The mass of a star in a galaxy far, far away,

is 3.12 × 105 Earth masses, and the mean
distance from the center of this star to the
center of a planet with the same mass as our
Earth is 2.46 × 108
km.
Treating this planet and star as particles,with each mass concentrated at its respective
geometric center, how far from the center of
the star is the center of mass of the planet-star
system?
Answer in units of km.

1 answer

If we set the center of mass at 0, then we have

3.12*10^5 * 2.46*10^8 = m*d

where m is earth's mass and d is the distance to the center of the planet.