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Homework Help: Red/blueshift due to orbiting planet

  1. Dec 27, 2016 #1
    1. The problem statement, all variables and given/known data
    The periodical oscillations of the spectral lines of a star incicate that there is a planet orbiting around it with a period of 2.1375 days.Calculate what the observed wavelengths of the NaI double line will be,which is normally observed at 586.6nm and 589.6nm.
    Star's mass=1.2 solar masses
    Assume the planet's mass is negligible compared to the mass of the star and that its orbit is circular.

    2. Relevant equations
    Kepler's law
    Doppler effect

    3. The attempt at a solution
    I would guess the shift in wavelength is because the gravitational force acting on the star because of the planet's mass at the points radially away from the earth,cause the star to move with a miniscule speed u,resulting in a red/blueshift.
    But,firstly, I can't calculate the mass of the planet in order to get the star's acceleration
    and secondly,even if I did,the phrase "Assume the planet's mass is negligible" leads me to believe this is not the correct solution.
  2. jcsd
  3. Dec 27, 2016 #2


    User Avatar
    Science Advisor

    You are right. If the planet's mass is negligible, then there is no Doppler shift. Whoever wrote the problem is confused.
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