Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Planetary orbital planes

  1. Nov 11, 2015 #1
    Hello every one. It's obvious that all of the major planetary objects in our solar system orbit the sun in roughly the same plane. My question is, is our solar system in a similar plane as it orbits the center of the Milky Way? If so, do most star systems share the same plane? This came to mind recently as I was thinking about the search for exo-planets. The two ways I am familiar with when searching for evidence of exo-planets are cyclical 'dimming' as an orbiting object passes between we observers here and the star in question and the 'wobble' effect as the orbiting object's gravity pulls it's star to and fro. The first way would seem to only be effective if the orbit of the system we observe is in roughly the same plane as we are.
  2. jcsd
  3. Nov 11, 2015 #2
    The milky way does have an axis of rotation, as would be expected for a disclike spiral galaxy.
    Most of the stars travel within the plane of the disc (approximately) and at any given distance from the center they travel at similar(ish) velocities.
    It's by no means exact though, just 'on average' most of them do, and there are some examples of stars traveling on trajectories which don't fit the general pattern at all.
    These anomalous stars result from unusual local gravitation interactions, such as a multiple star system from which one star has been ejected.

    The spin axis of the stars themselves and associated planetary systems it not at all aligned with the galaxy's axis of rotation.
    As seen from Earth the axis of rotation of other solar systems doesn't appear to fit any particular pattern, it can't be distinguished from random.

    You are correct in saying that the detection of exoplanets due to their occultation of the parent star can only happen when the plane of the exoplanet's orbit just happens to be nicely aligned with Earth.
    It's pure chance when that is the case, so many exoplanets are not detectable that way, but there are other ways such as detecting wobbling of the parent star due to the gravity of planets orbiting it.
    Last edited: Nov 11, 2015
  4. Nov 11, 2015 #3
    The sun's planetary plane is about 60 degrees offset from the galaxy.
  5. Nov 11, 2015 #4


    User Avatar
    Science Advisor
    Gold Member

    ...which can be observed with a naked eye by going out stargazing and comparing the angle the band of Milky Way makes with the ecliptic (whose position is easy to approximate by following the zodiacal constellations).
  6. Nov 11, 2015 #5
    That's great stuff to think about. So if we look at a system's suspected planet's 'occulation' effect how do we estimate, merely by it's relative brightness, the properties of the planetary body? It could be just barely grazing our line of sight or it could be being observed nearly edge on causing maximum effect.
  7. Nov 11, 2015 #6
    Yes, and there is another problem: even if you are on about the same plane, the planet doesn't necessarily occult. For example, Venus is more or less the same plane that we are, and we pass it every few years, the next time it'll actually transit the sun is in 2117.
  8. Nov 11, 2015 #7


    User Avatar
    Science Advisor
    Gold Member

    The depth of the luminosity dip, that a planet of a given size causes, is largely independent of where it transits. A grazing transit can net a shallower dip, but then the light curve is notably different, not to mention very rare.
    Use this transit simulator to find out how the position of a transit affects the light curve.
  9. Nov 11, 2015 #8
    A pic that show the 60° incline.

  10. Nov 11, 2015 #9


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Venus' orbital inclination to the ecliptic is 3.39 degrees. In terms of occluding the Sun, as seen from a distant star, it would not be about on the same plane . The sin of 3.39 degrees times the radius of Venus' orbit equals ~6.4 million km. that's better than 9 times the radius of the Sun. So assuming you were looking at the Sun from a distant star, that 3.39 degree difference would be more than enough to prevent you from seeing Venus occlude the Sun. To be sure to see it, the tilt would have to be under 0.36 degree.

    Seeing it occlude from the Earth is a bit different. When Venus passes the Earth, it is only some 41.4 million km apart. So let's assume, for the sake of argument, that Venus' inclination was that 0.36 degree value. This means that it would get as far as 695,000 km from the ecliptic and from the Earth at the time of passing could be as much as 0.96 degrees from the ecliptic. The Sun has only an angular size of ~0.5 degrees. So you still wouldn't be guaranteed to see Venus occlude the Sun every time you passed it.

    This is because the distance from Earth to Venus is smaller than Sun to Venus and this magnifies the effect of the inclination as seen from the Earth. The reason we do see Venus occlude the Sun from time to time, is because those times are when Venus passes Earth when it is near either its ascending or descending node.

    When we are viewing a planet occlude a distant star, our distance to the planet is much much larger than the planet's distance from its star, so we don't have to worry about how it effects things.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook