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Why do Jupiter's moons line up?

  1. Aug 14, 2009 #1
    So I took my telescope out last night and got to actually, really see Jupiter for the first time and it was amazing! While I was looking at it I could see it has 4 moons (though I later found out it has way more than that, but the ones I observed were the big Galilean moons). Another thing I noticed was that the moons were 'lined up'. That is to say, they were orbiting in a 2D plane it looked like (and further, I wasn't above or below but IN the plane).

    Why is this? Why do the moons orbit in one plane instead scattered all about in different orbits? Further, why is an observer from Earth parallel to the plane instead of viewing it from above or below?

    I'm looking for a physical explanation, which is why I didn't put this in the astronomy section.

    Please don't give an explanation that requires knowledge past a high school AP Physics class and Calculus I class.

  2. jcsd
  3. Aug 14, 2009 #2
    This is an extension of the fact that the planets are all roughly in the same plane, too. When the solar system formed, it was initially a large cloud of mass around a central proto-star. Conservation of angular momentum implied that because it was rotating, this cloud had to flatten out into a disc. As a result, all of the mass was in a (roughly) two-dimensional plane. This is why most of the planets are not inclined much to Earth's orbit, and the same should follow for Jupiter's moons, since they were formed from the same place.
  4. Aug 15, 2009 #3


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    Jupiter's plane of rotation is very close to its plane of orbit, closer than all the other planets. This is due to its size - it is not easily perturbed. Because of this, we will always see its equator and its moons' orbits edge-on.

    The reason its moons all orbit in the same plane is the same reason why all the planets are in the same plane in the Solar System - satellites that orbit off the plane of their parent are unstable. Due to a number of gravitational factors, they will eventually be nudged into equatorial orbits.
  5. Aug 15, 2009 #4
    What are the gravitational factors that nudge the the planets into equatorial orbits? Why aren't the moon's rotation around the planet, say, perpendicular to the equatorial plane?

    Would the Sun's gravitational pull on the moon of a planet be one of these nudging effects?
  6. Aug 15, 2009 #5
    If Jupiter was a perfect, non-rotating sphere then the moons could orbit with any inclination they please - a spherical mass has a perfectly symmetrical field. But Jupiter in fact spins very rapidly - 12.5 km/s at its equator - and this gives it a non-spherical shape that distorts the shape of its field. Thus unless a body is in the plane of its equator it will experience a small force towards the equatorial plane and over time it will eventually find itself orbitting in the plane.

    But the non-symmetrical component of the gravitational force drops off with distance more rapidly than the symmetrical component, thus at the distance of Jupiter's outer moon groups the plane-directing component of the gravitational force isn't strong enough to change the orbits of the irregular moons in a significant way since they entered those orbits aeons ago.

    The Sun does influence the orbits of the outer moons too, as do the other planets, especially Saturn.
  7. Aug 15, 2009 #6


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    The reason orbits become coplanar is because the rotating cloud of gas that collapses to form a planet rotates about an axis, so a moon above or below the planet wouldn't be orbiting the planet.

    Also, Jupiter's moons orbit in a plane near the ecliptic, but right now, they are almost exactly on the ecliptic. It is more aligned than ususal.
  8. Aug 15, 2009 #7
    Russ, the field of a rotating, collapsing mass pushes/pulls it all into the equatorial plane. That's how an initially spherical prestellar mass collapses into star-and-disk.
  9. Aug 24, 2009 #8
    Also, at this time the satellites are so nicely lined up that, on their orbits, they now are occasionally occulting each other. Seeing the satellites passing in front of Jupiter is quite common, but seeing one satellite passing behind another is quite rare.
  10. Aug 24, 2009 #9


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    If Jupiter were a perfect, non-rotating sphere, the moons would not be able to orbit in any inclination. Polar orbits, and any orbit with an inclination of above about 60 degrees would be unstable due to the Kozai mechanism. The influence of the Sun would cause any moon with a high inclination to periodically exchange inclination and eccentricity. As orbits get more and more eccentric over time, they will cross each other. This is why Jupiter has no moons with inclinations greater than 60 degrees. See here for more details.
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