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A105 Stars and Galaxies: Planets around Other Stars
Name _________________________
Reproduced below is a plot of observations of the radial velocity of the star 51
Pegasi, the first star discovered to have a planet. The observed velocity (in
meters per second) is plotted vs. the time (in days) when the observation was
made. The velocity of the star varies with time because the star and planet orbit
around their common center of mass. Remember the Doppler shift? The star
alternately moves toward and away from the Earth as it is tugged by the planet
orbiting around it. Use the velocity observations to estimate the period of the
orbit (the planet’s year).
What is the maximum velocity with which 51 Peg moves away from the Earth?
What is the minimum velocity with which the star moves toward the Earth?
Velocity (meters per second)
Sketch in a curve which oscillates between the maximum and minimum
velocities, and follows the points as closely as possible. You will need to
“estimate” where the curve should go during times when no observations were
taken (for example, between days 6 and 11, when the weather was cloudy).
Time (days)
Estimate the period of the orbit (the time for one complete orbit, or the time
between maximums (or minimums) in the velocity curve). What is the length of
the “year” of 51 Peg’s planet?
Next, estimate the mass of 51 Peg’s planet. The following graph relates the
mass of the planet to the orbital period, assuming that 51 Peg itself has a mass
of about equal to the Sun. The graph is determined using the laws of orbital
motion discussed in Chapter 5. The masses of extra-solar planets are usually
measured in units of the mass of Jupiter (e.g. 5 times the mass of Jupiter or 0.25
times the mass of Jupiter). Use the graph and the orbital period you measured to
estimate the mass of 51 Peg’s planet.
Planetary Mass (MJup)
Period (days)
How does the mass of 51 Peg’s planet compare to Jupiter?
Estimate the radius of the orbit of the planet around 51 Peg. Orbital radii are
usually measured in terms of AU (1 AU = 1 astronomical unit, the average
distance between the Earth and the Sun, about 150,000,000 kilometers). Again
we will use the laws of orbital motion from Chapter 5. The formula is
radius orbit 
6 * period
1000 * mass planet
What is the radius of the planet’s orbit (in AU)?
How does this compare to the distance from the Earth to the Sun (1 AU)?
How does it compare to the distance from Jupiter to the Sun (5 AU)?
What differences might you expect between 51 Peg’s planet and Jupiter based
on your calculations?