ESA - C. Carreau


It's not strictly correct to say that a planet
orbits its parent star.

Given that each exerts a gravitational pull
on the other, how could we expect the star
to remain absolutely fixed while the planet
orbits around it?

In fact both orbit; and they orbit around a
common point called the Center of Mass.


Wikipedia defines the Center of Mass (CM)
as, a

"specific point at which, for many purposes,
the system's mass behaves as if it were con-
centrated"


Physicists have used the concept of "balance"
to define CM,

"The center of mass is familiar to anyone
who has ever played on a see-saw. The fulcrum
point at which the see-saw will exactly balance
two people sitting on either end is the center
of mass for the two persons sitting on the
see-saw."


The figure below depicts a two-body system;
X marks its center of mass.


                                                  image credit

If X were the fulcrum of a see-saw, the two
bodies would balance.

For a two body system, the center of mass
can be determined through these equations:
where m denotes mass; d, distance; and R,
the total separation between the two bodies.

         
                               image credit


When the a planet and star orbit their center
of mass, CM will almost certainly be located
within the star itself, but not at its center, as
shown in the video below.