming planetary objects will seek to move at the same speed, bound by gravity, and should seek to compensate this orbital difference by rotating, again in the opposite direction of it’s orbital direction. Actually I’ve been searching for sources that might tie this with rotational tendency of planets, but have found none. Perhaps this difference is too small to make an impact.

    Anyway, hope I’m not overwhelming you. Internet sources are just not helping 8;

    Regards,
    Þorsteinn Sigurðsson

    • Jeff Mangum says:

      Hello Porsteinn,

      Let me take your questions in order:

      Correlation Between Planetary Orbits and Solar System Orbit in our Galaxy: It is the original angular momentum which existed during the formation of an object, whether it be a planet, star, or galaxy, which dictates the initial angular momentum, and thus rotation speed and direction, of an object in the universe. Since the formation of our galaxy and the formation of our solar system were two separate events, their relative directions of rotation are not related. By coincidence, though, the rotation direction of the planets is the same as that of the Milky Way as viewed from above, counter-clockwise.

      Orbital and Rotational Directions for the Planets: With only two exceptions, Venus and Uranus, the orbital direction of the planets is the same as their rotational directions, counter-clockwise. Venus rotates on its axis in a clockwise direction, while Uranus has a rotational axis that is tilted by almost 90 degrees relative to the orbital plane of the planets.

      Planets naturally rotate in the same direction that they orbit due to the fact that a point on a planet that is closest to the Sun will orbit slightly faster than a point on that planet which is on the opposite side of the planet, furthest away from the Sun. As a point on the planet moves from the slower outward facing direction to the faster inward facing direction, it speeds up, while the opposite happens for points on the planet that move from the faster inward facing direction to the slower outward facing direction. This results a rotation direction that is in the same direction as the orbit of the planet. If you want to see a nice summary with diagrams that do a good job of explaining this see http://www.sjsu.edu/faculty/watkins/solarspin.htm.

      The effects of impacts on these rotational and orbital motions are in most cases not significant, with the possible exception of the “oddball” planets Venus and Uranus. Early in the formation of the Sun and planets the frequency of impacts was probably quite large, but as the planets formed those impacts became less frequent, and the relative size of those impacting bodies became smaller.

      Thanks for your comment and I hope that this answers your questions.

      Jeff Mangum

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